56 多表連接查詢 索引
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主要內容
1 多表連接查詢
語法: select 字段列表
from 表一 inner/left/right join 表二
on 表一.字段 = 表二.字段
內連接: 直連接匹配的行:
select * from employee inner join department on employee.dep_id = department.id;
select * from employee , department where employee.dep_id = department.id;和上面的效果一樣.
左連接 : 顯示左表的全部記錄:
select * from employee left join department on employee.dep_id = department.id;
右連接 : 顯示右表的全部記錄:
select * from employee rightr join department on employee.dep_id = department.id;
顯示全部記錄:
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;
符合條件連接查詢:
示例1: 以內連接的方式查詢employee和department表,並且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門
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56 多表連接查詢 索引