大数据第24天-子查询-杨大伟

Posted 2020-12-18 shui68home

   1 cd /d D:MyWorkmysqlmysql-5.7.28-winx64in rem 目的是为了执行mysql.exe
   2 mysql -h127.0.0.1 -P3306 -uroot -p123456 rem 作用是为了让客户端能够正确的连接服务器
   3 mysql --host=127.0.0.1 --port=3306 --user=root --password=123456 world
   4 
   5 -- 查看服务器中有哪些数据库
   6 show databases;
   7 
   8 -- 创建新的数据库
   9 create database company;
  10 
  11 -- 切换工作数据库
  12 use company;
  13 
  14 -- 查看当前数据库中的表
  15 show tables;
  16 
  17 -- 导入.sql文件中的数据
  18 source d:/company.sql;
  19 
  20 -- 查看表中的数据
  21 select * from employees;
  22 
  23 -- 练习 : 创建world数据库, 并切换, 再导入world.sql文件中的数据到world库.
  24 create database world;
  25 
  26 use world;
  27 
  28 source d:/world.sql;
  29 
  30 -- 丢弃数据库
  31 drop database company;
  32 
  33 drop database world;
  34 
  35 -- 查看当前工作数据库
  36 select database();
  37 
  38 -- 查看当前工作数据库的版本
  39 select version();
  40 
  41 -- 查看服务器所有参数
  42 show variables;
  43 
  44 -- 和字符集设置相关的参数 : 
  45 character_set_client                                     | gbk
  46 character_set_connection                                 | gbk
  47 character_set_database                                   | utf8
  48 character_set_filesystem                                 | binary
  49 character_set_results                                    | gbk
  50 character_set_server                                     | utf8
  51 character_set_system                                     | utf8
  52 
  53 -- 如果client,connection和result的设置不是gbk, 必须执行以下语句
  54 
  55 -- 修改客户端编码
  56 set names gbk;
  57 
  58 数据库结构
  59     mysqld 服务器
  60         数据库1(目录形式)
  61             表1
  62                 数据(记录)1
  63                 数据(记录)2
  64                 数据(记录)3
  65                 ....
  66             表2 
  67             表3
  68             ....
  69             
  70         数据库2
  71         ......
  72         
  73 -- 查看表中数据
  74 select * from 表名; 
  75 
  76 -- 查看表结构
  77 describe employees;
  78 desc employees;
  79 
  80 +----------------+--------------+------+-----+---------+----------------+
  81 | Field          | Type         | Null | Key | Default | Extra          |
  82 +----------------+--------------+------+-----+---------+----------------+
  83 | employee_id    | int(6)       | NO   | PRI | NULL    | auto_increment |
  84 | first_name     | varchar(20)  | YES  |     | NULL    |                |
  85 | last_name      | varchar(25)  | YES  |     | NULL    |                |
  86 | email          | varchar(25)  | YES  |     | NULL    |                |
  87 | phone_number   | varchar(20)  | YES  |     | NULL    |                |
  88 | job_id         | varchar(10)  | YES  | MUL | NULL    |                |
  89 | salary         | double(10,2) | YES  |     | NULL    |                |
  90 | commission_pct | double(4,2)  | YES  |     | NULL    |                |
  91 | manager_id     | int(6)       | YES  |     | NULL    |                |
  92 | department_id  | int(4)       | YES  | MUL | NULL    |                |
  93 +----------------+--------------+------+-----+---------+----------------+
  94 
  95 create table customer(
  96     id int,
  97     name varchar(20),
  98     age int,
  99     phone varchar(15),
 100     birthday date
 101 );
 102 
 103 insert into customer (
 104     id,
 105     name,
 106     age,
 107     phone,
 108     birthday
 109 ) values (
 110     1,
 111     ‘张三‘,
 112     30,
 113     ‘134234234‘,
 114     ‘1992-5-8‘
 115 );
 116 
 117 insert into customer (
 118     id,
 119     name,
 120     age,
 121     phone,
 122     birthday
 123 ) values (
 124     2,
 125     ‘李四‘,
 126     40,
 127     ‘1599234234‘,
 128     ‘1991-5-8‘
 129 );
 130 
 131 insert into customer (
 132     id,
 133     name,
 134     age,
 135     phone,
 136     birthday
 137 ) values (
 138     3,
 139     ‘王五‘,
 140     50,
 141     ‘135234234‘,
 142     ‘1995-11-8‘
 143 );
 144 
 145 -- 更新记录(修改数据)
 146 -- 如果更新时不加where过滤, 会导致所有数据被修改
 147 update customer set 
 148     name = ‘某人‘,
 149     age = 10;
 150     
 151 update customer set 
 152     name = ‘张三‘,
 153     age = 30
 154 where 
 155     id = 1;
 156     
 157 -- 删除记录
 158 -- 如果没有where过滤, 会全部删除.
 159 delete from customer;
 160 
 161 delete from customer 
 162 where id = 2;
 163 
 164 
 165 DML 数据操纵语言.
 166 insert C
 167 select R 
 168 update U
 169 delete D
 170 
 171 +----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
 172 | Field          | Type                                                                                  | Null | Key | Default | Extra |
 173 +----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
 174 | Code           | char(3)   国家代码                                                                            | NO   | PRI |         |       |
 175 | Name           | char(52)  国家名称                                                                    | NO   |     |         |       |
 176 | Continent      | enum(‘Asia‘,‘Europe‘,‘North America‘,‘Africa‘,‘Oceania‘,‘Antarctica‘,‘South America‘) | NO   |     | Asia    |       |
 177 | Region         | char(26)          地区                                                                    | NO   |     |         |       |
 178 | SurfaceArea    | float(10,2)       国土面积m                                                                    | NO   |     | 0.00    |       |
 179 | IndepYear      | smallint(6)       独立年                                                                   | YES  |     | NULL    |       |
 180 | Population     | int(11)           国家人口                                                              | NO   |     | 0       |       |
 181 | LifeExpectancy | float(3,1)        平均寿命                                                                    | YES  |     | NULL    |       |
 182 | GNP            | float(10,2)       国民生产总值                                                                    | YES  |     | NULL    |       |
 183 | GNPOld         | float(10,2)                                                                           | YES  |     | NULL    |       |
 184 | LocalName      | char(45)                                                                              | NO   |     |         |       |
 185 | GovernmentForm | char(45)          政府组织                                                                    | NO   |     |         |       |
 186 | HeadOfState    | char(60)          领导人                                                                    | YES  |     | NULL    |       |
 187 | Capital        | int(11)           首都                                                                    | YES  |     | NULL    |       |
 188 | Code2          | char(2)                                                                                   | NO   |     |         |       |
 189 +----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
 190 city
 191 +-------------+----------+------+-----+---------+----------------+
 192 | Field       | Type     | Null | Key | Default | Extra          |
 193 +-------------+----------+------+-----+---------+----------------+
 194 | ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
 195 | Name        | char(35) | NO   |     |         |                |
 196 | CountryCode | char(3)  | NO   | MUL |         |                |
 197 | District    | char(20) | NO   |     |         |                |
 198 | Population  | int(11)  | NO   |     | 0       |                |
 199 +-------------+----------+------+-----+---------+----------------+
 200 SELECT *
 201 FROM   departments;
 202 
 203 SELECT department_id, location_id
 204 FROM   departments;
 205 
 206 select 
 207     code,
 208     name,
 209     continent
 210 from 
 211     country;
 212     
 213 select 
 214     continent,
 215     name,
 216     code
 217 from 
 218     country;
 219 
 220 SQL注意点 :     
 221     SQL 语言大小写不敏感。 
 222     SQL 可以写在一行或者多行
 223     关键字不能被缩写也不能分行
 224     各子句一般要分行写。
 225     使用缩进提高语句的可读性。
 226     
 227 -- 查询国家的人口,名称和代码及首都.
 228 select 
 229     population,
 230     name,
 231     code,
 232     capital
 233 from 
 234     country;
 235 
 236 -- AS用于给列起别名, AS关键字可以省略
 237 SELECT last_name AS name, commission_pct comm
 238 FROM   employees;
 239 
 240 -- 别名可以使用""包围, 目的是让它能原样显示或包含特殊的符号
 241 select 
 242     code 国家代码,
 243     name,
 244     continent as "国家 大洲"
 245 from 
 246     country;
 247 
 248 -- 过滤行 where 条件布尔 : 工作流程就是从基本表中测试每行数据, 都经过条件布尔测试一下, 如果结果为真留下, 为假则丢弃.     
 249 SELECT employee_id, last_name, job_id, department_id
 250 FROM   employees
 251 WHERE  department_id = 90 ;
 252 
 253 -- 查询所有亚洲国家
 254 select 
 255     code,
 256     population,
 257     name
 258 from 
 259     country 
 260 where 
 261     continent = ‘asia‘;
 262     
 263 -- 查询所有亚洲国家 , 下面的SQL是错误的, where中不可以使用列的别名, 因为where先执行.
 264 select 
 265     code,
 266     population,
 267     name,
 268     continent cont
 269 from 
 270     country 
 271 where 
 272     cont = ‘asia‘;
 273     
 274 执行顺序 : from => where => select 
 275 
 276 --查询所有中国城市的人口和名称和id号和国家代码, 给国家代码起别名, 尝试在where中使用别名.
 277 select 
 278     population,
 279     name,
 280     id,
 281     countrycode code 
 282 from 
 283     city 
 284 where 
 285     countrycode = ‘chn‘;
 286 
 287 -- between a and b --等效于salary >= a && salary <= 3500    
 288 SELECT last_name, salary
 289 FROM   employees
 290 WHERE  salary BETWEEN 2500 AND 3500;
 291 
 292 -- id in(a, b, c) -- 等效于id = a || id = b || id = c 
 293 SELECT employee_id, last_name, salary, manager_id
 294 FROM   employees
 295 WHERE  manager_id IN (100, 101, 201);
 296 
 297 select 
 298     code,
 299     continent,
 300     name,
 301     population
 302 from 
 303     country 
 304 where 
 305     name like ‘china‘; -- name = ‘china‘ 如果模糊查询中的字符串没有通配符, 和=效果一样.
 306     
 307 SELECT    first_name
 308 FROM     employees
 309 WHERE    first_name LIKE ‘S%‘;
 310 
 311 % 代表任意个任意字符
 312 _ 代表一个任意字符
 313 -- 查询国家名称中只要包含ch的都行
 314 
 315 select 
 316     code,
 317     population,
 318     name,
 319     continent 
 320 from 
 321     country 
 322 where 
 323     name like ‘%ch%‘
 324     
 325 -- 名字中第3个和第4个是in的国家
 326 select 
 327     code,
 328     population,
 329     name,
 330     continent 
 331 from 
 332     country 
 333 where 
 334     name like ‘__in%‘
 335     
 336 -- 查询城市表中, 名称的第2个和3个字母是or的城市.
 337 select 
 338     id,
 339     name,
 340     countrycode 
 341 from 
 342     city 
 343 where 
 344     name like ‘_or%‘;
 345     
 346 -- 查看哪些国家没有首都
 347 -- null和任意的比较运算, 结果一定是false, 处理null, 必须使用特殊的is来判断
 348 -- null值在进行统计时会被自动忽略.
 349 select 
 350     code,
 351     name,
 352     continent,
 353     capital
 354 from 
 355     country 
 356 where 
 357     capital = null;
 358     
 359 select 
 360     code,
 361     name,
 362     continent,
 363     capital
 364 from 
 365     country 
 366 where 
 367     capital is null;
 368     
 369 select 
 370     code,
 371     name,
 372     continent,
 373     capital
 374 from 
 375     country 
 376 where 
 377     capital is not null;
 378 
 379 -- 哪些国家尚未独立.
 380 select 
 381     name,
 382     code,
 383     indepYear
 384 from 
 385     country 
 386 where 
 387     indepYear is null;
 388 
 389 
 390 SELECT 
 391     employee_id, last_name, job_id, salary
 392 FROM   employees
 393 WHERE  
 394         salary >=10000
 395     AND    
 396         job_id LIKE ‘%MAN%‘;
 397 
 398 
 399 SELECT 
 400     employee_id, last_name, job_id, salary
 401 FROM   employees
 402 WHERE  
 403         salary >= 10000
 404     OR     
 405         job_id LIKE ‘%MAN%‘;
 406 
 407 -- 查询亚洲国家中人口大于5000万的国家.
 408 select 
 409     code,
 410     name,
 411     continent,
 412     population
 413 from 
 414     country 
 415 where 
 416         continent = ‘asia‘
 417     and 
 418         population > 50000000;
 419 
 420 -- 查询中国的城市人口小于20万的城市.
 421 select 
 422     *
 423 from 
 424     city 
 425 where 
 426     countrycode = ‘chn‘
 427     and 
 428     population < 200000;
 429     
 430 -- 去重 distinct, 后面的列最好是有重复数据的
 431 select 
 432     distinct 
 433         continent,
 434         region
 435 from 
 436     country;
 437     
 438 -- 查看中国各有哪些不同的省.
 439 select 
 440     distinct district
 441 from 
 442     city 
 443 where 
 444     countrycode = ‘chn‘;
 445 
 446 SELECT   last_name, job_id, department_id, salary
 447 FROM     employees
 448 ORDER BY salary asc;
 449 
 450 SELECT   last_name, job_id, department_id, salary
 451 FROM     employees
 452 ORDER BY salary desc;
 453 
 454 select 
 455     code,
 456     name,
 457     population pop 
 458 from 
 459     country 
 460 where 
 461     continent = ‘asia‘
 462 order by 
 463     pop;
 464 
 465 order by中可以使用列的别名, 原因是它最后执行
 466 
 467 顺序 : from => where => select => order by 
 468 
 469 
 470 SELECT last_name, department_id, salary
 471 FROM   employees
 472 ORDER BY department_id asc, salary DESC;
 473 
 474 -- 查询亚洲人口最少的国家
 475 select 
 476     code,
 477     name,
 478     continent,
 479     population pop 
 480 from 
 481     country 
 482 where 
 483     continent = ‘asia‘
 484 order by 
 485     pop desc;
 486 
 487 -- 查询山东省人口最多的城市
 488 select 
 489     id,
 490     name,
 491     district,
 492     population
 493 from 
 494     city 
 495 where 
 496         countrycode = ‘chn‘
 497     and 
 498         district = ‘shandong‘
 499 order by 
 500     population
 501     
 502 
 503     
 504 -- 多表联接 : 会产生笛卡尔集
 505 -- 为了学习, 创建简单表
 506 create table city2
 507 select * from city where name=‘london‘;
 508 +------+--------+-------------+----------+------------+
 509 | ID   | Name   | CountryCode | District | Population |
 510 +------+--------+-------------+----------+------------+
 511 |  456 | London | GBR         | England  |    7285000 |
 512 | 1820 | London | CAN         | Ontario  |     339917 |
 513 +------+--------+-------------+----------+------------+
 514 
 515 create table country2
 516 select * from country where code in(‘gbr‘, ‘can‘);
 517 +------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------
 518 | Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population | LifeExpectancy | GNP        | GNPOld     | LocalName
 519 +------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------
 520 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |           79.4 |  598862.00 |  625626.00 | Canada
 521 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |           77.7 | 1378330.00 | 1296830.00 | United Kingd
 522 +------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------
 523 
 524 select * from city2, country2; 
 525 结果集中的绝大多数都是垃圾. 必须要过滤掉
 526 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--
 527 | ID   | Name   | CountryCode | District | Population | Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population | L
 528 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--
 529 |  456 | London | GBR         | England  |    7285000 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
 530 | 1820 | London | CAN         | Ontario  |     339917 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
 531 |  456 | London | GBR         | England  |    7285000 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
 532 | 1820 | London | CAN         | Ontario  |     339917 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
 533 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--
 534 
 535 select 
 536     * 
 537 from 
 538     city2, 
 539     country2
 540 where 
 541     countrycode = code;
 542     
 543 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-
 544 | ID   | Name   | CountryCode | District | Population | Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population |
 545 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-
 546 | 1820 | London | CAN         | Ontario  |     339917 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
 547 |  456 | London | GBR         | England  |    7285000 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
 548 +------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-
 549 
 550 -- 下面的SQL是错误的, 因为列名模糊
 551 select 
 552     name,
 553     population,
 554     name,
 555     continent,
 556     population
 557 from 
 558     city2, 
 559     country2
 560 where 
 561     countrycode = code;
 562     
 563 select 
 564     city2.name as cityName,
 565     city2.population cityPop,
 566     country2.name countryName,
 567     country2.continent,
 568     country2.population countryPop
 569 from 
 570     city2, 
 571     country2
 572 where 
 573     city2.countrycode = country2.code;
 574     
 575 -- 表名的使用也麻烦了. 给表起别名    
 576 select 
 577     ci.name as cityName,
 578     ci.population cityPop,
 579     co.name countryName,
 580     co.continent,
 581     co.population countryPop
 582 from 
 583     city2 as ci , 
 584     country2 co 
 585 where 
 586     ci.countrycode = co.code;
 587 
 588 -- 表名一旦指定了别名, 原名就不可以使用了.
 589 select 
 590     city2.name as cityName,
 591     city2.population cityPop,
 592     country2.name countryName,
 593     country2.continent,
 594     country2.population countryPop
 595 from 
 596     city2 as ci, 
 597     country2 as co 
 598 where 
 599     city2.countrycode = country2.code;
 600     
 601 -- 查询所有国家的名称和国家的首都的名称.
 602 select 
 603     co.name country,
 604     ci.name captial,
 605     co.population countryPop,
 606     ci.population cityPop,
 607     co.continent 
 608 from 
 609     country co,
 610     city ci 
 611 where 
 612     co.capital = ci.id
 613     
 614 -- where中有多个条件.
 615 select 
 616     ci.name as cityName,
 617     ci.population cityPop,
 618     co.name countryName,
 619     co.continent,
 620     co.population countryPop
 621 from 
 622     city2 as ci , 
 623     country2 co 
 624 where 
 625         ci.countrycode = co.code -- 联接条件, 比普通过滤更重要.
 626     and 
 627         ci.population > 1000000
 628         
 629 +----------+---------+----------------+-----------+------------+
 630 | cityName | cityPop | countryName    | continent | countryPop |
 631 +----------+---------+----------------+-----------+------------+
 632 | London   | 7285000 | United Kingdom | Europe    |   59623400 |
 633 +----------+---------+----------------+-----------+------------+
 634 
 635 SQL99标准中 内联接不要用,号来写, 而是使用专门的关键字join.... on 联接条件
 636 select 
 637     ci.name as cityName,
 638     ci.population cityPop,
 639     co.name countryName,
 640     co.continent,
 641     co.population countryPop
 642 from 
 643     city2 as ci 
 644 inner join 
 645     country2 co 
 646 on 
 647     ci.countrycode = co.code 
 648 where 
 649     ci.population > 1000000;
 650     
 651 -- on 和 where 效果相同, 但是下面的写法不推荐
 652 select 
 653     ci.name as cityName,
 654     ci.population cityPop,
 655     co.name countryName,
 656     co.continent,
 657     co.population countryPop
 658 from 
 659     city2 as ci 
 660 inner join 
 661     country2 co 
 662 where 
 663         ci.countrycode = co.code 
 664     and 
 665         ci.population > 1000000;
 666     
 667 -- on 和 where 效果相同, 但是下面的写法不推荐    
 668 select 
 669     ci.name as cityName,
 670     ci.population cityPop,
 671     co.name countryName,
 672     co.continent,
 673     co.population countryPop
 674 from 
 675     city2 as ci 
 676 inner join 
 677     country2 co 
 678 on 
 679         ci.countrycode = co.code 
 680     and 
 681         ci.population > 1000000;
 682     
 683 -- 最好的写法是下面的.    inner关键字可以省略
 684 select 
 685     ci.name as cityName,
 686     ci.population cityPop,
 687     co.name countryName,
 688     co.continent,
 689     co.population countryPop
 690 from 
 691     city2 as ci 
 692 join 
 693     country2 co 
 694 on -- on后面只跟 联接条件 
 695     ci.countrycode = co.code 
 696 where -- where 后面只跟  普通行过滤
 697     ci.population > 1000000;
 698     
 699 -- 查询所有亚洲和欧洲国家的首都, 使用SQL99
 700 select 
 701     co.name country,
 702     co.continent,
 703     ci.name capital
 704 from 
 705     country co 
 706 join 
 707     city ci 
 708 on 
 709     co.capital = ci.id 
 710 where 
 711     co.continent in(‘asia‘, ‘europe‘);
 712     
 713 -- 查看所有国家名称和首都及官方语言
 714 select 
 715     co.name country,
 716     co.continent,
 717     ci.name capital,
 718     cl.language
 719 from 
 720     country co 
 721 join 
 722     city ci 
 723 on 
 724     co.capital = ci.id 
 725 join 
 726     countrylanguage cl 
 727 on 
 728     cl.countrycode = co.code 
 729 where 
 730     cl.isofficial = ‘t‘;
 731     
 732 -- 查询简单表的国家及首都
 733 -- 内联接的结果总是所有联接条件为真的记录. 为假的记录全部滤掉.
 734 select 
 735     co.name country,
 736     ci.name capital,
 737     ci.district
 738 from 
 739     country2 co 
 740 join 
 741     city2 ci 
 742 on 
 743     co.capital = ci.id;
 744     
 745     
 746 -- 外联接可以保证某张表的数据完整, 即使联接条件为假, 也要保留数据
 747 select 
 748     co.name country,
 749     ci.name capital,
 750     ci.district
 751 from 
 752     country2 co 
 753 left outer join 
 754     city2 ci 
 755 on 
 756     co.capital = ci.id;
 757     
 758 select 
 759     co.name country,
 760     ci.name capital,
 761     ci.district
 762 from 
 763     country2 co 
 764 right outer join 
 765     city2 ci 
 766 on 
 767     co.capital = ci.id;
 768     
 769 -- 查看所有国家名称和首都, 即使没有首都也要显示.
 770 select 
 771     co.name country,
 772     co.continent,
 773     ci.name capital
 774 from 
 775     country co 
 776 left join 
 777     city ci 
 778 on 
 779     co.capital = ci.id;
 780 
 781 -- 查询哪些国家没有首都
 782 select 
 783     co.name country,
 784     co.continent,
 785     ci.name capital
 786 from 
 787     country co 
 788 left join 
 789     city ci 
 790 on 
 791     co.capital = ci.id
 792 where 
 793     ci.name is null;
 794     
 795 // 伪代码
 796 // 内联接
 797 Set leftTable;
 798 Set rightTable;
 799 Set resultSet = new Set();
 800 for (int i = 0; i < leftTable.length; i++) {
 801     Row leftRow = leftTable[i];
 802     for (int j = 0; j < rightTable.length; j++) {
 803         Row rightRow = rightTable[j];
 804         if (联接条件(leftRow, rightRow)) {
 805             Row newRow = leftRow + rightRow;
 806             resultSet.add(newRow);
 807         }
 808     }
 809 }
 810 
 811 // 外联接 
 812 Set leftTable;
 813 Set rightTable;
 814 Set resultSet = new Set();
 815 for (int i = 0; i < leftTable.length; i++) {
 816     Row leftRow = leftTable[i];
 817     boolean flag = false;
 818     for (int j = 0; j < rightTable.length; j++) {
 819         Row rightRow = rightTable[j];
 820         if (联接条件(leftRow, rightRow)) {
 821             Row newRow = leftRow + rightRow;
 822             resultSet.add(newRow);
 823             flag = true; // 表明联接条件为真的记录保存起来
 824         }
 825     }
 826     
 827     if (!flag) {
 828         Row newRow = leftRow + 空行(右表);
 829         resultSet.add(newRow);
 830     }
 831 }
 832 
 833 -- 查询哪些国家没有官方语言
 834 select 
 835     co.name country,
 836     cl.language,
 837     cl.isofficial
 838 from 
 839     country co 
 840 left join 
 841     countrylanguage cl 
 842 on 
 843         co.code = cl.countrycode
 844     and 
 845         cl.isofficial = ‘T‘
 846 where 
 847     cl.isofficial is null
 848     
 849 -- 查询所有国家的首都和官方语言, 没有首都和官方语言的也要全部显示.
 850 
 851 -- 单行函数, 查询结果中对每一行都单独执行的函数
 852 select
 853     upper(name),
 854     lower(continent)
 855 from 
 856     county;
 857     
 858 select 
 859     concat(code, name),
 860     substr(name, 3, 2)
 861 from 
 862     country;
 863     
 864 -- 把国家的代码和名称及大洲用@符号连接. 如 : CHN@China@asia 
 865 select
 866     concat(concat(concat(concat(code, ‘@‘), name), ‘@‘), continent)
 867 from 
 868     country;
 869     
 870 select
 871     concat(code, ‘@‘, name, ‘@‘, continent)
 872 from 
 873     country;
 874 
 875 
 876 -- 分组函数 : 作用于一组数据. 默认情况下虚表中的所有记录被当作一个组.
 877 分组函数都是统计运算, 特点就是一组数据只能有一个结果. 结果也必须是所有数据处理完后才有的.
 878 
 879 max() 最大值 
 880 min() 最小值 
 881 avg() 平均值 
 882 sum() 求和
 883 count() 计数
 884 
 885 select 
 886     max(population),
 887     min(surfacearea)
 888 from 
 889     country ;
 890 
 891 
 892 select 
 893     -- name, 不可以再把普通的列放在这里
 894     max(name),
 895     max(population),
 896     min(surfacearea)
 897 from 
 898     country ;
 899 
 900 复习 :
 901 
 902 C / S 架构
 903 Server : mysqld 
 904 Client : mysql 
 905 
 906 客户端连接服务器必须要提供 : 主机地址, 端口号, 用户名, 密码, 默认数据库
 907 mysql --host=127.0.0.1 --port=3306 --user=root --password=123456 world
 908 
 909 -- 查看所有数据库
 910 show databases;
 911 
 912 -- 查看库中的表
 913 show tables;
 914 
 915 -- 查看其他库中的表
 916 show tables from 库名;
 917 
 918 -- 查看表结构 
 919 desc 表名;
 920 
 921 -- 查看表的建表语句
 922 show create table 表名;
 923 
 924 -- 通过建表语句可以看出表的存储引擎, 缺省字符集, 各种约束的名字....
 925 
 926 myisam引擎不支持高级特性 : 事务, 外键, 行级锁. 速度极快
 927 innodb引擎支持高级特性, 速度稍慢.
 928 
 929 ``的作用是专门用于包围数据库对象的名称. 
 930 数据库 
 931 表 
 932 列
 933 索引 
 934 约束
 935 函数
 936 ......
 937 
 938 C : insert  
 939 R : select 
 940 U : update 
 941 D : delete 
 942 以上语句统称为DML -> 数据操纵语言 
 943 
 944 
 945 create 
 946 drop 
 947 alter
 948 以上语句统称DDL语句 -> 数据定义语言
 949 
 950 
 951 跨库操作表 : 只需要在表名上加上库.限定
 952 select * from company.employees;
 953 
 954 select 
 955     *
 956 from 
 957     country;
 958     
 959 使用用户变量
 960 set @var1 = 200;
 961 set @var1 = ‘abc‘;
 962 
 963 -- 查询没有首都和官方语言的国家
 964 select 
 965     100, -- 常量 
 966     ‘abc‘,
 967     20 + 30 / (50 - 3), -- 常量
 968     @var1, -- 用户变量
 969     @@datadir, -- 系统变量
 970     database(), -- 函数
 971     now(),
 972     co.*, -- 普通列
 973     ci.*,
 974     cl.*
 975 from 
 976     country co 
 977 left join 
 978     city ci 
 979 on 
 980     co.capital = ci.id -- 联接条件
 981 left join 
 982     countrylanguage cl 
 983 on 
 984         co.code = cl.countrycode 
 985     and 
 986         cl.isofficial = ‘t‘
 987 where 
 988         cl.language is null
 989     and 
 990         ci.name is null
 991 order by 
 992     co.name desc; 
 993 
 994     
 995 -- 查询亚洲国家的平均人口
 996 select 
 997     avg(population)
 998 from 
 999     country 
1000 where 
1001     continent = ‘asia‘;
1002 
1003 -- 查询全球平均寿命最高和最低
1004 select 
1005     max(LifeExpectancy),
1006     min(LifeExpectancy)
1007 from 
1008     country;
1009     
1010 max() 最大值 
1011 min() 最小值 
1012 avg() 平均值 
1013 sum() 求和
1014 count() 计数
1015 
1016 select 
1017     sum(population),
1018     count(capital)
1019 from 
1020     country;
1021     
1022 -- 查看表中的记录数
1023 select 
1024     count(*)
1025 from 
1026     country;
1027     
1028 -- 解决的问题类似于, 查看 各大洲 的情况.
1029 select 
1030     continent, -- 群体概念
1031     max(name),
1032     avg(surfacearea)
1033 from 
1034     country 
1035 group by 
1036     continent;
1037     
1038 -- 查看各地区的最大人口数
1039 select
1040     continent,
1041     region,
1042     max(population)
1043 from 
1044     country 
1045 group by 
1046     continent,
1047     region;
1048     
1049 select
1050     max(continent),
1051     region,
1052     max(population)
1053 from 
1054     country 
1055 group by 
1056     region
1057 order by 
1058     max(continent);
1059 
1060 
1061 -- 查看各国的城市总人口数
1062 select 
1063     countrycode,
1064     sum(population)
1065 from 
1066     city 
1067 group by 
1068     countrycode 
1069 
1070 select
1071     name,
1072     population,
1073     sum(surfacearea)
1074 from 
1075     country 
1076 group by 
1077     name,
1078     population;
1079     
1080 -- 对分组的结果进行过滤, 只能使用having    
1081 select 
1082     countrycode,
1083     sum(population) sumPop
1084 from 
1085     city 
1086 group by 
1087     countrycode 
1088 having
1089     sumPop > 10000000
1090 order by 
1091     sumPop;
1092 
1093 -- 查看亚洲国家各省城市总人口和城市个数中城市总人口数大于100万的.
1094 select 
1095     co.name,
1096     ci.district,
1097     sum(ci.population) sumPop,
1098     count(ci.id) cities
1099 from 
1100     country co 
1101 join 
1102     city ci 
1103 on 
1104     co.code = ci.countrycode 
1105 where 
1106     co.continent = ‘asia‘
1107 group by 
1108     co.name,
1109     ci.district
1110 having 
1111     sumPop > 1000000
1112 order by 
1113     cities desc;
1114     
1115 解决SQL步骤 :     
1116 1) 最基础的基表, from 
1117 2) 考虑一张表的数据够不够, 如果不够进行连接 join 
1118 3) 再考虑是外还是内, 如果是外还得考虑保证哪张表完整
1119 4) 有join必须要有on 
1120 5) 是否需要对当前的大基表进行基础的行过滤. where 
1121 6) 是否需要分组, 分组依据的列是什么. group by 
1122 7) 如果有分组,第一时间,把分组的列放在 select 后面, 并同时继续分析要选择哪些列或统计运算.
1123 8) 是否要再对分组结果进行进一步过滤, 如果需要使用having
1124 9) 是否需要排序, 如果需要使用order by .
1125 
1126 练习 : 
1127 1 列出所有在超过10个国家中使用的语言。
1128 select
1129     language,
1130     count(countrycode) ct
1131 from 
1132     countrylanguage
1133 group by 
1134     language 
1135 having 
1136     ct > 10;
1137     
1138 2 查询每个大洲各有多少种政府组织形式和每个大洲最富有的收入
1139 select 
1140     continent,
1141     count(distinct GovernmentForm),
1142     max(gnp)
1143 from 
1144     country 
1145 group by 
1146     continent;
1147     
1148     
1149 3 列出不同的国家(country code)有居民超过7,000,000的城市， 它们有多少？
1150 select 
1151     countrycode,
1152     count(*)
1153 from 
1154     city 
1155 where 
1156     population > 7000000 
1157 group by 
1158     countrycode;
1159     
1160 4 查询中国的每个省的总城市数量和总人口数
1161 -- 能用where解决 不要用having解决.
1162 select 
1163     district,
1164     count(*),
1165     sum(population)
1166 from 
1167     city 
1168 where 
1169     countrycode = ‘chn‘
1170 group by 
1171     district
1172 order by 
1173     sum(population) desc;
1174     
1175 
1176 select
1177     countrycode,
1178     district,
1179     count(*),
1180     sum(population)
1181 from 
1182     city 
1183 group by 
1184     countrycode,
1185     district
1186 having 
1187     countrycode = ‘chn‘
1188 order by 
1189     sum(population) desc;
1190     
1191 5 Sweden国家说的是什么语言？
1192 select 
1193     co.name,
1194     cl.language,
1195     cl.isofficial
1196 from 
1197     countrylanguage cl 
1198 join 
1199     country co 
1200 on 
1201     cl.countrycode = co.code 
1202 where 
1203     co.name = ‘sweden‘;
1204     
1205 6 哪些国家没有列出任何使用语言？
1206 select 
1207     co.name,
1208     cl.language
1209 from 
1210     countrylanguage cl 
1211 right join 
1212     country co 
1213 on 
1214     cl.countrycode = co.code 
1215 where 
1216     cl.language is null;
1217     
1218 select 
1219     co.name,
1220     count(cl.language) ct 
1221 from 
1222     countrylanguage cl 
1223 right join 
1224     country co 
1225 on 
1226     cl.countrycode = co.code 
1227 group by 
1228     co.name 
1229 having 
1230     ct = 0;
1231 
1232 
1233 7 列出在城市表中80%人口居住在城市的国家
1234 select 
1235     co.name,
1236     sum(ci.population) / co.population rate
1237 from 
1238     country co 
1239 join 
1240     city ci 
1241 on 
1242     co.code = ci.countrycode 
1243 group by
1244     co.population,
1245     co.name 
1246 having 
1247     rate > 0.8;
1248     
1249     
1250     
1251     
1252 -- 子查询 : 解决一次查询不能解决的问题.
1253 select 
1254     name,
1255     surfacearea,
1256     continent
1257 from 
1258     country 
1259 where surfacearea >
1260                     (select 
1261                         avg(surfacearea)
1262                     from 
1263                         country) 
1264     
1265 -- 子查询用在比较运算中时, 必须返回一行一列. 下面的SQL是错误的    
1266 select 
1267     name,
1268     surfacearea,
1269     continent
1270 from 
1271     country 
1272 where surfacearea >
1273                     (select 
1274                         avg(surfacearea)
1275                     from 
1276                         country
1277                     group by continent) 
1278                     
1279 -- 找出哪个城市的人口是最多的.
1280 select 
1281     countrycode,
1282     name,
1283     population
1284 from 
1285     city  
1286 where     
1287     population = (select max(population) from city);
1288     
1289 -- 找出平均寿命最高的和最低的国家.
1290 select 
1291     continent,
1292     name,
1293     lifeExpectancy
1294 from 
1295     country 
1296 where 
1297     lifeExpectancy in ( (select max(lifeExpectancy) from country),
1298                         (select min(lifeExpectancy) from country))
1299                         
1300 -- 查看哪些国家的人口是大于本大洲平均人口.
1301 select 
1302     co1.name,
1303     co1.continent,
1304     co1.population,
1305     co2.avgPop 
1306 from 
1307     country co1
1308 join 
1309     (select 
1310         continent,
1311         avg(population) avgPop
1312     from 
1313         country 
1314     group by 
1315         continent) co2
1316 on 
1317     co1.continent = co2.continent
1318 where 
1319     co1.population > co2.avgPop
1320 order by  
1321     continent;
1322     
1323 -- 创建数据库
1324 create database if not exists school charset gbk;
1325 
1326 -- 修改数据库, 只能修改默认字符集
1327 alter database school charset utf8;
1328 
1329 -- 丢弃数据库
1330 drop database if exists school;
1331 
1332 -- 切换数据库
1333 use school;
1334 
1335 
1336 数据类型
1337 int                4字节整数
1338 bigint            8字节整数
1339 char(长度)        定长字符串, 长度最大255, 在实际插入数据时, 如果长度不够, 会自动补齐.
1340 varchar(长度)    变长字符串, 长度最大65535字节, 在实际插入数据时, 如果长度不够, 实际多少就占多少空间.
1341 longtext        超长文本
1342 double            8字节浮点数
1343 decimal            多字节定点数
1344 date            日期
1345 time            时间
1346 datetime        日期时间
1347 timestamp        时间戳
1348 
1349 -- 创建表, 必须至少要有一个列
1350 -- 主键 : 数据必须是非空且唯一.
1351 create table if not exists teacher(
1352     id int auto_increment, -- 将来这个列对应的数据可以自动生成. 必须作成主键
1353     name varchar(20), -- varchar数据类型必须要有长度.
1354     age int,
1355     salary double,
1356     gender enum(‘男‘, ‘女‘) default ‘男‘,
1357     primary key(id) -- 表级主键
1358 ) engine innodb charset utf8; -- 
1359 
1360 create table if not exists classes(
1361     id int auto_increment,
1362     name varchar(20) not null, -- not null 就是非空
1363     room char(3),
1364     begindate date,
1365     master int, --班主任是一个整数, 将来是某个老师的id 
1366     primary key(id)
1367 );
1368 
1369 -- 创建学生表, id, name, age, gender, phone, class_id
1370 create table if not exists student (
1371     id int auto_increment,
1372     name varchar(20),
1373     age int,
1374     gender enum(‘男‘, ‘女‘) default ‘男‘,
1375     phone varchar(15),
1376     class_id int,
1377     primary key(id)
1378 );
1379 
1380 insert into teacher(
1381     name,
1382     age,
1383     salary,
1384     gender
1385 ) values (
1386     ‘佟刚‘,
1387     40,
1388     2000,
1389     ‘男‘
1390 );
1391 
1392 insert into teacher(
1393     id,
1394     name,
1395     age,
1396     salary,
1397     gender
1398 ) values (
1399     null,
1400     ‘芳芳‘,
1401     20,
1402     50000,
1403     ‘女‘
1404 );
1405 
1406 
1407 -- 基于子查询建表, 把虚表变成实表
1408 create table country2
1409 as select * from world.country; -- 复制表, 只是机械的把虚表变实, 不能复制约束(主键)
1410 
1411 -- 复制表结构建表, 没有数据
1412 create table country3 
1413 like world.country;
1414 
1415 
1416 +----------+-----------------+------+-----+---------+----------------+
1417 | Field    | Type            | Null | Key | Default | Extra          |
1418 +----------+-----------------+------+-----+---------+----------------+
1419 | id       | int(11)         | NO   | PRI | NULL    | auto_increment |
1420 | name     | varchar(20)     | YES  |     | NULL    |                |
1421 | age      | int(11)         | YES  |     | NULL    |                |
1422 | gender   | enum(‘男‘,‘女‘) | YES  |     | 男      |                |
1423 | phone    | varchar(15)     | YES  |     | NULL    |                |
1424 | class_id | int(11)         | YES  |     | NULL    |                |
1425 +----------+-----------------+------+-----+---------+----------------+
1426 
1427 insert into student(
1428     name,
1429     age,
1430     phone,
1431     class_id,
1432     address 
1433 ) values(
1434     ‘小明‘,
1435     20,
1436     ‘2342342‘,
1437     1,
1438     ‘北京昌平‘
1439 );
1440 
1441 -- 修改表
1442 alter table 表名 
1443 -- 支持若干子名
1444 
1445 -- 添加新列
1446 add column 新列名 数据类型(长度) 其他选项.
1447 
1448 alter table student 
1449 add column address varchar(200) not null; -- 新列追加在所有列的最后.
1450 
1451 alter table student 
1452 add column email varchar(50) after age; -- 在指定列后面添加新列
1453 
1454 alter table student 
1455 add column cardNo char(18) first; -- 在最前面添加新列
1456 
1457 -- 修改列, 数据类型, 长度, 其他选项 
1458 modify 列名 新数据类型(新长度) 新其他选项
1459 
1460 alter table student 
1461 modify name varchar(10) not null;
1462 
1463 -- 重命名列
1464 change 列名 新列名 新数据类型(新长度) 新其他选项;
1465 
1466 alter table student 
1467 change phone mobile char(11) unique; -- unique就是唯一.
1468 
1469 -- 丢弃列
1470 drop column 列名; -- 会导致删除的列所对应的所有数据全部丢失, 无法找回
1471 
1472 alter table student
1473 drop column cardNo;
1474 
1475 -- 重新命名表
1476 rename to 新表名
1477 
1478 alter table student 
1479 rename to students;
1480 
1481 alter table teacher 
1482 rename to teachers;
1483 
1484 -- 修改约束
1485 以上都是alter table 的子句, 用于修改表
1486 ---------------------------------------------------------------
1487 
1488 -- 丢弃表 操作很危险, 如果没有数据备份, 就永远找不回来.
1489 drop table 表名;
1490 
1491 -- 丢弃表时可以批量丢弃
1492 drop table country2, country3;
1493 
1494 -- 清空表 DDL, 表中数据不能恢复. 效率高
1495 truncate table 表名;
1496 
1497 delete 是DML, 是可以后悔的. 效率低.
1498 
1499 
1500 insert into teachers(
1501     name,
1502     age,
1503     salary,
1504     gender
1505 ) values (
1506     ‘红康‘,
1507     30,
1508     3000,
1509     ‘男‘
1510 );
1511 
1512 insert into teachers(
1513     name,
1514     age,
1515     salary,
1516     gender
1517 ) value (
1518     ‘小柴‘,
1519     20,
1520     5000,
1521     ‘女‘
1522 );
1523 
1524 insert into teachers(
1525     name,
1526     age,
1527     salary,
1528     gender
1529 ) values (
1530     ‘贺飞‘,
1531     35,
1532     4000,
1533     ‘男‘
1534 ), (
1535     ‘老韩‘,
1536     40,
1537     3000,
1538     ‘男‘
1539 ), (
1540     ‘安妮‘,
1541     32,
1542     6000,
1543     ‘女‘
1544 );
1545 
1546 -- 不建议这样的写法. 因为它强烈依赖表的结构,如果表的结构发生变化, 下面的语句就容易失败!
1547 insert into teachers values(
1548     null,
1549     ‘小邱‘,
1550     40,
1551     7000,
1552     ‘男‘
1553 );
1554 
1555 +-----------+-------------+------+-----+---------+----------------+
1556 | Field     | Type        | Null | Key | Default | Extra          |
1557 +-----------+-------------+------+-----+---------+----------------+
1558 | id        | int(11)     | NO   | PRI | NULL    | auto_increment |
1559 | name      | varchar(20) | NO   |     | NULL    |                |
1560 | room      | char(3)     | YES  |     | NULL    |                |
1561 | begindate | date        | YES  |     | NULL    |                |
1562 | master    | int(11)     | YES  |     | NULL    |                |
1563 +-----------+-------------+------+-----+---------+----------------+
1564 
1565 insert into classes (
1566     name,
1567     room,
1568     begindate,
1569     master 
1570 ) values (
1571     ‘Java0725‘,
1572     ‘305‘,
1573     now(), -- 值的部分可以使用函数调用
1574     2
1575 );
1576 
1577 -- 实现表的完全克隆!!!
1578 create table if not exists country2 like world.country;
1579 
1580 -- 基于子查询插入数据, 好处就是一次性插入多条记录.
1581 insert into country2 
1582 select * from world.country;
1583 
1584 
1585 -- 事务 : 一组DML, 在执行时, 要么全部失败, 要么全部成功, 使得数据从一种状态转换为另一种状态. 数据没有丢失!!
1586 
1587 A Atomic 原子性             所有操作不可分割.
1588 C Consistence 一致性        数据在事务前和事务后数据是一致的.
1589 I ISOLATION    独立性             不同事务之间具有隔离性
1590 D Duration    持久性            事务一旦提交, 永久生效.
1591 
1592 DCL : 数据控制语言 : commit, rollback, grant, revoke...
1593 
1594 显式启动事务 : 
1595     set autocommit = false;
1596     set @@autocommit = off;
1597     
1598     多条DML语句;
1599     
1600     commit; 事务提交, 成功
1601     rollback; 事务回滚, 失败
1602     ddl语句执行 : 事务提交
1603     会话正常结束 提交
1604     异常结束 : 回滚
1605     
1606     
1607     还原设置
1608     set autocommit = true;
1609     set @@autocommit = on;
1610 
1611 
1612     
1613