sql 內鏈接 左右連接

inner join(等值鏈接) 只返回兩個表中聯結字段相等的行

left join(左聯接) 返回包括左表中的全部記錄和右表中聯結字段相等的記錄

right join(右聯接) 返回包括右表中的全部記錄和左表中聯結字段相等的記錄

2.操做實例

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

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

實驗以下:
1.left join

sql語句以下:
select * from A
left join B
on A.aID = B.bID

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

結果說明:
               left join是以A表的記錄爲基礎的,A能夠當作左表,B能夠當作右表,left join是以左表爲準的.
換句話說,左表(A)的記錄將會所有表示出來,而右表(B)只會顯示符合搜索條件的記錄(例子中爲: A.aID = B.bID).
B表記錄不足的地方均爲NULL.

2.right join
sql語句以下:
select * from A
right join B
on A.aID = B.bID
結果以下:
aID               aNum                          bID                  bName
1                   a20050111                1                      2006032401
2                   a20050112                2                     2006032402
3                   a20050113                3                     2006032403
4                   a20050114                4                     2006032404
NULL           NULL                          8                     2006032408
（所影響的行數爲 5 行）
結果說明:
        仔細觀察一下,就會發現,和left join的結果恰好相反,此次是以右表(B)爲基礎的,A表不足的地方用NULL填充.

3.inner join
sql語句以下:
select * from A
innerjoin B
on A.aID = B.bID

結果以下:aID               aNum                          bID                  bName1                   a20050111                1                      20060324012                   a20050112                2                     20060324023                   a20050113                3                     20060324034                   a20050114                4                     2006032404