根據PostgreSQL中的2個表計算兩個count（*）查詢的結果之間的差異

Question

我想計算在PostgreSQL數據庫的2個獨立表上執行的2 count(*)-type SELECT查詢的結果之間的差異。

這我目前使用的（但我應該能夠這一切包裝成一個單一的SELECT語句）什麼：

SELECT "count"(*) AS val1 FROM tab1; 
SELECT "count"(*) AS val2 FROM tab2; 
SELECT val2-val1; 

在此先感謝

Answer 1

試試這個方法：

select 
(SELECT "count"(*) as val1 from tab1) - (SELECT "count"(*) as val2 from tab2) as total_count 

Answer 2

select t1.cnt - t2.cnt 
from (select count(*) as cnt from tab1) as t1 
    cross join (select count(*) as cnt from tab2) as t2