标量子查询与改写逻辑的一致性
当标量子查询中有聚合函数时,很多人在改写时,很喜欢把聚合函数写在主查询里,大概是认为这样简单。
SELECT *
FROM (SELECT * FORM t1 WHERE col4_1 = '0') a
LEFT JOIN (SELECT col3_1,
col3_2 coll,
col2,
SELECT DISTINCT col_1
FROM t2
WHERE col2_2 = a.col2
AND col2_3 <= to_date('2012-09-03','YYYY-MM-DD')
AND col2_4 >= to_date('2012-09-03','YYYY-MM-DD'))
FROM t3 a
WHERE col2_3 <= to_date('2012-09-03','YYYY-MM-DD')
AND col2_4 >= to_date('2012-09-03','YYYY-MM-DD'))b
ON a.col4 = b.col3_1;
网友在改写这个查询时,直接把DISTINCT 放在了主查询SELECT关键字后面
SELECT *
FROM (SELECT * FORM t1 WHERE col4_1 = '0') a
LEFT JOIN (SELECT DISTINCT a.col3_1,
a.col3_2 coll,
a.col2,
b.col2_1 AS col3
FROM t3 a
JOIN (SELECT *
FROM t2 b
WHERE b.col2_3 <= to_date('2012-09-03','YYYY-MM-DD')
AND b.col2_4 >= to_date('2012-09-03','YYYY-MM-DD')) b
ON a.col2 =b.col2_2
AND a.col2_3 <= to_date('2012-09-03','YYYY-MM-DD')
AND a.col2_4 >= to_date('2012-09-03','YYYY-MM-DD')) b
ON a.col4 = b.col3_1;
笔者不赞成这么改写,因为改写之前有很多重复数据 ,改写之后就没有了,可以看下面的等价例子
原查询可以表示为:
SELECT a.job,
a.dept,
(SELECT DISTINCT dname FROM dept2 b WHERE b.deptno = a.deptno) AS dename,
FROM emp a
ORDER BY 1,2,3;
按上面的方法更改:
SELECT DISTINCT a.job,
a.deptno,
b.dname
FROM emp a
LEFT JOIN dept2 b ON b.deptno = a.deptno;
除非确认主查询没有重复数据,否则不要这样改写,正确的改写如下:
SELECT a.job,
a.deptno,
b.dname
FROM emp a
LEFT JOIN (SELECT dname,deptno FROM dept2 GROUP BY dname,deptno) b
ON b.deptno = a.deptno;