如何使用 UNION 组合两个具有 ORDER BY 的查询？

Question

我有两个查询，它们每个都有自己的order by，如下所示：

查询1：

SELECT id, name, title, content 
FROM table where match(title, content) against('anything') 
Order By title

查询1：

SELECT id, tag, question, answer 
FROM table 
Where tag like '%anything' 
Order By tag, question

现在如何使用UNION ALL 组合它们？

Answer 1

您需要对结果进行排序：

1. 结果类型（匹配或喜欢）
2. 标题（MATCH）或标签（LIKE）
3. NULL（对于 MATCH）或 question（对于 LIKE）

您可以使用嵌套查询：

SELECT * FROM (
    SELECT 1 AS result_type, id, name, title, content 
    FROM table
    WHERE MATCH (title, content) AGAINST ('anything') 
    UNION ALL
    SELECT 2, id, tag, question, answer 
    FROM table 
    WHERE tag LIKE '%anything' 
) AS foobar
ORDER BY
    result_type,
    CASE result_type WHEN 1 THEN title ELSE tag END,
    CASE result_type WHEN 1 THEN NULL ELSE question END

或者您可以添加排序辅助列：

(
SELECT 1 AS sort_1, title AS sort_2, NULL     AS sort_3, id, name, title, content 
FROM table
WHERE MATCH (title, content) AGAINST ('anything') 
) UNION ALL (
SELECT 2 AS sort_1, tag   AS sort_2, question AS sort_3, id, tag, question, answer 
FROM table 
WHERE tag LIKE '%anything' 
)
ORDER BY sort_1, sort_2, sort_3

Answer 2

如果您想保持相同的顺序，那么以下通常有效：

(SELECT id, name, title, content
 FROM table
 where match(title, content) against('anything')
 order by title
) union all
(SELECT id, tag, question, answer
 FROM table
 where tag like '%anything'
 order by tag, question
);

这在实践中有效，因为实际上第一个子查询在第二个子查询之前执行。但是，我认为 MySQL 文档并不能保证两者的处理顺序。为了保证，你需要一个外部的order by：

(SELECT id, name, title, content, 1 as priority
 FROM table
 where match(title, content) against('anything')
) union all
(SELECT id, tag, question, answer, 2 as prioirty
 FROM table
 where tag like '%anything'
)
ORDER BY priority, title, content