left join后边跟on...and 和where...and的区别

文章目录

1. 建表

testA

CREATE TABLE testA (
  Id int ( 10 ) PRIMARY KEY,
  NAME VARCHAR ( 10 )
)

INSERT INTO testA
VALUES
  ( 1, '小黄' );
INSERT INTO testA
VALUES
  ( 2, '小绿' );
INSERT INTO testA
VALUES
  ( 3, '小白' );
INSERT INTO testA
VALUES
  ( 4, '小黑' );
INSERT INTO testA
VALUES
  ( 5, '小花' );

testB

create TABLE testB (
  Id int ( 10 ) PRIMARY KEY,
  age int ( 10 )
)

INSERT INTO testB
VALUES
  ( 1, 10 );
INSERT INTO testB
VALUES
  ( 2, 11 );
INSERT INTO testB
VALUES
  ( 3, 12 );
INSERT INTO testB
VALUES
  ( 4, 13 );

两表如下：

2. on…and

先来一个常用的​​left join...on​​

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id

​​on​​ 后边加 ​​and a.id = 1​​

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id 
and a.id = 1  # 连接限制条件  a.id = 1

​​on​​ 后边加 ​​and b.id = 1​​

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id 
and b.id = 1

​​on​​ 后边加 ​​and b.age = a.id​​

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id 
and b.age = a.id

on 后的 and 有两表关联的字段： b.age 和 a.id，由于b.age 没有与 a.id相等的数据

所以​​a​​表全部显示，​​b​​表内容为空

3. where…and

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id 
and b.id > 1

where b.id =2

以上sql分解一下：首先看

SELECT
  * 
FROM
  testA a
LEFT JOIN 
  testB b 
ON a.id = b.id 
and b.id > 1

然后再加上​​where b.id =2​​

由此可见，on…and是连接。where…and 是过滤！