SQL联合一些表在一起

Question

I'm trying to get the total count from 4 different tables using the union but i'm not sure how to get this to work..

My code for the single table count which works fine,

SELECT (Select count(*) 
        FROM (SELECT RN, DP, DT, Min(ED) AS MinDate 
              FROM ECount1 
              GROUP BY RN, DP, DT) as x 
        WHERE DT = 'STR'  
          AND MONTH(MinDate) >= '07' 
          AND Month(MinDate) <= '09') as StrAFT

i thought i could just add a union but this doesn't work? Something like this?

SELECT (Select count(*) FROM 
(SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount1 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
UNION
SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount2 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
UNION
SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount3 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
UNION
SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount4 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
) as StrAFT

Answer 1

I believe the problem you have is UNION , it will remove duplicate rows
You can use UNION ALL to keep those duplicate rows

Select count(*) FROM 
(
    SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount1 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
    UNION ALL
    SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount2 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
    UNION ALL
    SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount3 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
    UNION ALL
    SELECT RN, DP, DT, Min(ED) AS MinDate FROM ECount4 GROUP BY RN, DP, DT) as x WHERE DT = 'STR' AND MONTH(MinDate) >= '07' and Month(MinDate) <= '09'
) as StrAFT

Simplify the case

-- This will return 1 row with 1
SELECT 1 UNION SELECT 1

-- This will return 2 rows with 1
SELECT 1 UNION ALL SELECT 1

As suggested by e_i_pi, you can remove the first SELECT () , it done nothing in your query

Answer 2

As suggested for Prissioner use UNION ALL

WITH cte as (
    SELECT * FROM ECount1 
    UNION ALL
    SELECT * FROM ECount2
    UNION ALL
    SELECT * FROM ECount3 
    UNION ALL
    SELECT * FROM ECount4
), calculate as (
    SELECT RN, DP, DT, Min(ED) AS MinDate 
    FROM cte 
    WHERE DT = 'STR'  
      AND MONTH(ED) >= '07' 
      AND MONTH(ED) <= '09
    GROUP BY RN, DP, DT
    HAVING MIN(ED) IS NOT NULL
)
SELECT COUNT(*) 
FROM calculate

For count for each month use GROUP BY

 SELECT MONTH(ED), COUNT(*)
 FROM calculate
 GROUP BY MONTH(ED)

OR use conditional count:

 SELECT COUNT(*), 
        COUNT (CASE WHEN MONTH(ED) = '07' THEN 1 END) as count_07,
        COUNT (CASE WHEN MONTH(ED) = '08' THEN 1 END) as count_08,
        COUNT (CASE WHEN MONTH(ED) = '09' THEN 1 END) as count_09
 FROM calculate
 GROUP BY MONTH(ED)