MySQL查询
Posted NachoLau
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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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