oracle的left join，right join和full join的一點介紹(R1)
以下是摘自oracle ocp9i文件：
outer join syntax:
1)you use an outer join to also see rows that do not meet the join condition
2)the outer join operator is the plus sign(+)
outer join restrictions:
1)the outer join operator can appear on only one side of the expression:the side that has information missing.it returns those rows from one table that have no direct match in the other table.
2)a condition involving an outer join cannot use the IN operator or be linked to another condition by the OR operator.
配置實驗環境：
hr@ORCL> drop table a;
hr@ORCL> drop table b;
hr@ORCL> create table a(id number,name varchar2(10));
hr@ORCL> create table b(id number,name varchar2(10));
hr@ORCL> insert into a values(1,'a');
hr@ORCL> insert into a values(2,'b');
hr@ORCL> insert into a values(3,'c');
hr@ORCL> insert into b values(1,'a');
hr@ORCL> insert into b values(2,'b');
hr@ORCL> insert into b values(4,'d');
hr@ORCL> select * from a;
ID NAME
---------- ----------
1 a
2 b
3 c
hr@ORCL> select * from b;
ID NAME
---------- ----------
1 a
2 b
4 d
hr@ORCL> commit;
--全外連線的結果是：
1）選出所有滿足條件的結果
2）以左表為準，將左表不滿足條件的結果接在左邊
3）以右表為準，把右表不滿足條件的結果接在右邊
4）將以上結果全部合起來
hr@ORCL> select a.id,b.id from a full join b on a.id=b.id;
ID ID
---------- ----------
1 1
2 2
3
4
--left out join和oracle的加號在右結果是相同。同理，right out join和加號在左是一樣的。（sql99的語法和oracle私有語法的比較）
hr@ORCL> select a.id,b.id from a,b where a.id=b.id(+);
ID ID
---------- ----------
1 1
2 2
3
hr@ORCL> select a.id,b.id from a left outer join b on a.id=b.id;
ID ID
---------- ----------
1 1
2 2
3
--結果類似於from a left join b on a.col=b.col and a.coln=....。單個列選擇條件的列是基表（加號在誰身上誰是從表，沒有加號的一方是基表）的用decode和+改寫，不能用一般的改寫，若不是基表的可以簡單用+改寫。比如：
SQL> SELECT a.ID,b.ID
2 FROM a FULL JOIN b
3 ON a.ID=b.ID AND a.NAME='a';
ID ID
---------- ----------
1 1
2
3
1 1
4
2
改寫之：
SQL> SELECT a.ID,b.ID
2 FROM a,b WHERE a.id=decode(a.NAME,'a',b.ID(+))
3 UNION ALL
4 SELECT a.ID,b.ID
5 FROM a,b WHERE a.ID(+)=b.ID AND a.NAME(+)='a' AND a.ID IS NULL;
ID ID
---------- ----------
1 1
2
3
1 1
2
4
用union來實現上面例子的full join結果，需要考慮表的關係。
1）如果是兩表1:1
加號在左，以右為準（相當於sql99的right join）
加號在右，以左為準（相當於sql99的left join）
hr@ORCL> select a.id,b.id from a,b where a.id=b.id(+)
2 union
3 select a.id,b.id from a,b where a.id(+)=b.id;
ID ID
---------- ----------
1 1
2 2
3
4
2）如果是兩表1:n，用union剔重就不正確。
hr@ORCL> insert into a values(1,'a');
hr@ORCL> commit;
hr@ORCL> select * from a;
ID NAME
---------- ----------
1 a
2 b
3 c
1 a
hr@ORCL> select * from b;
ID NAME
---------- ----------
1 a
2 b
4 d
--1:n用UNION不正確
SQL> SELECT a.ID,b.ID
2 FROM a,b WHERE a.ID=b.ID(+)
3 UNION
4 SELECT a.ID,b.ID
5 FROM a,b WHERE a.ID(+)=b.ID;
ID ID
---------- ----------
1 1
2 2
3
4
--正確解法有三。注意，在使用sql時，任何時候，任何地方，一定要考慮null！！！切記。
法一：
SQL> SELECT a.ID,b.ID
2 FROM a,b WHERE a.ID=b.ID(+)
3 UNION ALL
4 SELECT a.ID,b.ID
5 FROM a,b WHERE a.ID(+)=b.ID AND a.ID IS NULL;
ID ID
---------- ----------
1 1
1 1
2 2
3
4
法二：
SQL> SELECT a.ID,b.ID
2 FROM a,b WHERE a.ID=b.ID(+) AND b.ID IS NULL
3 UNION ALL
4 SELECT a.ID,b.ID
5 FROM a,b WHERE a.ID(+)=b.ID;
ID ID
---------- ----------
3
1 1
1 1
2 2
4
法三：
SQL> SELECT a.ID,b.ID
2 FROM a FULL JOIN b
3 ON a.ID=b.ID;
ID ID
---------- ----------
1 1
1 1
2 2
3
4
--逗號和full join是不一樣的。另外，full join須加上關鍵字on，才是完整的語句。
hr@ORCL> select p.id,t.id from p,t where p.id=t.id;
ID ID
---------- ----------
1 1
3 3
hr@ORCL> select p.id,t.id from p full join t on p.id=t.id;
ID ID
---------- ----------
1 1
3 3
2
5
全外連線和union連線都可以實現相同結果。我們來看一下他們的執行計劃。
全外連線的執行計劃：
hr@ORCL> select a.id,b.id from a full join b on a.id=b.id;
ID ID
---------- ----------
1 1
2 2
3
4
Execution Plan
----------------------
Plan hash value: 2192011130
-----------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------
| 0 | SELECT STATEMENT | | 4 | 104 | 13 (8)| 00:00:01 |
| 1 | VIEW | | 4 | 104 | 13 (8)| 00:00:01 |
| 2 | UNION-ALL | | | | | |
|* 3 | HASH JOIN OUTER | | 3 | 312 | 7 (15)| 00:00:01 |
| 4 | TABLE ACCESS FULL| A | 3 | 195 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL| B | 3 | 117 | 3 (0)| 00:00:01 |
|* 6 | HASH JOIN ANTI | | 1 | 26 | 7 (15)| 00:00:01 |
| 7 | TABLE ACCESS FULL| B | 3 | 39 | 3 (0)| 00:00:01 |
| 8 | TABLE ACCESS FULL| A | 3 | 39 | 3 (0)| 00:00:01 |
-----------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("A"."ID"="B"."ID"(+))
6 - access("A"."ID"="B"."ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------
0 recursive calls
0 db block gets
29 consistent gets
0 physical reads
0 redo size
520 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
union的執行計劃：
hr@ORCL> select a.id,b.id from a,b where a.id=b.id(+)
2 union
3 select a.id,b.id from a,b where a.id(+)=b.id;
ID ID
---------- ----------
1 1
2 2
3
4
Execution Plan
----------------------
Plan hash value: 891669117
-----------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------
| 0 | SELECT STATEMENT | | 6 | 156 | 15 (60)| 00:00:01 |
| 1 | SORT UNIQUE | | 6 | 156 | 15 (60)| 00:00:01 |
| 2 | UNION-ALL | | | | | |
|* 3 | HASH JOIN OUTER | | 3 | 78 | 7 (15)| 00:00:01 |
| 4 | TABLE ACCESS FULL| A | 3 | 39 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL| B | 3 | 39 | 3 (0)| 00:00:01 |
|* 6 | HASH JOIN OUTER | | 3 | 78 | 7 (15)| 00:00:01 |
| 7 | TABLE ACCESS FULL| B | 3 | 39 | 3 (0)| 00:00:01 |
| 8 | TABLE ACCESS FULL| A | 3 | 39 | 3 (0)| 00:00:01 |
-----------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("A"."ID"="B"."ID"(+))
6 - access("A"."ID"(+)="B"."ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------
0 recursive calls
0 db block gets
28 consistent gets
0 physical reads
0 redo size
520 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk)
4 rows processed
顯然，union連線的cpu代價要比full join連線來得多。此外，union還會暗含一個排序操作。當資料量海量時，估計會對效能帶來一定的影響。而且，在oracle的私有語法裡，是沒有全外連線的，只能透過union連線來模擬full join。所以，建議需要使用外連線時，請使用full join，不要用union模擬。
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/26613085/viewspace-1104739/，如需轉載，請註明出處，否則將追究法律責任。
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