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表A記錄如下：
aID               aNum
1                  a20050111
2                  a20050112
3                  a20050113
4                  a20050114
5                  a20050115

表B記錄如下:
bID               bName
1                   2006032401
2                  2006032402
3                  2006032403
4                  2006032404
8                  2006032408


實(shí)驗如下:
1.left join

sql語(yǔ)句如下:
select * from A
left join B
on A.aID = B.bID

結果如下:
aID               aNum                          bID                  bName
1                   a20050111                1                      2006032401
2                   a20050112                2                     2006032402
3                   a20050113                3                     2006032403
4                   a20050114                4                     2006032404
5                   a20050115                NULL              NULL
（所影響的行數為 5 行）

結果說(shuō)明:
               left join是以A表的記錄為基礎的,A可以看成左表,B可以看成右表,left join是以左表為準的.
換句話(huà)說(shuō),左表(A)的記錄將會(huì )全部表示出來(lái),而右表(B)只會(huì )顯示符合搜索條件的記錄(例子中為: A.aID = B.bID).
B表記錄不足的地方均為NULL.

2.right join
sql語(yǔ)句如下:
select * from A
right join B
on A.aID = B.bID
結果如下:
aID               aNum                          bID                  bName
1                   a20050111                1                      2006032401
2                   a20050112                2                     2006032402
3                   a20050113                3                     2006032403
4                   a20050114                4                     2006032404
NULL           NULL                          8                     2006032408
（所影響的行數為 5 行）
結果說(shuō)明:
        仔細觀(guān)察一下,就會(huì )發(fā)現,和left join的結果剛好相反,這次是以右表(B)為基礎的,A表不足的地方用NULL填充.

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