SQL对另一个表中的多个字段进行计数

Question

我想计算表B idA_1-3字段中存在的表A idA数，并在输出中将0替换为null。

A       B
idA     idB   idA_1 idA_2 idA_3
1       1     2     3     3
2       2     2     3     3
3       3     1     3     2
        4     3     1     2
        5     1     1     2

所需的输出：

idA     cA_1  cA_2  cA_3
1       2     2    
2       2           3
3       1     3     2

仅计算cA_1的查询如下所示，但我未能对其进行修改以包含更多列：

SELECT IdA, COUNT(*) AS cA_1 FROM
(SELECT A.IdA, B.IdB
FROM A LEFT JOIN B ON A.IdA = B.IdA_1)
WHERE IdB IS NOT NULL
GROUP BY IdA
UNION ALL SELECT IdA, NULL AS cA_1 FROM
(SELECT A.IdA, B.IdB
FROM A LEFT JOIN B ON A.IdA = B.IdA_1)
WHERE IdB IS NULL
GROUP BY IdA;

IIF也有一个选项，但是工作太慢：

SELECT A.IdA, IIf((SELECT COUNT(*) FROM B WHERE IdA_1 = A.IdA)=0, NULL, (SELECT COUNT(*) FROM B WHERE IdA_1 = A.IdA)) AS cA_1
FROM A;

Answer 1

我建议相关的子查询：

select a.id,
       (select count(*) from b where b.idA_1 = a.id) as a_1,
       (select count(*) from b where b.idA_2 = a.id) as a_2,
       (select count(*) from b where b.idA_3 = a.id) as a_3
from a;

您可以使用use iif()将NULL转换为0：

select id, iif(a_1 > 0, a_1, NULL) as a_1,
       iif(a_2 > 0, a_2, NULL) as a_2,
       iif(a_3 > 0, a_3, NULL) as a_3
from (select a.id,
             (select count(*) from b where b.idA_1 = a.id) as a_1,
             (select count(*) from b where b.idA_2 = a.id) as a_2,
             (select count(*) from b where b.idA_3 = a.id) as a_3
      from a
     ) as a;

大多数数据库都有一个称为nullif()的函数，但我认为MS Access中没有相应的函数。