Mysql INNER,LEFT ,RIGHT join的使用
JOIN 按照功能大致分为如下三类:
- INNER JOIN(内连接,或等值连接):获取两个表中字段匹配关系的记录。
- LEFT JOIN(左连接):获取左表所有记录,即使右表没有对应匹配的记录。
- RIGHT JOIN(右连接): 与 LEFT JOIN 相反,用于获取右表所有记录,即使左表没有对应匹配的记录。
INNER JOIN
首先,这样是寻找两个表之间的一个关联的字段,然后就能够得到两个表的功用数据。
SELECT
a.UserId,
a.UserNo,
b.ty_name
FROM
user a,
type b
WHERE
a.UserId = b.ty_id;
等同于
select a.UserId,a.UserNo FROM user a
用查出来的a.UserId查询出来的list,做一个循环,将主键带入到另一个表中查询出数据
SELECT
b.ty_name
from
type b
where
b.ty_id = ?
这样也能达到一样的效果。
同样等价于:
SELECT t1.UserNo,b.ty_name FROM
(select a.UserId,a.UserNo FROM user a) t1
,type b
where
t1.UserId = b.ty_id
表示,将一个表里面查出来的数据作为一张临时的表,然后和另一张表进行外键的链接。
同样等价于这里所谓的INNER JOIN内链接,
SELECT
a.UserId,
a.UserNo,
b.ty_name
FROM
user a
INNER JOIN
type b ON a.UserId = b.ty_id;
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LEFT JOIN(左链接)
SELECT
a.UserId,
a.UserNo,
b.ty_name
FROM
user a
LEFT JOIN
type b ON a.UserId = b.ty_id;
MySQL LEFT JOIN 会读取左边数据表的全部数据,即便右边表无对应数据
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RIGHT JOIN 会读取右边数据表的全部数据,即便左边边表无对应数据
SELECT
a.UserId,
a.UserNo,
b.ty_name
FROM
user a
right JOIN
type b ON a.UserId = b.ty_id;
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" alt="" />
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