MySQL查询

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单表查询

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

(1)where 约束 

where子句中可以使用
1.比较运算符:>、<、>=、<=、<>、!=
2.between 80 and 100 :值在80到100之间
3.in(80,90,100)值是10或20或30
4.like xiaomagepattern: pattern可以是%或者_。%小时任意多字符,_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

(2)group by 分组查询

如果想分组,则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
mysql> set global sql_mode=ONLY_FULL_GROUP_BY;
Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默认的sql_mode如下:
mysql> select @@global.sql_mode;
+--------------------+
| @@global.sql_mode  |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in set (0.00 sec)

mysql> exit;#设置成功后,一定要退出,然后重新登录方可生效

 

mysql> select * from emp group by post;# 报错
ERROR 1054 (42S22): Unknown column post in group statement



mysql>  select post from employee group by post;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| operation                               |
| sale                                    |
| teacher                                 |
| gardener                                |
+-----------------------------------------+
rows in set (0.00 sec)

 

(3)聚合函数

max()求最大值
min()求最小值
avg()求平均值
sum() 求和
count() 求总个数

#聚合函数聚合的是组的内容,若是没有分组,则默认一组
# 每个部门有多少个员工
select post,count(id) from employee group by post;
# 每个部门的最高薪水
select post,max(salary) from employee group by post;
# 每个部门的最低薪水
select post,min(salary) from employee group by post;
# 每个部门的平均薪水
select post,avg(salary) from employee group by post;
# 每个部门的所有薪水
select post,sum(age) from employee group by post;

 

(4)HAVING过滤

HAVING与WHERE不一样的地方在于

#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

 

验证:
mysql> select * from employee where salary>1000000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | qwe  | male |  78 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
row in set (0.00 sec)

mysql> select * from employee having salary>1000000;
ERROR 1463 (42000): Non-grouping field salary is used in HAVING clause

# 必须使用group by才能使用group_concat()函数,将所有的name值连接
mysql> select post,group_concat(name) from emp group by post having salary > 10000; ##错误,分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column post in field list

 

正确:

select post,group_concat(name),count(id) from employee group by post;
select post,group_concat(name),count(id) from employee group by post having count(id)<2;
select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;

 

(5)order by 查询排序

按单列排序
    SELECT * FROM employee ORDER BY age;
    SELECT * FROM employee ORDER BY age ASC;
    SELECT * FROM employee ORDER BY age DESC;
按多列排序:先按照age升序排序,如果年纪相同,则按照id降序
    SELECT * from employee
        ORDER BY age ASC,
        id DESC;

 

(5)limit  限制查询的记录数:

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

 

多表查询

多表连接查询

SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

(1)交叉连接:不适用任何匹配条件。生成笛卡尔积

select * from employee,department;

 

(2)内连接:只连接匹配的行

select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;

 

(3)外链接之左连接:优先显示左表全部记录

#以左表为准,即找出所有员工信息,当然包括没有部门的员工
#本质就是:在内连接的基础上增加左边有,右边没有的结果

select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;

 

(4) 外链接之右连接:优先显示右表全部记录

#以右表为准,即找出所有部门信息,包括没有员工的部门
#本质就是:在内连接的基础上增加右边有,左边没有的结果

select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;

 

(5) 全外连接:显示左右两个表全部记录

#外连接:在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#注意:mysql不支持全外连接 full JOIN
#强调:mysql可以使用此种方式间接实现全外连接

语法:select * from employee left join department on employee.dep_id = department.id 
       union all
      select * from employee right join department on employee.dep_id = department.id;

 mysql> select * from employee left join department on employee.dep_id = department.id
          union
        select * from employee right join department on employee.dep_id = department.id
           ;

#注意 union与union all的区别:union会去掉相同的纪录

 

符合条件连接查询

select employee.name,department.name from employee inner join department
  on employee.dep_id = department.id
  where age > 25;

 

select employee.id,employee.name,employee.age,department.name from employee,department
    on employee.dep_id = department.id
    where age > 25
    order by age asc;

子查询

#1:子查询是将一个查询语句嵌套在另一个查询语句中。
#2:内层查询语句的查询结果,可以为外层查询语句提供查询条件。
#3:子查询中可以包含:IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4:还可以包含比较运算符:= 、 !=、> 、<等

(1)带in关键字的子查询

#查询平均年龄在25岁以上的部门名
select id,name from department
    where id in 
        (select dep_id from employee group by dep_id having avg(age) > 25);
# 查看技术部员工姓名
select name from employee
    where dep_id in 
        (select id from department where name=技术);
#查看不足1人的部门名
select name from department
    where id not in 
        (select dep_id from employee group by dep_id);

 

(2)带比较运算符的子查询

#比较运算符:=、!=、>、>=、<、<=、<>
#查询大于所有人平均年龄的员工名与年龄
mysql> select name,age from employee where age > (select avg(age) from employee);
+---------+------+
| name    | age  |
+---------+------+
| qwe     |   48 |
| asd     |   38 |
+---------+------+

#查询大于部门内平均年龄的员工名、年龄
思路:
      (1)先对员工表(employee)中的人员分组(group by),查询出dep_id以及平均年龄。
       (2)将查出的结果作为临时表,再对根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
       (3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。



mysql> select t1.name,t1.age from employee as t1
             inner join
            (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
            on t1.dep_id = t2.dep_id
            where t1.age > t2.avg_age;
+------+------+
| name | age  |
+------+------+
| qwe  |   48 |

 

(3)带EXISTS关键字的子查询

#EXISTS关字键字表示存在。在使用EXISTS关键字时,内层查询语句不返回查询的记录。而是返回一个真假值。True或False
#当返回True时,外层查询语句将进行查询;当返回值为False时,外层查询语句不进行查询
#department表中存在dept_id=203,Ture
mysql> select * from employee  where exists (select id from department where id=200);
+----+----------+--------+------+--------+
| id | name     | sex    | age  | dep_id |
+----+----------+--------+------+--------+
|  1 | qwe      | male   |   18 |    200 |
|  2 | asd      | female |   48 |    201 |
|  3 | zxc      | male   |   38 |    201 |
+----+----------+--------+------+--------+
#department表中存在dept_id=205,False
mysql> select * from employee  where exists (select id from department where id=204);
Empty set (0.00 sec)

 





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