两个SQL COUNT()查询？

Question

我想计算表中记录的总数,以及匹配特定条件的记录总数.我可以通过两个单独的查询来完成这些

SELECT COUNT(*) AS TotalCount FROM MyTable;
SELECT COUNT(*) AS QualifiedCount FROM MyTable
  {possible JOIN(s) as well e.g. JOIN MyOtherTable mot ON MyTable.id=mot.id} 
  WHERE {conditions};

有没有办法将这些组合成一个查询,以便我在一行中得到两个字段？

SELECT {something} AS TotalCount, 
  {something else} AS QualifiedCount 
  FROM MyTable {possible JOIN(s)} WHERE {some conditions}

如果没有,我可以发出两个查询并将它们包装在一个事务中,这样它们就是一致的,但我希望能用一个查询.

编辑:我最关心原子性; 如果有两个子SELECT语句需要,只要有来自某个地方的INSERT,它就不会使两个响应不一致.

编辑2:CASE答案是有帮助的,但在我的具体实例中,条件可能包括与另一个表的JOIN(忘了在我原来的帖子中提到,抱歉)所以我猜这种方法不起作用.

Answer 1

一种方法是将表加入自身:

select
   count(*) as TotalCount,
   count(s.id) as QualifiedCount
from
   MyTable a
left join
   MyTable s on s.id = a.id and {some conditions}

另一种方法是使用子查询:

select
   (select count(*) from Mytable) as TotalCount,
   (select count(*) from Mytable where {some conditions}) as QualifiedCount

或者你可以把条件放在一个案例中:

select
   count(*) as TotalCount,
   sum(case when {some conditions} then 1 else 0 end) as QualifiedCount
from
   MyTable

有关:

SQL组合几个SELECT结果

Answer 2

在Sql Server或MySQL中,您可以使用CASE语句执行此操作:

select 
    count(*) as TotalCount,
    sum(case when {conditions} then 1 else 0 end) as QualifiedCount
from MyTable

编辑:如果您在条件中使用JOIN,这也有效:

select 
    count(*) as TotalCount,
    sum(case when {conditions} then 1 else 0 end) as QualifiedCount
from MyTable t
left join MyChair c on c.TableId = t.Id
group by t.id, t.[othercolums]

GROUP BY用于确保您只从主表中找到一行.

Answer 3

如果你只计算行,你可以使用嵌套查询.

select 
    (SELECT COUNT(*) AS TotalCount FROM MyTable) as a,
    (SELECT COUNT(*) AS QualifiedCount FROM MyTable WHERE {conditions}) as b