表A记录如下:
aID aNum
1 a11
2 a22
3 a33
4 a44
5 a55
表B记录如下:
bID bName
1 b11
2 b22
3 b33
4 b44
8 b88
创建这两个表SQL语句如下:
CREATE TABLE a
aID int( 1 ) AUTO_INCREMENT PRIMARY KEY ,
aNum char( 20 )
)
CREATE TABLE b(
bID int( 1 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
bName char( 20 )
)
INSERT INTO a
VALUES ( 1, 'a11' );
commit;
.....
INSERT INTO b
VALUES ( 1, 'b11' );
commit;
.....
1.left join(左联接)
sql语句如下:
SELECT * FROM a
LEFT JOIN b
ON a.aID =b.bID
结果如下:
aID aNum bID bName
1 a11 1 b11
2 a22 2 b22
3 a33 3 b33
4 a44 4 b44
5 a55 NULL NULL
(所影响的行数为 5 行)
结果说明:
left join是以A表的记录为基础的,A可以