多表查询
连表操作
完整版
执行顺序: FROM >ON>JOIN> WHERE>GROUP BY >HAVING>SELECT>DISTINCT>ORDERBY>LIMIT>SELECT
语法顺序: SELECT [DISTINCT]<name> FROM tablename <join type> [JOIN] <right table name> [ON]<condition on>[WHERE]<condition where> [GROUP BY]<name> HAVING <condition having> ORDER BY <name> LIMIT<int>
笛卡尔积
select * from employee, dep #笛卡尔积
内连接
select * from employee inner join department on employee.dep_id= dep.id;
左连接
内连接的基础上保留左表的内容
select * from employee left join department on employee.dep_id= dep.id;
右连接
内连接的基础上保留右表的内容
select * from employee left join department on employee.dep_id= dep.id;
全外连接, full join , mysql不支持
内连接的基础上保留左右表没有对应关系的内容
select * from employee left join department on employee.dep_id= dep.id
union
select * from employee right join department on employee.dep_id= dep.id;
查询员工平均年龄大于30岁的部门名称
select dep.name,avg(age) from employee inner join dep on employee.dep_id = dep.id
group by dep.name
having avg(age) > 30;