SQL ORDER BY 带 CASE 带 UNION ALL

Question

运行 PostgreSQL(7.4 和 8.x)，我认为这是可行的，但现在我遇到了错误。

我可以单独运行查询，它工作得很好，但如果我使用 UNION 或 UNION ALL，它会抛出错误。

这个错误:(警告:pg_query():查询失败:错误:列“Field1”不存在......按情况排序“Field1”W ...)

SELECT "Field1" AS field_1, "Field2" AS field_2,
"Field3" AS field_3, "Field4" AS field_4
FROM "TableName" 
WHERE condition
AND other_condition
UNION ALL
SELECT "Field1" AS field_1, "Field2" AS field_2,
"Field3" AS field_3, "Field4" AS field_4
FROM "TableName" 
WHERE yet_another_condition
AND yet_another_other_condition
ORDER BY CASE "Field1"
    WHEN 'A' THEN 1
    WHEN 'B' THEN 2
    WHEN 'C' THEN 3
    ELSE 4
END

这个有效:

SELECT "Field1" AS field_1, "Field2" AS field_2,
"Field3" AS field_3, "Field4" AS field_4
FROM "TableName" 
WHERE yet_another_condition
AND yet_another_other_condition
ORDER BY CASE "Field1"
    WHEN 'A' THEN 1
    WHEN 'B' THEN 2
    WHEN 'C' THEN 3
    ELSE 4
END

这也行得通:

SELECT "Field1" AS field_1, "Field2" AS field_2,
"Field3" AS field_3, "Field4" AS field_4
FROM "TableName" 
WHERE condition
AND other_condition
ORDER BY CASE "Field1"
    WHEN 'A' THEN 1
    WHEN 'B' THEN 2
    WHEN 'C' THEN 3
    ELSE 4
END

如果我不使用 ORDER BY 而只使用 UNION 或 UNION ALL，它也能正常工作。

有什么想法吗？

Answer 1

将所有内容放在另一个 SELECT 中:

SELECT * FROM (
  SELECT "Field1" AS field_1, "Field2" AS field_2,
  "Field3" AS field_3, "Field4" AS field_4
  FROM "TableName" 
  WHERE condition
  AND other_condition
  UNION ALL
  SELECT "Field1" AS field_1, "Field2" AS field_2,
  "Field3" AS field_3, "Field4" AS field_4
  FROM "TableName" 
  WHERE yet_another_condition
  AND yet_another_other_condition
) As A
ORDER BY CASE field_1
    WHEN 'A' THEN 1
    WHEN 'B' THEN 2
    WHEN 'C' THEN 3
    ELSE 4
END

或者，更好的是，在 ORDER BY 中使用别名，因为它在 UNION 的末尾传递:

  SELECT "Field1" AS field_1, "Field2" AS field_2,
  "Field3" AS field_3, "Field4" AS field_4
  FROM "TableName" 
  WHERE condition
  AND other_condition
  UNION ALL
  SELECT "Field1" AS field_1, "Field2" AS field_2,
  "Field3" AS field_3, "Field4" AS field_4
  FROM "TableName" 
  WHERE yet_another_condition
  AND yet_another_other_condition
  ORDER BY CASE field_1
    WHEN 'A' THEN 1
    WHEN 'B' THEN 2
    WHEN 'C' THEN 3
    ELSE 4
  END