一,单表查询的语法
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
二,关键字的执行优先级(重点)
重点中的重点:关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit
1.找到表:from
2.拿着where指定的约束条件,去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组
4.将分组的结果进行having过滤
5.执行select
6.去重
7.将结果按条件排序:order by
8.限制结果的显示条数
三,简单查询
1,准备表和记录
#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
)
;
#ps:如果在windows系统中,插入中文字符,select的结果为空白,可以将所有字符编码统一设置成gbk
2,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;
结合CASE语句:
SELECT
(
CASE
WHEN NAME = 'egon' THEN
NAME
WHEN NAME = 'alex' THEN
CONCAT(name,'_BIGSB')
ELSE
concat(NAME, 'SB')
END
) as new_name
FROM
emp;
四,WHERE约束
1,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__';
五,分组查询:GROUP BY
1,什么是分组,为什么要分组?
1、首先明确一点:分组发生在where之后,即分组是基于where之后得到的记录而进行的
2、分组指的是:将所有记录按照某个相同字段进行归类,比如针对员工信息表的职位分组,
或者按照性别进行分组等
3、为何要分组呢?
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数
小窍门:‘每’这个字后面的字段,就是我们分组的依据
4、大前提:
可以按照任意字段分组,但是分组完毕后,比如group by post,只能查看post字段,
如果想查看组内信息,需要借助于聚合函数
2,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
#!!!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。
#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式):
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';
!!!SQL_MODE设置!!!
#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
#!!!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。
#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式):
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';
!!!SQL_MODE设置!!!
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mysql> select @@global.sql_mode;
+-------------------+
| @@global.sql_mode |
+-------------------+
| |
+-------------------+
row in set (0.00 sec)
mysql> select * from emp group by post;
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
rows in set (0.00 sec)
#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果,默认都是组内的第一条记录,但其实这是没有意义的
mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)
mysql> quit #设置成功后,一定要退出,然后重新登录方可生效
Bye
mysql> use db1;
Database changed
mysql> select * from emp group by post; #报错
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
mysql> select post,count(id) from emp group by post; #只能查看分组依据和使用聚合函数
+----------------------------+-----------+
| post
3,GROUP BY
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
4,聚合函数
强调:聚合函数聚合的是组的内容,若是没有分组,则默认一组
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;
!!!执行优先级从高到低:where > group by > having
1. Where 发生在分组group by之前,因而Where中可以有任意字段,但是绝对不能使用聚合函数。
2. Having发生在分组group by之后,因而Having中可以使用分组的字段,
无法直接取到其他字段,可以使用聚合函数
六,HAVING过滤
1,HAVING与WHERE不一样的地方在于下面
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