mysql left join,right join,inner join用法分析
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下面是两个表表A和表B
表A记录如下:
aID aNum
5 a20050115
B表记录不足的地方均为NULL.
2.right join(右联接)
sql语句如下:
SELECT * FROM a
RIGHT JOING b
ON a.aID = b.bID
结果如下:
aID aNum bID bName
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
NULL NULL 8 2006032408
(所影响的行数为 5 行)
结果说明:
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
3.inner join(相等联接或内联接)
sql语句如下:
SELECT * FROM a
INNER JOIN b
ON a.aID =b.bID
等同于以下SQL句:
SELECT *
FROM a,b
WHERE a.aID = b.bID
结果如下:
aID aNum bID bName
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
结果说明:
很明显,这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.
LEFT JOIN操作用于在任何的 FROM 子句中,
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