Summing common columns of multi table parent-child relation

问题

I have four tables which are structured in a parent-child relation. One table has a primary key and the other three tables, otherwise unrelated to each other, have that primary key as a foreign key. That is, Table A has a primary key called customer ID. Tables B, C, and D also have customer ID and some other information.

What I want to achieve is to sum column2 of tables B, C, and D. I have tried:

SELECT dbo.a.primary_key,
       SUM(dbo.b.column2) AS Expr1,
       SUM(dbo.c.column2) AS Expr2, 
       SUM(dbo.d.column2) AS Expr3
FROM   dbo.a 
INNER JOIN dbo.b ON dbo.a.primary_key = dbo.b.column1 
INNER JOIN dbo.c ON dbo.a.primary_key = dbo.c.column1  
INNER JOIN dbo.d ON dbo.a.primary_key = dbo.d.column1
GROUP BY dbo.a.primary_key

This returns nothing. I have also tried a left outer join which returns nulls. I have also tried it with the below line, which is what I really want to achieve:

SUM(dbo.b.column2) + SUM(dbo.c.column2) + SUM(dbo.d.column2) AS Expr1

This may be down to poor design, as I could effectively join these three tables into one table, but believe it is tidier like this.

Once I have this problem solved, I would like to expand this as column3 of tables B, C, and D, so that they have a date field that is an int and links back to a date table (E). I would like to join tables A, B, C, and D to a newly created E at the same time, grouped by date and a.primary_key. Is this possible, or will I need to rethink the design?

回答1:

You can use APPLY instead of JOINs in case you just need to SELECT a single Column.
EDIT: If there are any NULL values this query should get you a SUM...
EDIT2: Changed the CROSS APPLY to OUTER APPLY, because in case one of the SUMS is non existent, the whole ROW will be dropped from the result.

SELECT a.primary_key,
    b.Sum AS Expr1,
    c.Sum AS Expr2, 
    d.Sum AS Expr3,
    ISNULL(b.Sum, 0) + ISNULL(c.Sum, 0) + ISNULL(d.Sum, 0) as [GrandTotal]
FROM dbo.Agency_Table a
    OUTER APPLY (SELECT SUM(ISNULL(b.column2, 0)) [Sum] FROM dbo.b WHERE a.primary_key = b.column1) as b
    OUTER APPLY (SELECT SUM(ISNULL(c.column2, 0)) [Sum] FROM dbo.c WHERE a.primary_key = c.column1) as c
    OUTER APPLY (SELECT SUM(ISNULL(d.column2, 0)) [Sum] FROM dbo.d WHERE a.primary_key = d.column1) as d