如何在 Postgres 中为联合查询自定义排序顺序

Question

使用这样的查询（为清楚起见进行了简化）：

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。类似于：

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;

但是，Postgres 抱怨：

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

将其包装在派生的 table 中（这就是“提示：....或将 UNION 移入 FROM 子句”的建议）

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

我会添加一个额外的列来显示所需的顺序，然后在 ORDER BY 中使用序数列位置，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并集中的每个集合无论如何都必须不同。

Answer 3

通过添加一个额外的列用于排序目的，但是它使 UNION 子句与 UNION ALL 完全一样工作（它不会从结果中消除重复的行）。

如何在 Postgres 中为联合查询自定义排序顺序

How to have a custom sort order for a union query in Postgres

postgresql

union

sql-order-by