SQL PostgreSQL中基于2张表计算两次count(*)查询结果的差异
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Calculate the difference between results of two count(*) queries based on 2 tables in PostgreSQL
提问by Ucef
I want to calculate the difference between the results of 2 count(*)-type SELECT queries executed on 2 separate tables of my PostgreSQL database.
我想计算在count(*)我的 PostgreSQL 数据库的 2 个单独表上执行的 2类型 SELECT 查询的结果之间的差异。
This what I'm currently using (however I should be able to wrap it all up into a single SELECT statement):
这是我目前正在使用的（但是我应该能够将其全部包装到一个 SELECT 语句中）：
SELECT "count"(*) AS val1 FROM tab1;
SELECT "count"(*) AS val2 FROM tab2;
SELECT val2-val1;
Thanks in advance
提前致谢
回答by Robert
Try this way:
试试这个方法：
select
(
SELECT
"count"(*) as val1
from
tab1
) - (
SELECT
"count"(*) as val2
from
tab2
) as total_count
回答by a_horse_with_no_name
select t1.cnt - t2.cnt
from (select count(*) as cnt from tab1) as t1
cross join (select count(*) as cnt from tab2) as t2
回答by Aditya Tomar
for same table you can do this.
对于同一张桌子，你可以这样做。
select (count(abc)-count(DISTINCT xyz)) from tab;
select (count(abc)-count(DISTINCT xyz)) from tab;
回答by Lukasz Szozda
Another option is to use SUM:
另一种选择是使用SUM：
SELECT SUM(val) AS result
FROM (SELECT count(*) AS val FROM tab2
UNION ALL
SELECT -count(*) FROM tab1) sub;
