SQL 计数聚合函数和子查询

Question

我只是在学习SQL专门学习聚合函数和子查询

我有一个包含列 c0, c1, c2, c3 的数据库 table。

我的查询：

SELECT ID, count(ID) 
FROM ((select ID from tbl1 where c0 BETWEEN 4 and 7) 
UNION ALL
(select ID from tbl1 where c1 BETWEEN 5 and 7) 
UNION ALL
(select ID from tbl1 where c2 BETWEEN 6 and 10) 
UNION ALL
(select ID from tbl1 where c3 BETWEEN 1 and 5)) AS tbl
GROUP BY ID HAVING count(ID) >= 2

能否重写上面的查询以使结果更快？或者如何使我的查询更快？

Answer 1

我认为您根本不需要聚合。只需计算每一行中的匹配项。

假设id在table中是唯一的：

select id,
       ((c0 between 4 and 7) + (c1 between 5 and 7) + (c2 between 6 and 10) +
        (c3 between 1 and 5)
       ) as cnt
from tbl1
having cnt >= 2;

MySQL 将数字上下文中的布尔值视为数字，1 表示真，0 表示假，因此表达式 c0 between 4 and 7 基本上等同于 case when c0 between 4 and 7 then 1 else 0 end，但更容易编写.

MySQL 还扩展了 having 子句，因此它可以在没有 group by 的情况下工作。在这种情况下，它的行为类似于 where，但您可以使用 select.

中定义的别名

注意：如果列有 NULL 个值，这会稍微复杂一些。

如果id不是唯一的，那么你基本上可以用聚合做同样的事情：

select id,
       sum((c0 between 4 and 7) + (c1 between 5 and 7) + (c2 between 6 and 10) +
           (c3 between 1 and 5)
          ) as cnt
from tbl1
group by id
having cnt >= 2;

但是，名为 id 的列应该是唯一的。

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