PostgreSQL中如何计算不同表两个日期之间的天数差？

PostgreSQL 未内置datediff函数，两个date类型字段直接做减法运算，返回的结果就是间隔的天数，不需要调用专门的函数。

select 
   t1."group",
   t2.start_date - t1.event_date as date_diff_days,
   count(distinct t1.employee_id)
from t1 join t2 on t1.project_id = t2.id
where true
  and t1.cummulative_manhour > 0
  and t1.company_id in ('a', 'b')
  and t1.event_date = '2021-05-17'
  and t1.project_id = '2900'
group by 
  t1."group",
  t2.start_date,
  t1.event_date

• 如果需要不区分日期前后顺序取正数的天数差，可以用abs(t2.start_date - t1.event_date)替代原有差值计算逻辑
• 如果待计算的字段是timestamp类型而非date类型，计算天数差可以用EXTRACT(DAY FROM (t2.start_timestamp - t1.event_timestamp))实现

内容的提问来源于stack exchange，提问作者nomnom3214