Question

The query below selects the rows from table_1 which do not exist in table_2 based on values in the 4 columns. The mismatch could be due to one or more columns.

I would like to extend the query to tell me which column(s) had the mismatched values, either by showing the column name or its value. I can do this in a cursor but prefer to do it in a set based operation if possible.

SELECT  i.agent ,
        i.agency ,
        i.customer ,
        i.Company
FROM    table_1 AS i
WHERE   NOT EXISTS ( SELECT p.agent ,
                            p.agency ,
                            p.customer ,
                            p.Company
                     FROM   table_2 AS p
                     WHERE  i.Agent = p.Agent
                            AND i.agency = p.Agency
                            AND i.customer = p.customer
                            AND i.Company = p.Company )

Update:
I guess this needs more refinement. Let’s add that 3 out of 4 columns need to match.

Answer 1

You can simplify this problem drastically if you require that certain columns do match, or at least start with some expectations of which columns should match. In other words, instead of looking at this as a non-matching problem, redefine it as a partial matching problem.

Let’s say you expect agent and agency to match, but customer and company might not. This isn’t too difficult:

SELECT
    i.agent, i.agency, i.customer, i.company, p.customer, p.company,
    CASE
        WHEN i.customer = p.Customer THEN 'Y'
        ELSE 'N'
    END AS matchescustomer,
    CASE
        WHEN i.company = p.Company THEN 'Y'
        ELSE 'N'
    END AS matchescompany
FROM table1 i
INNER JOIN table2 p
    ON p.agent = i.agent
    AND p.agency = i.agency

If you want to check for other partial matches, just reorder the columns. Instead of joining on agent and agency, join on agent and customer, or whatever.

If you only expect a few different kinds of partial matches, you can write a few different queries similar to the one above and put them together with a UNION (or UNION ALL if you don’t mind duplicates). In other words:

SELECT (columns)
FROM table1 i INNER JOIN table2 p
    ON p.agent = i.agent AND p.agency = i.agency
UNION
SELECT (columns)
FROM table1 i INNER JOIN table2 p
    ON p.agent = i.agent AND p.customer = i.customer

Now if you’re looking to get every conceivable mismatch, then this is quickly going to get out of control, so you might want to adapt a more heuristic method, and search for partial matches that match at least a certain number of columns (say 3). Then you can restrict the obnoxiousness to at most the number of columns you have to compare:

;WITH PartialMatches_CTE AS
(
    SELECT i.agent AS iagent, p.agent AS pagent, ... (etc.)
    FROM table1 i INNER JOIN table2 p ON p.agent = i.agent
    UNION ALL
    SELECT (...) FROM table1 INNER JOIN table2 ON p.agency = i.agency
    UNION ALL
    SELECT (...) FROM table1 INNER JOIN table2 ON p.company = i.company
    ... and so on
),
ResolvedMatches_CTE AS
(
    SELECT DISTINCT
        iagent, pagent, iagency, pagency, ...,
        CASE WHEN pagent = iagent THEN 'Y' ELSE 'N' END AS agentmatch,
        CASE WHEN pagency = iagency THEN 'Y' ELSE 'N' END AS agencymatch,
        ...,
        (CASE WHEN pagent = iagent THEN 1 ELSE 0 END +
         CASE WHEN pagency = iagency THEN 1 ELSE 0 END +
         ...) AS MatchCount
    FROM PartialMatches_CTE
)
SELECT *
FROM ResolvedMatches_CTE
WHERE MatchCount >= 3

Now, having said all this, there’s one thing I’m wondering…

These two data tables aren’t, perchance, sequentially related, are they? As in, the 3rd row in table1 always maps to the 3rd row in table 2, but may not match all columns? It’s a shot in the dark, but if that is indeed the case, then we could make this way simpler. Otherwise, the last query here should probably do what you want without becoming too much of an unmaintainable mess.

Note that performance is likely going to stink for all of these queries. Hopefully your data sets aren’t too large. AFAIK there is no easy way to really optimize this kind of thing.