标签:io ar 使用 java 数据 on ef as sql
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;
标签:io ar 使用 java 数据 on ef as sql
原文地址:http://my.oschina.net/aptx4869/blog/348727