如何组合两个不同的mysql where子句每个结果为5行答案

【问题标题】：How to combine two different mysql where clauses each results in 5 rows如何组合两个不同的mysql where子句每个结果为5行
【发布时间】：2023-03-29 23:22:01
【问题描述】：

我使用以下两个查询从每个查询中获取 5 行并显示在一个列表中。有没有可能组合两个查询的方法，这样我就不必单独运行它们。但是仍然得到相同的结果，首先是第一个 where 子句的结果，然后是第二个 where 子句的结果。

    SELECT *
    FROM
    institutes
    LEFT JOIN city ON institutes.city_id = city.city_id 
    LEFT JOIN district ON city.district_id = district.district_id 
    WHERE
    city.city_id = $current_city_id ORDER BY RAND() DESC LIMIT 5


    SELECT *
    FROM
    institutes
    LEFT JOIN city ON institutes.city_id = city.city_id 
    LEFT JOIN district ON city.district_id = district.district_id 
    WHERE
    district.district_id = $current_district_id ORDER BY RAND() DESC LIMIT 5

【问题讨论】：

为什么需要 2query？

标签： mysql select left-join union where-clause

【解决方案1】：

将UNION ALL 与包装器SELECTs 一起使用：

SELECT * FROM (
    SELECT *
    FROM institutes
    LEFT JOIN city ON institutes.city_id = city.city_id 
    LEFT JOIN district ON city.district_id = district.district_id 
    WHERE city.city_id = $current_city_id
    ORDER BY RAND() DESC LIMIT 5
) x1
UNION ALL
SELECT * FROM (
    SELECT *
    FROM institutes
    LEFT JOIN city ON institutes.city_id = city.city_id 
    LEFT JOIN district ON city.district_id = district.district_id 
    WHERE district.district_id = $current_district_id
    ORDER BY RAND() DESC LIMIT 5 
) x2

需要包装器SELECTs，因为您不能将ORDER BY ... LIMIT 和UNION 组合在同一级别，但您可以将ORDER BY ... LIMIT 放入子查询。

需要UNION ALL 而不仅仅是UNION，因为UNION 会删除重复项，而您的问题需要保留重复项。

UNION ALL 也比UNION 快（因为它不需要重复数据删除，这通常需要排序），但在这种情况下，行数太少以至于不会注意到性能差异，但是在以后的应用中记住这一点是很好的。

【讨论】：

请注意，每个子查询中都有一个多余的 LEFT JOIN

【解决方案2】：

我尝试了@Bohemian 的回答，但它对我不起作用。感谢他的简单解释，并按照link 进行操作，我能够想出下面对我有用的代码。但我不知道是否有办法可以摆脱代码中与左连接相关的冗余

sql = '(
  SELECT *
  FROM institutes
  LEFT JOIN city ON institutes.city_id = city.city_id 
  LEFT JOIN district ON city.district_id = district.district_id 
  WHERE city.city_id = $current_city_id
  )
  UNION
  (
  SELECT *
  FROM institutes
  LEFT JOIN city ON institutes.city_id = city.city_id 
  LEFT JOIN district ON city.district_id = district.district_id 
  WHERE district.district_id = $current_district_id
)
ORDER BY RAND() DESC LIMIT 10'

【讨论】：