How to create fast database queries

Matching whole sets

Comments enabled. I *really* need your comment

From Stack Overflow:

Given the following:

declare @a table
    pkid int,
    value int

declare @b table
    otherID int,
    value int

insert into @a values (1, 1000)
insert into @a values (1, 1001)
insert into @a values (2, 1000)
insert into @a values (2, 1001)
insert into @a values (2, 1002)

insert into @b values (-1, 1000)
insert into @b values (-1, 1001)
insert into @b values (-1, 1002)

, how do I query for all the values in @a that completely match up with @b?

{@a.pkid = 1, @b.otherID = -1} would not be returned (only 2 of 3 values match)

{@a.pkid = 2, @b.otherID = -1} would be returned (3 of 3 values match)

We need to compare each set of values from @a to each set of @b and return the id's of non-matching sets.

Two sets are considered matching when for each row from the first set there is an equal row from the second set and vise versa. That is, both the values and the count of the rows should match in both sets.

First thing that comes to mind is to calculate COUNT(*)'s of both sets, then calculate the COUNT(*) for the result of both sets joined on value and make sure all three are equal.

But we don't actually need to COUNT(*) all values. We just need to find out the first row from either of the subsets that has no match in another subset. As soon as we find such a row, we can tell that the sets don't match; if we don't find one, then the sets do match.

This is where FULL OUTER JOIN comes handy. If there is a record in either of the sets that doesn't have a match in another set, the join will return NULL in one of the fields. We can match it in WHERE clause and return just as soon as we encounter a NULL id returned in the resultset.

Finally, we need to produce all possible set combinations to compare. We can do it by cross-joining @a and @b, selecting DISTINCT pkid, otherid from the resulting recordset and using it in a subquery to produce all possible set combinations.

The resulting query looks like this:

FROM    (
        SELECT  ab.pkid, ab.otherID,
                SELECT  TOP 1 COALESCE(ai.value, bi.value)
                FROM    (
                        SELECT  *
                        FROM    @a aii
                        WHERE   aii.pkid = ab.pkid
                        ) ai
                FULL OUTER JOIN
                        SELECT  *
                        FROM    @b bii
                        WHERE   bii.otherID = ab.otherID
                        ) bi
                ON      ai.value = bi.value
                WHERE   ai.pkid IS NULL OR bi.otherID IS NULL
                ) unmatch
                SELECT  DISTINCT pkid, otherid
                FROM    @a a , @b b
                ) ab
        ) q
pkid otherid unmatch
1 -1 1002

As a free bonus, it also returns the first non-matching value.

Written by Quassnoi

March 31st, 2009 at 11:00 pm

Posted in SQL Server

Leave a Reply