如何将2个select语句合并为一个？

Question

当谈到SQL语法时，我是一个noob。

我有一个表，有很多场的行和列：P 可以说，它看起来像这样：

 AAA BBB CCC DDD 
----------------------- 
Row1 | 1 A D X 
Row2 | 2 B C X 
Row3 | 3 C D Z 

现在我想建立一个先进的select语句，让我这个组合（伪SQLish这里）：

select 'Test1', * from TABLE Where CCC='D' AND DDD='X' 
select 'Test2', * from TABLE Where CCC<>'D' AND DDD='X' 

输出将是：

Test1, 1, A, D, X 
Test2, 2, B, C, X 

如何将这两个select语句合并成一个很好的select语句？

它会工作，如果我像下面复杂的SQL（因为我自己的SQL语句包含一个存在的声明）？我只想知道如何组合选择，然后尝试将其应用于我的更高级的SQL。

select 'Test1', * from TABLE Where CCC='D' AND DDD='X' AND exists(select ...) 
select 'Test2', * from TABLE Where CCC<>'D' AND DDD='X' AND exists(select ...) 

我真正的SQL语句是这个：

select Status, * from WorkItems t1 
where exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 

，给了我一个结果。但是我想把它和这个select语句的一个副本结合起来，并且'Status'字段会被一个字符串'DELETED'改变。

select 'DELETED', * from WorkItems t1 
where exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 
AND NOT (BoolField05=1) 

Answer 1

在这里你有两个选择。首先是有两个结果集，其将根据WHERE子句中的条件中设置“测试1”或“Test2的”，然后UNION在一起：

select 
    'Test1', * 
from 
    TABLE 
Where 
    CCC='D' AND DDD='X' AND exists(select ...) 
UNION 
select 
    'Test2', * 
from 
    TABLE 
Where 
    CCC<>'D' AND DDD='X' AND exists(select ...) 

这可能是一个问题，因为你会在TABLE上有效地扫描/查找两次。

其他的解决办法是从表中一次基于表的条件来选择，并设置“测试1”或“Test2的”：

select 
    case 
     when CCC='D' AND DDD='X' AND exists(select ...) then 'Test1' 
     when CCC<>'D' AND DDD='X' AND exists(select ...) then 'Test2' 
    end, 
    * 
from 
    TABLE 
Where 
    (CCC='D' AND DDD='X' AND exists(select ...)) or 
    (CCC<>'D' AND DDD='X' AND exists(select ...)) 

这里的渔获是，你将不得不重复CASE声明中的筛选条件和WHERE声明。

Answer 2

如果他们是来自同一个表，我想UNION是你要找的命令。

（如果你会永远需要选择不同的表列的值，你应该看看JOIN，而不是...）

Answer 3

select Status, * from WorkItems t1 
where exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 

UNION 

select 'DELETED', * from WorkItems t1 
where exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 
AND NOT (BoolField05=1) 

或许会做的伎俩。我无法从这里测试它，但我不确定你正在使用哪种版本的SQL。

Answer 4

The Union command is what you need.如果不工作，你可能需要改进你在什么样的环境。

Answer 5

使用的情况下，进入选择，并在那里接近A或

像这样使用，我没有测试，但它应该工作，我想......

select case when CCC='D' then 'test1' else 'test2' end, * 
from table 
where (CCC='D' AND DDD='X') or (CCC<>'D' AND DDD='X') 

Answer 6

我以为这就是你要找的内容：

SELECT CASE WHEN BoolField05 = 1 THEN Status ELSE 'DELETED' END AS MyStatus, t1.* 
FROM WorkItems t1 
WHERE (TextField01, TimeStamp) IN(
    SELECT TextField01, MAX(TimeStamp) 
    FROM WorkItems t2 
    GROUP BY t2.TextField01 
) 
AND TimeStamp > '2009-02-12 18:00:00' 

如果你在Oracle或在MS SQL 2005及以上版本，那么你可以做：

SELECT * 
FROM (
    SELECT CASE WHEN BoolField05 = 1 THEN Status ELSE 'DELETED' END AS MyStatus, t1.*, 
    ROW_NUMBER() OVER (PARTITION BY TextField01 ORDER BY TimeStamp DESC) AS rn 
    FROM WorkItems t1 
) to 
WHERE rn = 1 

，它更有效率。

Answer 7

感谢您的意见。尝试已经在这里提到的东西，这些是2我开始工作：

(
select 'OK', * from WorkItems t1 
where exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 
AND (BoolField05=1) 
) 
UNION 
(
select 'DEL', * from WorkItems t1 
where exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00' 
AND NOT (BoolField05=1) 
) 

和

select 
    case 
     when 
      (BoolField05=1) 
    then 'OK' 
    else 'DEL' 
     end, 
     * 
from WorkItems t1 
Where 
      exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1)) 
      AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
      AND TimeStamp>'2009-02-12 18:00:00' 

这将是最有效的，这些（编辑：第二，因为它只能扫描表一次），，有可能使它更有效率？ （BoolField = 1）实际上是一个变量（dyn sql），可以在表中包含任何where语句。

我在MS SQL 2005上运行。试过Quassnoi的例子，但没有按预期工作。

Answer 8

select t1.* from 
(select * from TABLE Where CCC='D' AND DDD='X') as t1, 
(select * from TABLE Where CCC<>'D' AND DDD='X') as t2 

另一种方法可以做到这一点！