SQL计数聚合函数和子查询

Question

我只是学习SQL专门的聚合函数和子查询

我有列c0, c1, c2, c3数据库表。

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 

可以在上面重写，结果查询会更快？或者如何让我的查询更快？

Answer 1

我不认为你需要聚合。只需计算每一行中的匹配。

假设id是表中唯一的：

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的对待布尔在数值上下文为数字，以1为真，0代表假，所以表达式c0 between 4 and 7基本上等同于case when c0 between 4 and 7 then 1 else 0 end，但更容易写。

MySQL还扩展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，它的行为如同where，但您可以使用select中定义的别名。

注意：如果列的值为NULL，则稍微复杂一点。

如果id不是唯一的，那么你基本上可以做同样的事情与聚集：

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; 

然而，一列名为id应该是唯一的。