PostgreSQL的计数（布尔表达式）

Question

什么是PostgreSQL中的最佳方式，我想在一个领域与value = 1算多少算多少数与value = 0在同一领域A.

是这样的：

select 
    count (field1 = value1) as filed1_ok, 
    count (field1 = value2) as filed1_bad, 
    extract(MONTH from rpt_date) AS mth 
where rpt_date between frdate and todate 
group by mth 

Answer 1

使用count(condition or null)

select 
    count(field1 = 1 or null) as filed1_ok, 
    count(field1 = 0 or null) as filed1_bad, 
    extract(MONTH from rpt_date) AS mth 
where rpt_date between frdate and todate 
group by mth 

true or null评估为true。 false or null评估为null。由于count不计算正是你想要的空值。

在其他SQL方言，并在PostgreSQL中可以使用case

select 
    coalesce(sum(case field1 when 1 then 1 end), 0) as filed1_ok, 
    coalesce(sum(case field1 when 0 then 1 end), 0) as filed1_bad, 
    extract(MONTH from rpt_date) AS mth 
where rpt_date between frdate and todate 
group by mth 

我认为这是冗长和不透明相比count(condition or null) PostgreSQL的选项来完成。