Question

我有两张表table1和table2。 table1列id和table2_id，而table2有id和category。我需要根据table1中包含值table2.category或Regular的两个单独值来计算来自Special的行。

我在两个查询中完成了这个，但我想知道是否可以在单个sql中完成。我的疑问是：

"SELECT COUNT(t1.id) AS regular FROM table1 t1 LEFT OUTER JOIN table2 t2 ON t1.t2_id = t2.id WHERE t2.category = 'Regular'";
"SELECT COUNT(t1.id) AS special FROM table1 t1 LEFT OUTER JOIN table2 t2 ON t1.t2_id = pr.id WHERE t2.category = 'Special'";

感谢。

修改

第二个查询JOIN应该是ON t1.t2_id = t2.id而不是ON t1.t2_id = pr.id。很抱歉可能造成的混乱。请相应地更新/编辑您的答案/评论。

Answer 1

将Where条件移至CASE语句并执行计数

以下是使用Conditional Aggregate

的一种方法

SELECT
COUNT(case when t2.category = 'Regular' then t1.id end) AS Regular,
COUNT(case when t2.category = 'Special' then t1.id end) AS special 
FROM table1 t1 
INNER JOIN table2 t2 ON t1.t2_id = pr.id 
Where t2.category IN ('Regular','Special' )

注意：我已将LEFT JOIN更改为INNER JOIN，因为您只想在table2.category为'Regular'或{{1}时进行计数所以不要在这里使用'Special'

Answer 2

而不是

"SELECT COUNT(t1.id) AS regular FROM table1 t1 LEFT OUTER JOIN table2 t2 ON t1.t2_id = t2.id WHERE t2.category = 'Regular'";
"SELECT COUNT(t1.id) AS special FROM table1 t1 LEFT OUTER JOIN table2 t2 ON t1.t2_id = pr.id WHERE t2.category = 'Special'";

你可以这样做：

select t2.category, count(t1.id)
from table1 t1
left outer join table2
on t1.t2_id = t2.id
group by t2.category
having t2.category in ('Regular', 'Special')

建议的查询对已连接的记录进行分组，过滤组并选择类别名称及其计数。

根据另一个表中的2个条件计算mysql表行

2 个答案: