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MySQL 查询

MySQL 单表多表查询

select 字段名 from 表名 where 条件

例如：select id from user where name='张三'

​ 查询出 user表中 name为张三 的id

条件查询关键字

= 等于
<> 或 != 不等于
>、 < 、>= 、<=  大于、小于、大于等于、小于等于

between ... and .... 两个值之间，但前值必须小于后值

is null 和 is not null  判断空的

and 并且  ， or 或者

in 包含 例如 in('a','b');

not 用于取非，主要用于 is 和 in

like 模糊查询 例如 username like %c% 或 username like _c%
     %匹配任意个字符 、下划线_ 匹配单个字符
其它关键字
distinct 去重 例如，select distinct dep_name from ....

desc 降序
asc  升序（默认）
order by age asc,name desc; 按年龄升序，当年龄一样时，按名字降序
例子：找出工资在1250到3000之间的员工的姓名和薪资，要求按薪资进行降序排序。

select name,sal 
from emp 
where sal between 1250 and 3000 
order by sal desc;

单行处理函数

数据处理函数又被成为单行处理函数，特点是一个输入对应一个输出。 
常见的有
lower 转小写 lower('A')
upper 转大写 lower('a')
substr 截取字符串 substr(字段名,起始位置从1开始,截取的长度)
length 取长度 
trim 去两边的空格
str_to_date 字符串转日期 （'字符串日期','日期格式'）%Y %m %d %h %i分 %s秒
            str_to_date（'01-10-1999','%d-%m-%Y'）
            这种'1999-01-01' 正确格式无需使用函数，mysql会自动转换
date_format （'日期类型数据','日期格式'） 日期转字符串 date_format(birth,'%m/%d/Y') 这函数没啥用，存正确格式的日期，mysql会自动转换
rand 生成随机数
round 四舍五入 round(1.23, 2) 保留2位小数
concat 对字符串进行拼接 concat(name1,name2)
ifnull 将null转换成一个具体指值 ifnull(age,0) ，表示如果年龄是空，用0代替

case 字段名 when 条件 then 做什么 else 做什么 end 
例子：当sex为1时，表示为男，否则表示为 女
case sex when '1' then '男' else '女' end

多行处理函数（分组函数）

分组函数又被称为多行处理函数，特点：多个输入对应一个输出,自动忽略null的记录，不能使用在where后。
使用方式：函数名(字段名)，例如 sum(score) ，表示对 分数进行求和。

常见的有 
max 取最大值
min 取最小值
avg 求平均值
sum 求和

count 计数 
    count(*)、count(字段名)和 count(1) 的区别？ 

    答：(*)会计算包含null的记录，count(字段名)则忽略null的记录，使用*效率更低
        列名为主键，count(列名) 会比 count(1) 快 ；
        列名不为主键，count(1) 会比 count(列名) 快 ；
        如果表多个列并且没有主键，则 count(1) 的执行效率优于 count() ；
        如果有主键，则 select count（主键）执行效率最优；
        如果表只有一个字段，则 select count(*)最优。

分组查询（重要）

select 字段名 from 表名 where 条件 order by 字段名

SQL语句执行顺序： from → where → group by → select → order by

例子1：按照工作岗位分组，对工资进行求和，查询出岗位和工资总和
     select job,sum(sal) from emp group by job;

例子2：按照部门编号分组，找出每个部门的最高薪资
     select depno,max(sal) from emp group by depno;

例子3: 找出每个部门，不同工作岗位的最高薪资（按照部门编号分组）
     select depno,job,max(sal) from emp group by depno,job;

例子4: 找出每个部门的最高薪资大于3000的信息（按照部门编号分组）
     方式1：select depno,max(sal) from emp where sal>3000 group by depno;
     方式2：select depno,sal from emp group by depno having max(sal)>3000;
     having 可以对分完组后的数据进行再次过滤，不能单独使用，得和group by配合。
     优先使用方式1，where，效率更高

例子5：找出每个部门的平均薪资大于2500的信息（按照部门编号分组） 
     select depno,avg(sal) from emp group by depno having avg(sal)>2500;
     此例子只能用having，无法用where

备注：用了 group by后，select 后面只能跟 参与分组的字段 或 分组函数，否则无意义；

注意：使用分组函数前必须要先分组，所以不能在where 后使用分组函数，例如不能 ...where min(x);

为什么min(x) 不能用在where 的后面，因为使用min(x)的时候，还没有进行分组！请参考SQL的执行顺序。

Mysql 多表查询

内连接

等值连接

查询每个员工所在的部门名称，显示员工名和部门名

select e.ename,d.dname
from emp e
join dept d
on e.deptno = d.deptno;

非等值连接

找出每个员工的薪资等级，要求显示员工名、薪资、薪资等级？

select e.ename,e.sal,s.grade
from emp e
join salgrade s
on e.sal between s.losal and s.hisal;

自连接

查询员工的上级领导，要求员工名和对应的领导名(同  张表)

select a.ename,b.ename
from emp a
join emp b
on a.mgr = b.empno

外连接

sql语句中的inner和outer可以省略，带有right或left的是外连接。

左外连接

select e.ename,d.dname
from dept d
 left join emp e
on e.deptno = d.deptno;

右外连接

select e.ename,d.dname
from dept d
right join emp e
on e.deptno = d.deptno;

right 代表什么，表示将join关键字右边的这张表看成主表，主要是为了将右边的这张表的数据全部查询出来，捎带着关联查询左边的表，在外连接当中，两张表连接，产生了主次关系。

三表连接

找出每个员工的部门名称以及工资等级，要求显示员工名、部门名、薪资、薪资等级

select e.ename,d.dname,e.sal,s.grade
from emp e
join dept d
on e.deptno = d.deptno
join salgrade s
on e.sal between s.losal and hisal;
找出每个员工的部门名称以及工资等级，还有上级领导，要求显示员工名、领导名、部门名、薪资、薪资等级

select e.ename,l.ename,d.dname,e.sal,s.grade
from emp e
join dept d
on e.deptno = d.deptno
join salgrade s
on e.sal between s.losal and hisal;
left join emp l //left 保证emp表的数全部查出来
on e.mgr = l.empno

MySQL 子查询

select 中 嵌套 select语句，被嵌套的select 语句成为子查询

可嵌套的位置：select (select) from (select) where (select)

where后出现子查询

找出比最低工资高的员工姓名和工资

select ename,sal from emp where sal > (select min(sal) from emp);

from 后出现子查询

找出每个岗位的平均薪资的薪资等级

select t.*,s.grade
from (select job,avg(sal) as avgsal from emp group by job) t
join salgrade s
on t.avgsal between s.losal and s.highsal;

select 后出现子查询

找出每个员工的部门名称，要求显示员工名，部门名（此题了解即可）

select e.ename,(select d.dname from dept d where e.deptno=d.deptno) as dname
from emp e;

此情况的子查询只能一次返回1条结果，如果结果是多条会报错，因为和其它字段结果行数不匹配

其它知识

union的例子

第1种：select ename,job from emp where job='Manager' or job='Salesman';

第2种：select ename,job from emp where in('Manager','Salesman');

第3种：
select ename,job from emp where job='Manager';
union
select ename,job from emp where job='Salesman'

第3种使用 union的效率要高一些，对于表连接来说，每连接一次新表则匹配的次数满足笛卡尔积，成倍的翻，但是union可以减少匹配的次数，还可以完成两结果的拼接。
注意，使用union 进行结果合并时，要求两语句的列数和各字段相同
limit

limit startIndex,length;

limit 5; 前5条记录
limit 2,5; 从第3条开始，往后取5条

注意：limit在order by后执行！

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