Question

我有两个问题，每个问题都有自己的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

现在我如何使用UNION ALL组合它们？

Answer 1

如果您想保持相同的顺序，则以下内容通常有效：

(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，我不认为MySQL文档保证了两者的处理顺序。对于该保证，您需要外部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

Answer 2

您需要按以下方式对结果进行排序：

结果类型（MATCH或LIKE）
title（用于MATCH）或标签（用于LIKE）
NULL（用于MATCH）或问题（用于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助手列：

(
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

如何使用UNION组合两个具有ORDER BY的查询？

2 个答案: