兩個不同的條件

Question

獲取計數假設我有一個表states如下：

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

我想在其狀態爲1，表中的行數，並在表中的行數其狀態2.兩個單獨的查詢這樣做很簡單：

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

但似乎很傻要經過整個表的兩倍。有人能想到一些聰明的伎倆，讓我能夠在一個聲明中獲得相同的信息，並且只通過一次表格嗎？

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與聚合函數：

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] 

假設您正在使用SQL Server

Answer 4

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

使用SQL組通過