单表查询

Posted 2020-10-05 —super—

一 介绍

本节内容:

查询语法

关键字的执行优先级

简单查询

单条件查询:WHERE

分组查询:GROUP BY

HAVING

查询排序:ORDER BY

限制查询的记录数:LIMIT

使用聚合函数查询

使用正则表达式查询

 

 

创建表
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum(\'male\',\'female\') not null default \'male\', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);

#插入记录
#三个部门：教学，销售，运营
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
(\'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
(\'alex\',\'male\',78,\'20150302\',\'teacher\',1000000.31,401,1),
(\'wupeiqi\',\'male\',81,\'20130305\',\'teacher\',8300,401,1),
(\'yuanhao\',\'male\',73,\'20140701\',\'teacher\',3500,401,1),
(\'liwenzhou\',\'male\',28,\'20121101\',\'teacher\',2100,401,1),
(\'jingliyang\',\'female\',18,\'20110211\',\'teacher\',9000,401,1),
(\'jinxin\',\'male\',18,\'19000301\',\'teacher\',30000,401,1),
(\'成龙\',\'male\',48,\'20101111\',\'teacher\',10000,401,1),

(\'歪歪\',\'female\',48,\'20150311\',\'sale\',3000.13,402,2),#以下是销售部门
(\'丫丫\',\'female\',38,\'20101101\',\'sale\',2000.35,402,2),
(\'丁丁\',\'female\',18,\'20110312\',\'sale\',1000.37,402,2),
(\'星星\',\'female\',18,\'20160513\',\'sale\',3000.29,402,2),
(\'格格\',\'female\',28,\'20170127\',\'sale\',4000.33,402,2),

(\'张野\',\'male\',28,\'20160311\',\'operation\',10000.13,403,3), #以下是运营部门
(\'程咬金\',\'male\',18,\'19970312\',\'operation\',20000,403,3),
(\'程咬银\',\'female\',18,\'20130311\',\'operation\',19000,403,3),
(\'程咬铜\',\'male\',18,\'20150411\',\'operation\',18000,403,3),
(\'程咬铁\',\'female\',18,\'20140512\',\'operation\',17000,403,3)
;

二 查询语法

SELECT 字段1,字段2... FROM 表名
                  WHERE 条件
                  GROUP BY field
                  HAVING 筛选
                  ORDER BY field
                  LIMIT 限制条数

三 关键字的执行优先级(重点)

from
where
group by
having
select
distinct
order by
limit

1.找到表:from

2.按条件过滤表记录:where（过滤的结果是一条条记录）

3.将查出的结果按条件分组：group by

4.将分组的结果再次过滤：having（与where相同之处是都可以进行过滤，不同之处是having是基于分组之后的结果进行过滤，过滤的结果也是一个个的组）

5.查出结果：select

6.去重

7.将6的结果按条件排序：order by

8.将7的结果限制显示条数

详细见：http://www.cnblogs.com/linhaifeng/articles/7372774.html

简单查询

#简单查询
    SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT name,salary FROM employee;

#避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

#通过四则运算查询
    SELECT name, salary*12 FROM employee;
    SELECT name, salary*12 AS Annual_salary FROM employee;
    SELECT name, salary*12 Annual_salary FROM employee;

#定义显示格式
   CONCAT() 函数用于连接字符串
   SELECT CONCAT(\'姓名: \',name,\'  年薪: \', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(\':\',name,salary*12)  AS Annual_salary 
   FROM employee;

View Code

五 单条件查询:WHERE

where字句中可以使用：

1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like \'egon%\'
    pattern可以是%或_，
    %表示任意多字符
    _表示一个字符 
5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

#1:单条件查询
    SELECT name FROM employee
        WHERE post=\'sale\';
        
#2:多条件查询
    SELECT name,salary FROM employee
        WHERE post=\'teacher\' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)
    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT name,post_comment FROM employee 
        WHERE post_comment=\'\'; 注意\'\'是空字符串，不是null
    ps：
        执行
        update employee set post_comment=\'\' where id=2;
        再用上条查看，就会有结果了

#5:关键字IN集合查询
    SELECT name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE name LIKE \'eg%\';

    通配符’_’
    SELECT * FROM employee 
            WHERE name LIKE \'al__\';

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六 分组查询:GROUP BY

大前提：可以按照任意字段分组,但分完组后，只能查看分组的那个字段，要想取的组内的其他字段信息，需要借助函数

单独使用GROUP BY关键字分组
    SELECT post FROM employee GROUP BY post;
    注意：我们按照post字段分组，那么select查询的字段只能是post，想要获取组内的其他相关信息，需要借助函数

GROUP BY关键字和GROUP_CONCAT()函数一起使用
    SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
    SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;

GROUP BY与聚合函数一起使用
    select post,count(id) as count from employee group by post;#按照岗位分组，并查看每个组有多少人

View Code

#参考链接：http://www.ywnds.com/?p=8184
#分组查询的常见问题：
mysql> select id,count from tt group by id;
ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column \'test.tt.count\' which is 
not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

#查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

#去掉ONLY_FULL_GROUP_BY模式，如下操作：
mysql> set global sql_mode=\'STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION\';


#！！！注意
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。

注意

七 HAVING

HAVING与WHERE用法一样
select * from employee where salary > 10000;
select * from employee having salary > 10000;

不同之处在于：having是分组后过滤，而wehre是分组前过滤
语法：having可以放到group by之后，而where只能放到group by之前
select post,group_concat(name) from employee group by post having salary > 10000;#错误，分组后无法直接取到salary字段
select post,group_concat(name) from employee group by post having avg(salary) > 10000;

执行优先级：having比where的执行优先级更低
select count(id) from employee where salary > 10000; #正确：where先执行，后执行select
select count(id) from employee having salary > 10000;#错误：select先执行，后执行having，所有无法统计count

#以上两条sql的顺序是
#1：
找到表employee--->用where过滤---->没有分组则默认一组--->select执行查看组内id数目
#2
找到表employee--->用where过滤---->没有分组则默认一组--->select执行查看组内id数目---->having 基于上一步得到的结果进行过滤，无法从组内组取到salary这个值

八 查询排序:ORDER BY

按单列排序
    SELECT * FROM employee ORDER BY salary;
    SELECT * FROM employee ORDER BY salary ASC;
    SELECT * FROM employee ORDER BY salary DESC;

按多列排序:先按照age排序，如果年纪相同，则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;

九 限制查询的记录数:LIMIT

示例：
    SELECT * FROM employee ORDER BY salary DESC 
        LIMIT 3;                    #默认初始位置为0 
    
    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条

十 使用聚合函数查询

示例：
    SELECT COUNT(*) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    SELECT MAX(salary) FROM employee;
    SELECT MIN(salary) FROM employee;
    SELECT AVG(salary) FROM employee;
    SELECT SUM(salary) FROM employee;
    SELECT SUM(salary) FROM employee WHERE depart_id=3;

十一 使用正则表达式查询

SELECT * FROM employee WHERE name REGEXP \'^ale\';

SELECT * FROM employee WHERE name REGEXP \'on$\';

SELECT * FROM employee WHERE name REGEXP \'m{2}\';


小结：对字符串匹配的方式
WHERE name = \'egon\';
WHERE name LIKE \'yua%\';
WHERE name REGEXP \'on$\';