Question

我有以下查询，它将两个较小的查询合并：

flat_list=[item for sublist in l foo(item) for item in sublist]

如果查询返回一行，而查询2返回一行，那么我将有两行是这样的：

SELECT 'Static 1' type, somefield, some_id   
      FROM (SELECT some_id, somefield
               FROM sometable
                WHERE someothercolumn = 'Static 1'
                and someid = :id)
     UNION ALL
     SELECT 'Static 1' type, somefield, some_id   
      FROM (SELECT some_id, somefield
               FROM sometable
                WHERE someothercolumn = 'Static 2'
                and someid = :id)

我想要哪个输出。如果这两个查询之一不返回行（或者即使两个都不返回行），那么我将返回零行。这不是我想要的。我一直希望'type'和'some_id'返回如果没有数据。因此，如果这两个查询都没有任何数据，我的预期结果将是：

type      |      somefield     |     some_id
---------------------------------------------
Static 1  |       somevalue    |      1
Static 2  |       somevalue    |      1

Answer 1

一种选择是选择带有NOT EXISTS的 header dummy 行，以确定是否返回它（是，如果表中的实际行不存在；否，如果行存在）。

看看示例：请看第4行，该行当前被注释，这意味着您的表不包含满足该条件的行，因此将显示一个虚拟的 header 行：

SQL> with test (type, some_id) as
  2    (select 'Static 99', 44 from dual union all
  3     select 'Static 2' , 57 from dual
  4     --union all select 'Static 1', 66 from dual   -- toggle to see the difference
  5    )
  6  -- This is a header which won't be displayed if table contains
  7  -- type = 'Static 1' and some_id = 66
  8  select 'Static 1' type, 1 some_id
  9  from dual
 10  where not exists (select null
 11                    from test
 12                    where type = 'Static 1'
 13                      and some_id = 66
 14                   )
 15  union all
 16  -- This returns actual rows (if they exist)
 17  select 'Static 1' type, some_id
 18  from (select some_id
 19        from test
 20        where type = 'Static 1'
 21          and some_id = 66
 22       );

TYPE        SOME_ID
-------- ----------
Static 1          1

SQL>

但是，如果存在（未注释第4行），那么将显示实际数据，而没有该虚拟标题行：

SQL> with test (type, some_id) as
  2    (select 'Static 99', 44 from dual union all
  3     select 'Static 2' , 57 from dual
  4     union all select 'Static 1', 66 from dual   -- toggle to see the difference
  5    )
  6  -- This is a header which won't be displayed if table contains
  7  -- type = 'Static 1' and some_id = 66
  8  select 'Static 1' type, 1 some_id
  9  from dual
 10  where not exists (select null
 11                    from test
 12                    where type = 'Static 1'
 13                      and some_id = 66
 14                   )
 15  union all
 16  -- This returns actual rows (if they exist)
 17  select 'Static 1' type, some_id
 18  from (select some_id
 19        from test
 20        where type = 'Static 1'
 21          and some_id = 66
 22       );

TYPE        SOME_ID
-------- ----------
Static 1         66

SQL>

如果这样做可以满足您的要求，则将相同的原理应用于您正在使用的第二个select（至Static 2）。

Answer 2

也许是这样的：

WITH a AS (/* subquery 1 */),
     b AS (/* subquery 2 */)
SELECT (SELECT a.col1 FROM a),
       (SELECT a.col2 FROM a),
       ...
       FROM dual
UNION ALL
SELECT (SELECT b.col1 FROM b),
       (SELECT b.col2 FROM b),
       ...
       FROM dual

Answer 3

为您的查询创建一个CTE，为另外2行添加另一个CTE，并使用UNION ALL并且不存在，如下所示：

WITH 
  cte1 AS (
    <your query here>
  ),
  cte2 AS (
    SELECT 'Static 1' type, null somefield, :id FROM dual
    UNION ALL
    SELECT 'Static 2' type, null somefield, :id FROM dual 
  )
SELECT * FROM cte1
UNION ALL
SELECT * FROM cte2
WHERE NOT EXISTS (SELECT 1 FROM cte1)

Oracle SQL-UNION ALL返回行，即使它们不存在

3 个答案: