JOIN對於接觸過數據庫的人,這個詞都不陌生,而且很多人很清楚各種JOIN,還有很多人對這個理解也不是很透徹。
假設我們有兩個表,Table_A和Table_B。這兩個表中的數據如下所示:
TABLE_A
PK Value
---- ----------
1 FOX
2 COP
3 TAXI
6 WASHINGTON
7 DELL
5 ARIZONA
4 LINCOLN
10 LUCENT
TABLE_B
PK Value
---- ----------
1 TROT
2 CAR
3 CAB
6 MONUMENT
7 PC
8 MICROSOFT
9 APPLE
11 SCOTCH
Join 語法:
join_table:
table_reference JOIN table_factor [join_condition] //內連接
| table_reference {LEFT|RIGHT|FULL} [OUTER] JOIN table_reference join_condition //外連接
| table_reference LEFT SEMI JOIN table_reference join_condition //左半連接
| table_reference CROSS JOIN table_reference [join_condition] (as of Hive 0.10)
table_reference:
table_factor //表
| join_table //join語句
table_factor:
tbl_name [alias] //表名[別名]
| table_subquery alias //子查尋[別名]
| ( table_references ) //帶空號的table_reference
join_condition:
ON expression //on開頭的條件語句
1、Inner JOIN: (內連接)
這是最簡單、最容易理解的連接,也是最常見的連接。此查詢將返回左表(表A)中具有右表(表B)中匹配記錄的所有記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
INNER JOIN Table_B B
ON A.Key = B.Key
-- Inner JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
INNER JOIN Table_B B
ON A.PK = B.PK
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
1 FOX TROT 1
2 COP CAR 2
3 TAXI CAB 3
6 WASHINGTON MONUMENT 6
7 DELL PC 7
(5 row(s) affected)
2、Left JOIN: (左連接)
此查詢將返回左表(表A)中的所有記錄,而不管這些記錄是否與右表(表B)中的任何記錄匹配。它還將從正確的表中返回任何匹配的記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
-- Left JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
1 FOX TROT 1
2 COP CAR 2
3 TAXI CAB 3
4 LINCOLN NULL NULL
5 ARIZONA NULL NULL
6 WASHINGTON MONUMENT 6
7 DELL PC 7
10 LUCENT NULL NULL
(8 row(s) affected)
3、Left Excluding JOIN: (左連接排除內連接結果)
此查詢將返回左表(表A)中與右表(表B)中的任何記錄都不匹配的所有記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key IS NULL
-- Left Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK
WHERE B.PK IS NULL
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
4 LINCOLN NULL NULL
5 ARIZONA NULL NULL
10 LUCENT NULL NULL
(3 row(s) affected)
4、Right JOIN: (右連接)
此查詢將返回右表(表B)中的所有記錄,而不管這些記錄中是否有任何記錄與左表(表A)中的記錄相匹配。它還將返回左表中的任何匹配記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
-- Right JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
1 FOX TROT 1
2 COP CAR 2
3 TAXI CAB 3
6 WASHINGTON MONUMENT 6
7 DELL PC 7
NULL NULL MICROSOFT 8
NULL NULL APPLE 9
NULL NULL SCOTCH 11
(8 row(s) affected)
5、Right Excluding JOIN: (右連接排除內連接結果)
此查詢將返回右表(表B)中與左表(表A)中的任何記錄都不匹配的所有記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL
-- Right Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
NULL NULL MICROSOFT 8
NULL NULL APPLE 9
NULL NULL SCOTCH 11
(3 row(s) affected)
6、Outer JOIN: (外連接)
此聯接也可以稱爲完全外聯接或完全聯接。此查詢將返回兩個表中的所有記錄,連接左表(表A)中與右表(表B)中的記錄相匹配的記錄。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
-- Outer JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
1 FOX TROT 1
2 COP CAR 2
3 TAXI CAB 3
6 WASHINGTON MONUMENT 6
7 DELL PC 7
NULL NULL MICROSOFT 8
NULL NULL APPLE 9
NULL NULL SCOTCH 11
5 ARIZONA NULL NULL
4 LINCOLN NULL NULL
10 LUCENT NULL NULL
(11 row(s) affected)
7、Outer Excluding JOIN: (外連接排除內連接結果)
此查詢將返回左表(表A)中的所有記錄和右表(表B)中不匹配的所有記錄。我還不需要使用這種類型的聯接,但所有其他類型的聯接我都相當頻繁地使用。此連接寫成如下:
SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL OR B.Key IS NULL
-- Outer Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
OR B.PK IS NULL
A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
NULL NULL MICROSOFT 8
NULL NULL APPLE 9
NULL NULL SCOTCH 11
5 ARIZONA NULL NULL
4 LINCOLN NULL NULL
10 LUCENT NULL NULL
(6 row(s) affected)
注意,在外部聯接上,首先返回內部連接記錄,然後返回右連接記錄,最後返回左連接記錄(至少,我的Microsoft SQL Server就是這樣做的;當然,這不需要使用任何ORDERBY語句)。您可以訪問維基百科文章以獲得更多信息(但是,條目不是圖形化的)。我還創建了一個備忘單,您可以在需要時打印出來。如果您右鍵單擊下面的圖像並選擇“將目標保存爲.”,您將下載完整大小的圖像。