在使用implict update table 時,發現其中一個表一定要有唯一約束,否則會報錯!但是oracle可以使用hints:/*+ BYPASS_UJVC*/ 屏蔽掉隊唯一性的檢查。具體測試過程如下:
SQL> CREATE TABLE test_a(
2 id NUMBER ,
3 score NUMBER );
Table created.
Elapsed: 00:00:00.10
SQL>
CREATE TABLE test_b(
2 id NUMBER);
Table created.
Elapsed: 00:00:00.01
SQL>
SQL> INSERT INTO test_a VALUES(1,100);
INSERT INTO test_a VALUES(2,200);
1 row created.
Elapsed: 00:00:00.01
SQL>
INSERT INTO test_a VALUES(3,300);
1 row created.
Elapsed: 00:00:00.00
SQL>
1 row created.
Elapsed: 00:00:00.00
SQL> INSERT INTO test_b VALUES(1);
1 row created.
Elapsed: 00:00:00.01
SQL> INSERT INTO test_b VALUES(2);
1 row created.
Elapsed: 00:00:00.00
SQL> INSERT INTO test_b VALUES(3);
1 row created.
Elapsed: 00:00:00.01
SQL> INSERT INTO test_b VALUES(4);
1 row created.
Elapsed: 00:00:00.01
SQL>
SQL> SELECT * FROM test_a ORDER BY 1;
SELECT * FROM test_b ORDER BY 1;
ID SCORE
---------- ----------
1 100
2 200
3 300
3 rows selected.
Elapsed: 00:00:00.01
SQL>
ID
----------
1
2
3
4
4 rows selected.
Elapsed: 00:00:00.01
現在更新a表的字段:(先不考慮SQL的功能,只是測試這種方法)
SQL> UPDATE (
2 SELECT a.id,a.score,b.id AS b_id
3 FROM test_a a,
4 test_b b
5 WHERE a.id=b.id
6 )
7 SET id=b_id;
SET id=b_id
*
ERROR at line 7:
ORA-01779: cannot modify a column which maps to a non key-preserved table
報錯了,需要對b表加唯一索引。加入hints執行
SQL> UPDATE (
2 SELECT/*+ BYPASS_UJVC*/
a.id,a.score,b.id AS b_id
3 FROM test_a a,
4 test_b b
5 WHERE a.id=b.id
6 )
7 SET id=b_id;
3 rows updated.
Elapsed: 00:00:00.01
說明:oracle可以跳過檢查唯一約束。
繼續往b表添加一條記錄,使b表的記錄不唯一。
SQL> INSERT INTO test_b VALUES(3);
1 row created.
Elapsed: 00:00:00.00
SQL> COMMIT;
Commit complete.
Elapsed: 00:00:00.00
SQL> SELECT * FROM TEST_B ORDER BY 1;
ID
----------
1
2
3
3
4
5 rows selected.
Elapsed: 00:00:00.00
SQL> UPDATE (
2 SELECT/*+ BYPASS_UJVC*/ a.id,a.score,b.id AS b_id
3 FROM test_a a,
4 test_b b
5 WHERE a.id=b.id
6 )
7 SET id=b_id;
4 rows updated.
Elapsed: 00:00:00.01
SQL> SELECT * FROM TEST_A;
ID SCORE
---------- ----------
1 100
2 200
3 300
3 rows selected.
Elapsed: 00:00:00.00
總結:使用了該hints,oracle就完全放棄了檢查唯一性。因此SQL可以執行通過。
但要注意,是否能達到我們希望的目的,例如:下面進行一個有意義的更新,將a表中只要id在b表中出現就更新score,將score加1.
執行SQL如下:
SQL> UPDATE (
2 SELECT/*+ BYPASS_UJVC*/ a.id,a.score,b.id AS b_id
3 FROM test_a a,
4 test_b b
5 WHERE a.id=b.id
6 )
7 SET score=score+1;
4 rows updated.
Elapsed: 00:00:00.00
SQL> SELECT * FROM TEST_A;
ID SCORE
---------- ----------
1 101
2 201
3 302
3 rows selected.
Elapsed: 00:00:00.01
發現對於id=3的記錄多加了個1。因此並沒有得到我們想要的結果。這種情況應該先對b表的id進行去重。然後再更新。(其實完全也可以換種寫法,呵呵) ,在此就不寫了
總結:雖然對於此種應用,oracle跳過了檢查唯一性,但是我們要注意在具體使用時,是否真的就達到了我們的效果!
要巧用並且活用現在已經有的功能。