两个不同的条件

Question

获取计数假设我有一个表states如下：

create table states (id int4 unique not null, state int4 not null); 

我想在其状态为1，表中的行数，并在表中的行数其状态2.两个单独的查询这样做很简单：

select count(*) from states where state = 1; 
select count(*) from states where state = 2; 

但似乎很傻要经过整个表的两倍。有人能想到一些聪明的伎俩，让我能够在一个声明中获得相同的信息，并且只通过一次表格吗？

Answer 1

select 
    count((state = 1)::integer or null) state_1, 
    count((state = 2)::integer or null) state_2 
from states 

或

select 
    sum((state = 1)::integer) state_1, 
    sum((state = 2)::integer) state_2 
from states 

Answer 2

可以使用CASE与聚合函数：

select 
    sum(case when state=1 then 1 else 0 end) State1, 
    sum(case when state=2 then 1 else 0 end) State2 
from states 

Answer 3

SELECT [state], COUNT(*) TotalRows 
FROM [Expense].[dbo].[state] 
GROUP BY [state] 

假设您正在使用SQL Server

Answer 4

select count(*) from states group by state having state = 1 or state=2; 

使用SQL组通过