如何组合两个不同的 mysql where 子句，每个子句产生 5 行

Question

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。有没有一种可能的方法来组合两个查询，这样我就不必运行他们分开。但仍然得到相同的结果，第一个 where 子句的结果和第二个 where 子句的结果相同。

    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

Answer 1

将 UNION ALL 与包装器一起使用 SELECTs:

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

包装器 SELECT 是必需的，因为您不能在同一级别组合 ORDER BY ... LIMIT 和 UNION，但可以将 ORDER BY ... LIMIT 放入 子查询 .

需要

UNION ALL 而不仅仅是 UNION，因为 UNION 删除了重复项，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，性能差异不大不会被注意到，但在以后的应用中记住这一点是很好的。

Answer 2

我尝试了@Bohemian 的回答，但它对我不起作用。多亏了他简单的解释，并遵循这个 link 我能够想出下面对我有用的代码。但我不知道是否有办法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'

如何组合两个不同的 mysql where 子句，每个子句产生 5 行

How to combine two different mysql where clauses each results in 5 rows

mysql

union

select

left-join

where-clause