创建表：
CREATE TABLE emp (
ename varchar(10) DEFAULT NULL,
hiredate date DEFAULT NULL,
sal decimal(10,2) DEFAULT NULL,
deptno int(2) DEFAULT NULL,
age int(3) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE dept (
deptno int(2) DEFAULT NULL,
deptname varchar(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表连接：当需要同时显示多个表中的字段时，使用表连接

内连接：仅选出两张表中互相匹配的记录
外连接：既选出匹配的记录，也选出不匹配的记录

    左连接：包含所有的左表中的记录，即使右表中没有和它相匹配的记录
    例如，用户表中有部门id，使用左连接查询右表，得到相应的部门名称，对于用户表中的id在右表中找不到对应的记录时结果为NULL
    select ename,hiredate,sal,emp.deptno,age,deptname from emp left join dept on emp.deptno=dept.deptno;
    +-------+------------+------+--------+------+----------+
    | ename | hiredate   | sal  | deptno | age  | deptname |
    +-------+------------+------+--------+------+----------+
    | zzxl  | 2000-01-01 | 2000 |      1 | NULL | dept1    |
    | lisa  | 2003-01-01 | 3000 |      2 |   20 | dept2    |
    | dony  | NULL       | 2000 |      5 | NULL | dept5    |
    +-------+------------+------+--------+------+----------+

    右连接：包含所有的右表中的记录，即使左表中没有和它相匹配的记录
    select ename,emp.deptno,deptname from emp right join dept on emp.deptno=dept.deptno;
    +-------+--------+----------+
    | ename | deptno | deptname |
    +-------+--------+----------+
    | dony  |      5 | dept5    |
    | NULL  | NULL   | dept6    |
    | zzxl  |      1 | dept1    |
    | lisa  |      2 | dept2    |
    +-------+--------+----------+

子查询：进行查询时，当需要的条件是另外一个查询的结果是，使用子查询
查询所有部门中的用户

select * from emp where deptno in (select deptno from dept);
+-------+------------+------+--------+------+
| ename | hiredate   | sal  | deptno | age  |
+-------+------------+------+--------+------+
| zzxl  | 2000-01-01 | 2000 |      1 | NULL |
| lisa  | 2003-01-01 | 3000 |      2 |   20 |
| dony  | NULL       | 2000 |      5 | NULL |
+-------+------------+------+--------+------+
如果子查询结果唯一，可以用=代替in
select * from emp where deptno = (select deptno from dept limit 1);
+-------+----------+------+--------+------+
| ename | hiredate | sal  | deptno | age  |
+-------+----------+------+--------+------+
| dony  | NULL     | 2000 |      5 | NULL |
+-------+----------+------+--------+------+

注意：表连接用于优化子查询

记录联合：将多次查询结果合并显示

select deptno from emp union select deptno from dept;//去除重复数据
select deptno from emp union all select deptno from dept;

create table salary(userid int,salary decimal(10,2));
insert into salary values(1,1000),(2,2000),(3,3000),(4,4000),(1,null);

if判断

select *,if(salary>2000,‘high‘,‘low‘) as statu from salary;
+--------+--------+-------+
| userid | salary | statu |
+--------+--------+-------+
|      1 | 1000   | low   |
|      2 | 2000   | low   |
|      3 | 3000   | high  |
|      4 | 4000   | high  |
|      1 | NULL   | low   |
+--------+--------+-------+

ifnull判断

select ifnull(salary,0) from salary;
+------------------+
| ifnull(salary,0) |
+------------------+
| 1000             |
| 2000             |
| 3000             |
| 4000             |
| 0                |
+------------------+

case when

select case when salary<=2000 then ‘low‘ else ‘high‘ end from salary;
select case salary when 1000 then ‘low‘ when 2000 then ‘mid‘ else  ‘high‘ end from salary;

mysql表连接，子查询以及if判断