Question

使用这样的查询（为简洁起见而简化）：

SELECT 'East' AS name, *
FROM events 
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00' 

UNION

SELECT 'West' AS name, *
FROM events 
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00'

UNION

SELECT 'Both' AS name, *
FROM events 
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00'

我想自定义结果行的顺序。类似的东西：

ORDER BY name='East', name='West', name='Both'

或者

ORDER BY 
    CASE
        WHEN name='East' THEN 1 
        WHEN name='West' THEN 2
        WHEN name='Both' THEN 3
        ELSE 4
    END;

然而，Postgres抱怨说：

ERROR:  invalid UNION/INTERSECT/EXCEPT ORDER BY clause
DETAIL:  Only result column names can be used, not expressions or functions.
HINT:  Add the expression/function to every SELECT, or move the UNION into a FROM clause.

我有其他选择吗？

Answer 1

将其包装在派生表中（这是“ HINT：......或将UNION移动到FROM子句中”建议）

select *
from (
  ... your union goes here ... 
) t
order by
    CASE
        WHEN name='East' THEN 1 
        WHEN name='West' THEN 2
        WHEN name='Both' THEN 3
        ELSE 4
    END;

Answer 2

我添加了一个显示所需排序的额外列，然后使用ORDER BY中的序号列位置，例如

SELECT 1, 'East' AS name, *
...
UNION ALL
SELECT 2, 'West' AS name, *
...
ORDER BY 1

请注意，您可能还需要UNION ALL，因为您添加的列确保联合中的每个集合都必须是不同的。

Answer 3

通过添加额外的列来进行排序，但是它使UNION子句完全像UNION ALL一样工作（它不会从结果中消除重复的行）。

如何在Postgres中为联合查询设置自定义排序顺序

3 个答案: