如何使用 COUNT 和 WHERE 子句从两个不同的表中选择 2 个不同的列

Question

I really need your help on this, I have these queries where I want to select 2 columns of 2 different tables with count and where statement..

SELECT Error_Code as ISERR, 
       COUNT(Error_Code) as A 
FROM COMPIS 
WHERE test_date BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
  AND Error_Code <> ' ' 
GROUP BY Error_Code

SELECT error_code as SCERR, 
       COUNT(error_code) as B 
FROM COMPSER 
WHERE test_date_time BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
  AND error_code <> ' ' 
GROUP BY error_code

I have tried these queries, but this only showed me null values.

SELECT A, a.ISERR, B, b.SCERR
FROM ( SELECT Error_Code as ISERR, 
              COUNT(Error_Code) as A 
       FROM COMPIS 
       WHERE test_date BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
         AND Error_Code <> ' ' 
       GROUP BY Error_Code ) a,
     ( SELECT error_code as SCERR, 
              COUNT(error_code) as B 
       FROM COMPSER 
       WHERE test_date_time BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
         AND error_code <> ' ' 
       GROUP BY error_code ) b

OUTPUT was:

A    | ISERR | B   | SCERR
-----+-------+-----+------
null   null   null   null

Your help will be appreciated. Thanks

Answer 1

SELECT error_code, 
       SUM(CASE WHEN table='A' THEN cnt END) cnt_A, 
       SUM(CASE WHEN table='B' THEN cnt END) cnt_B
FROM ( SELECT Error_Code as ISERR, 
              COUNT(Error_Code) as cnt,
              `A` as table 
       FROM COMPIS 
       WHERE test_date BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
         AND Error_Code <> ' ' 
       GROUP BY Error_Code
     UNION ALL
       SELECT error_code, 
              COUNT(error_code),
              'B'
       FROM COMPSER 
       WHERE test_date_time BETWEEN '2020-01-01 06:00:00' AND '2020-01-10 18:00:00' 
         AND error_code <> ' ' 
       GROUP BY error_code ) total
GROUP BY error_code

If you need NULL instead of zeros use MAX instead SUM.