Oracle标量子查询
Oracle标量子查询
ORACLE允许在select子句中包含单行子查询, 使用标量子查询可以有效的改善性能，当使用到外部连接，或者使用到了聚合函数，就可以考虑标量子查询的可能性。
下面来看几个例子,来了解标量子查询的用法：
1.生成模拟数据
CREATE TABLE TEST1(ID INT);
CREATE TABLE TEST2(ID INT);
INSERT INTO TEST1 VALUES(1);
INSERT INTO TEST1 VALUES(2);
INSERT INTO TEST1 VALUES(3);
INSERT INTO TEST2 VALUES(1);
INSERT INTO TEST2 VALUES(2);
COMMIT;
admin@ORCL> SELECT ID,(SELECT ID FROM TEST2 WHERE TEST1.ID = TEST2.ID) ID2 FROM TEST1;
ID ID2
---------- ----------
1 1
2 2
3
从上面的结果可以看出标量子查询的表相当于是外连接中的内表，数据结果等价于下面的sql：
admin@ORCL> SELECT * FROM TEST1,TEST2 WHERE TEST1.ID = TEST2.ID(+);
ID ID
---------- ----------
1 1
2 2
3
2.
admin@ORCL> SELECT A.USERNAME,
2 (SELECT COUNT(*) FROM ALL_OBJECTS B WHERE A.USERNAME = B.OWNER) AS OBJ_CNT
3 FROM ALL_USERS A
4 WHERE A.USERNAME IN ('DIP', 'BI', 'HR');
USERNAME OBJ_CNT
------------------------------ ----------
BI 8
DIP 0
HR 35
查询结果等价于
admin@ORCL> SELECT A.USERNAME, COUNT(B.OBJECT_ID)
2 FROM ALL_USERS A, ALL_OBJECTS B
3 WHERE A.USERNAME = B.OWNER(+)
4 AND A.USERNAME IN ('DIP', 'BI', 'HR')
5 GROUP BY A.USERNAME;
USERNAME COUNT(B.OBJECT_ID)
------------------------------ ------------------
DIP 0
HR 35
BI 8
PS:
若把COUNT(B.OBJECT_ID)换成" * " 的话，DIP用户的COUNT值为1，这对于实际结果是不对的。
