MySQL 多表where查询与单表select where in哪一个效率高

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SELECT b.id FROM t_address a, t_unit b WHERE b.utj=1 AND b.ufreeze=2 AND a.id=b.uads and a.sid=3
就这个啦!!
原因很简单
b.utj=1不满足的话,那么系统就直接找下一个数据
用的in的话,如果第一个不满足,那么就匹配第二个,第二个不满足,在匹配第三个
,全部不匹配 才找下一条数据!!
这个就可以看到了,可能用IN的话,效率可能会减慢3倍,打个比方
参考技术A 多表 inner join 比 in 快

python开发mysql:单表查询&多表查询

一 单表查询,以下是表内容

技术分享

 

  1 一 having 过滤
  2 1.1 having和where
  3     select * from emp where id > 15;
  4     解析过程;from > where 找到数据 > 分组(没有默认一个组)> select 打印   where是出结果之前
  5     select * from emp having id > 15;
  6     解析过程;from > where 找到数据(没有约束条件,就是整个表)) > 分组(没有默认一个组)> select 打印 > having   where是出结果之后
  7     上面2个输出是一样的,因为没有分组
  8     select depart_id,count(id) from emp group by depart_id; 分组完后3个组,就是3个记录,就要通过聚合取值,通过分组字段取值
  9 
 10     select depart_id,count(id) from emp group by depart_id where depart_id=1; 报错,这里改成having就好了
 11     from emp group by depart_id 得出一张虚拟的表在内存里面,有2个字段depart_id,count(id)
 12     在往后接就是针对这块虚拟的表,也就是where针对的是这个虚拟表,所以报错,因为where针对的是硬盘表
 13 
 14     小结;where和having的区别
 15           where是约束来自数据库的数据,是返回结果之前起作用的,从硬盘把数据where约束拿到内存,在分组之前
 16           having是过滤声明,是结果已经到内存,在分组之后
 17 
 18 
 19 
 20 1.2 聚合 把内容压成一个字段一条记录
 21     select depart_id,count(id) from emp group by depart_id having name like ego%; 报错,结果针对虚拟表,只有depart_id,count(id)2个字段
 22     select depart_id,count(id) from emp group by depart_id having count(id) > 2; 在having里面的聚合最后就是字符串字段
 23     from > where > groupby > 聚合  > select > having
 24     select count(id) from emp where id > 15;   解析 from > where > groupby(默认是一组) > 聚合 > select
 25     select count(id) from emp; 解析 from > where (没有约束条件,就是整个表) > groupby(默认是一组) > 聚合 > select
 26     select count(id) from emp having id > 15; 报错 解析 因为having是groupby后的结果,只有count(id)这个字段
 27     select * from emp having avg(salary) > 10000; 报错 聚合把内容压成一个字段一条记录,这条记录的名字是avg(salary)
 28     select avg(salary) from emp;
 29     select max(salary) from emp having avg(salary) > 10000;
 30     解析 from > where (没有约束条件,就是整个表) > groupby(默认是一组) > 聚合max > 聚合avg > select > having
 31     也就是过滤针对的是select打印做出的限制,限制为真,就打印select,否则为空
 32     select 10000 from emp having 20000 > 10000;  从表中拿到数据,然后where,groupby然后判断过滤后面的,如果为真,就输出select
 33 
 34     **顺序
 35     select max(salary) from emp where id > 2 group by depart_id having avg(salary) > 10000;
 36     from emp > where id>2 到内存 > group by depart_id 只有depart_id字段(select只能靠聚合) > 算出结果max(salary) avg(salary) > having 满足 > select
 37     depart_id  max(id)  avg(salary)
 38     1          3        10
 39     2          1        11
 40 
 41 1.3 练习
 42     1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
 43         select post,count(id),group_concat(name) from emp group by post having count(id) < 2;
 44     3. 查询各岗位平均薪资大于10000的岗位名、平均工资
 45         select post,avg(salary),group_concat(name) from emp group by post having avg(salary) > 10000;
 46         select post 部门,avg(salary) 平均工资,group_concat(name) 职工 from emp group by post having avg(salary) > 10000;
 47     3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
 48         select post,avg(salary),group_concat(name) from emp group by post having avg(salary) between 10000 and 20000);
 49 
 50 
 51 
 52 二 order by  关键字 排序 默认升序,asc
 53 2.1 select * from emp order by salary; 按照薪资排序,从小到大,升序
 54     select * from emp order by salary asc; 升序
 55     select * from emp order by salary desc; 降序
 56 
 57     年龄从小到大,出现相同再按照薪资去排
 58     select * from emp order by age,salary;
 59     select * from emp order by age(asc,desc),salary(asc,desc);
 60 
 61 2.2 优先级
 62     在having后面执行的,对打印出来的限制,也就是在select之前
 63 
 64 2.3 练习
 65     1. 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
 66         select * from employee ORDER BY age asc,hire_date desc;
 67     2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
 68         select post 岗位名,avg(salary) 平均工资 from emp group by post having avg(salary) > 10000 order by 平均工资 asc;
 69     3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
 70         select post 岗位名,avg(salary) 平均工资 from emp group by post having avg(salary) > 10000 order by 平均工资 desc;
 71 
 72 ** select name from emp where id > 5 having id > 16;
 73 ** select 后面决定了有什么字段,having,等限制打印的一定要限制后面的字段,不然就报错,因为没有这个字段
 74 
 75 
 76 三 limit 限制查询记录数
 77 3.1 select * from emp limit 3; 打印升序(默认就是升序)前三条
 78     select * from emp order by id desc limit 3;打印后三条
 79 
 80 3.2 分页查询,从哪开始,往后取几条
 81     select * from emp limit 0,3; 0代表第一条,3代表查询几条
 82     select * from emp limit 3,3; 3代表第四条
 83     select * from emp limit 6,3;
 84 
 85 四 使用正则表达式查询
 86 4.1 like 模糊查询,% 匹配多个 _ 匹配一个
 87     select * from emp where name like eg%;
 88     select * from emp where name like _g%;
 89 
 90 4.2 正则 regexp 支持正则匹配
 91     select * from emp where name regexp ^eg;
 92     select * from emp where name regexp eg;
 93 
 94 
 95 五 distinct 去重
 96     select distinct sex from emp;
 97 
 98 六 数学运算符
 99     select name,salary from emp; 每个人名字,薪资
100     select name,salary*12 年薪 from emp;

 

二 多表查询,一下是表内容

技术分享

 

 1 多表查询
 2 一  简单查询 多张表 提取相同的 假设department id 和 employee dep_id 关联
 3 1.1 select * from department,employee; 结果显示一张表上,是左右字段一次排开拼接这种
 4     遍历第一张表,循环第二张,配对一遍,显示出来,拼接方式叫做 笛卡尔积
 5     select * from department,employee where department.id=employee.dep_id;
 6     select * from employee,department where department.id=employee.dep_id;  提取2表相同的部分
 7 
 8 
 9 二  连表操作
10 2.1 内链接 inner join department on
11     按照on条件只取2表的相同部分,链接成一张虚拟的表
12     select * from employee inner join department on department.id=employee.dep_id;   提取2表相同的部分
13 
14 2.2 左链接 left join department on
15     按照on条件取2表的相同部分基础上,保留左表有,右边没有的记录
16     select * from employee left join department on department.id=employee.dep_id;
17 
18 2.3 右链接 right join department on
19     按照on条件取2表的相同部分基础上,保留右表有,左边没有的记录
20     select * from employee right join department on department.id=employee.dep_id;
21 
22 2.4 全链接 full join 在mysql里面没有这个功能,只是个概念,还要考左,右链接拼凑出来
23     有对应关系,和没对应关系的都取出来
24     select * from employee left join department on department.id=employee.dep_id
25     union                 联合的意思,将2表联合,去掉重复的部分
26     select * from employee right join department on department.id=employee.dep_id;
27 
28 **  优先级
29     from > join on 后面的条件联合 > where > gourp by > 聚合 > having > order by > limit > select
30 
31 三  子查询
32 3.1 带in关键字的子查询
33     select id from department where name in (技术,销售);  查看2个部门的d
34     select * from employee where dep_id in (200.202);  查询部门id对应的员工
35     问题;怎么知道员工的id,应该是从部门表得到id
36     select * from employee where dep_id in (select id from department where name in (技术,销售));
37 
38 3.2 练习
39     1. 查询平均年龄在25岁以上的部门名
40         select name from department where id in (select dep_id from employee group by dep_id having avg(age) > 25);
41     2. 查看技术部员工姓名
42         select name from employee where dep_id in (select id from department where name=技术);
43     3. 查看人数小于2的部门,不准确,因为还有空的存在,看 4 544         select name from department where id in (select dep_id from employee group by dep_id having count(dep_id) < 2);
45     4. 提取空部门,就是没有人的部门
46         select distinct dep_id from employee; 去重,有人的部门
47         select name from department where id not in (select distinct dep_id from employee);  非有人的部门,空部门
48     5. 查看人数小于2的部门,但是有空的情况
49         select name from department where id in (select dep_id from employee group by dep_id having count(dep_id) < 2)
50         union
51         select name from department where id not in (select distinct dep_id from employee); 没有人的部门
52 
53         或者:
54         select name from department where id in
55         (
56         select dep_id from employee group by dep_id having count(id) < 2
57         union
58         select id from department where id not in (select distinct dep_id from employee)
59         );
60 
61 3.3  exists存在,判断
62     select * from employee where exists (select id from department where name=技术);
63     select id from department where name=技术 存在 就打印前面的select,不存在就返回空
64 
65 **  子查询,括号内的查询都要select出2表相同的字段,这样才可以,语法:查询 where 相同字段 in 子查询

 

三 总结&表创建过程

 1 总结;
 2 查询语法
 3 SELECT 字段1,字段2... FROM 表名
 4                   JOIN ON
 5                   WHERE 条件
 6                   GROUP BY field
 7                   HAVING 筛选
 8                   ORDER BY field
 9                   LIMIT 限制条数
10 
11 关键字的执行优先级
12 from
13 join on
14 where
15 ---------------
16 group by
17 ---------------
18 聚合
19 having
20 select
21 ---------------
22 限制打印
23 distinct
24 order by
25 limit

 

单表创建

 1 #创建表
 2 create table employee(
 3 id int not null unique auto_increment,
 4 name varchar(20) not null,
 5 sex enum(male,female) not null default male, #大部分是男的
 6 age int(3) unsigned not null default 28,
 7 hire_date date not null,
 8 post varchar(50),
 9 post_comment varchar(100),
10 salary double(15,2),
11 office int, #一个部门一个屋子
12 depart_id int
13 );
14 
15 #插入记录
16 #三个部门:教学,销售,运营
17 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
18 (egon,male,18,20170301,老男孩驻沙河办事处外交大使,7300.33,401,1), #以下是教学部
19 (alex,male,78,20150302,teacher,1000000.31,401,1),
20 (wupeiqi,male,81,20130305,teacher,8300,401,1),
21 (yuanhao,male,73,20140701,teacher,3500,401,1),
22 (liwenzhou,male,28,20121101,teacher,2100,401,1),
23 (jingliyang,female,18,20110211,teacher,9000,401,1),
24 (jinxin,male,18,19000301,teacher,30000,401,1),
25 (成龙,male,48,20101111,teacher,10000,401,1),
26 
27 (歪歪,female,48,20150311,sale,3000.13,402,2),#以下是销售部门
28 (丫丫,female,38,20101101,sale,2000.35,402,2),
29 (丁丁,female,18,20110312,sale,1000.37,402,2),
30 (星星,female,18,20160513,sale,3000.29,402,2),
31 (格格,female,28,20170127,sale,4000.33,402,2),
32 
33 (张野,male,28,20160311,operation,10000.13,403,3), #以下是运营部门
34 (程咬金,male,18,19970312,operation,20000,403,3),
35 (程咬银,female,18,20130311,operation,19000,403,3),
36 (程咬铜,male,18,20150411,operation,18000,403,3),
37 (程咬铁,female,18,20140512,operation,17000,403,3)
38 ;

 

多表创建

 1 多表查询
 2 整体是一个数据,方便管理,节省空间拆成不同的表
 3 company.employee
 4 company.department
 5 
 6 ==============================被关联表
 7 创建 被关联表
 8 create table department(
 9 id int,
10 name varchar(20)
11 )charset utf8;
12 插入数据
13 insert into department values
14 (200,技术),
15 (201,人力资源),
16 (202,销售),
17 (203,运营);
18 
19 
20 ===============================关联表
21 创建 关联表
22 create table employee(
23 id int primary key auto_increment,
24 name varchar(20),
25 sex enum(male,female) not null default male,
26 age int,
27 dep_id int
28 )charset utf8;
29 插入数据
30 insert into employee(name,sex,age,dep_id) values
31 (egon,male,18,200),
32 (alex,female,48,201),
33 (wupeiqi,male,38,201),
34 (yuanhao,female,28,202),
35 (liwenzhou,male,18,200),
36 (jingliyang,female,18,204)
37 ;
38 
39 ** 暂时不加入外键,如果假如外键,关联表在拆入数据的时候,插入204,上面的表没有下面的表对应关系
40    上有下没有,下有上没有,就会报错
41    上表有203下面没有,下面204上面没有
42 ** 虽然没有外键,咱们假设department id 和 employee dep_id 关联

 

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