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：

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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。