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操作：

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答案很有帮助，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

或者您可以将条件放在案例中：

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

相关：

SQL Combining several SELECT results

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：如果你在条件中使用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

如果您只是计算行数，则可以使用嵌套查询。

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

Answer 4

在Oracle SQL Developer中，我必须在选择中添加* FROM，否则我遇到语法错误：

select * FROM 
    (select COUNT(*) as foo FROM TABLE1),
    (select COUNT(*) as boo FROM TABLE2);

Answer 5

MySQL不计算NULL，所以这也应该有效：

SELECT count(*) AS TotalCount, 
  count( if( field = value, field, null)) AS QualifiedCount 
  FROM MyTable {possible JOIN(s)} WHERE {some conditions}

如果QuailifiedCount字段来自LEFT JOIN，那么效果很好，你只关心它是否存在。获取用户数量以及填写其地址的用户数量：

SELECT count( user.id) as NumUsers, count( address.id) as NumAddresses
  FROM Users
  LEFT JOIN Address on User.address_id = Address.id;

两个SQL COUNT（）查询？

5 个答案: