Question

新手问题。我想做点什么：

SELECT c1,c2,c3 FROM TABLE t1
UNION
SELECT c1,c2,c3 FROM TABLE t2 WHERE t1.c1 IS NOT NULL AND t1.c2 IS NULL;

所以，如果我有t1：

c1|c2|c3
1 | a|v1
2 | b|v2

和t2：

c1|c2|c3
1 | a|v3
2 | b|v4
2 | c|v5
3 | d|v6

我会得到：

c1|c2|c3
1 | a|v1
2 | b|v2
2 | c|v5

有人知道怎么做吗？

Answer 1

查询应该类似于下面的内容。但是，我不确定它的效率如何。

SELECT 
    T1.c1,
    T1.c2,
    T1.c3
FROM T1
UNION     
SELECT 
    T2.c1,
    T2.c2,
    T2.c3
FROM T2 
WHERE ((T2.c1,T2.c2) NOT IN (SELECT t1.c1,t1.c2 FROM t1)) AND 
(T2.c1 IN (SELECT t1.c1 FROM t1))

Answer 2

根据SQL文档，JOIN比SubQueries更快。试试这个：

 SELECT c1, c2, c3
 FROM   t1
 UNION
 SELECT b.c1, b.c2, b.c3
 FROM   t1 a inner join t2 b ON
         (a.c1 = b.c1) and (a.c1 = b.c1)
 WHERE  (b.c1,b.c2) NOT IN (SELECT c1,c2 FROM t1)

要证明，请参见此处：http://sqlfiddle.com/#!2/50a4e/23

Answer 3

select t2.c1,t2.c2,t2.c3 
from t1,t2 
where t1.c1=t2.c2;

选择匹配的列

3 个答案: