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主要内容
1 多表连接查询
语法: select 字段列表
from 表一 inner/left/right join 表二
on 表一. 字段 = 表二. 字段
内连接: 直连接匹配的行:
select * from employee inner join department on employee.dep_id = department.id;
select * from employee , department where employee.dep_id = department.id; 和上面的效果一样.
左连接 : 显示左表的全部记录:
select * from employee left join department on employee.dep_id = department.id;
右连接 : 显示右表的全部记录:
select * from employee rightr join department on employee.dep_id = department.id;
显示全部记录:
- select * from employee left join department on employee.dep_id = department.id;
- union
- select * from employee rightr join department on employee.dep_id = department.id;
符合条件连接查询:
示例 1: 以内连接的方式查询 employee 和 department 表, 并且 employee 表中的 age 字段值必须大于 25, 即找出年龄大于 25 岁的员工以及员工所在的部门
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