Question

I am trying to spot some broken records in a MS-SQL Database.

In a simplified example, the scenerio is this:

I have 2 tables, simply put:

• Table_1 : Id,Date,OpId

• Table_2 : Date,OpId,EventName

And I have this business rule: If there is a record in Table_1 THEN at least 1 row should exist in the Table_2 for the Table_1.Date and Table.OpId.

If there is a row in Table_1 and if there is no row matching with that row in Table_2 THEN there is a broken data -whatever the reason-.

To find out the incorrect data, I use:

SELECT *
FROM table_1 t1
LEFT JOIN table_2 t2 ON t1.Date = t2.Date AND t1.OpId = t2.OpId
WHERE t2.OpId IS NULL -- So, if there is no 
--                           matching row in table_2 then this is a mistake

But it takes too long to have the query completed.

Can there be a faster or better way to approach similar scenerios?

Answer 1

To do an anti semi join NOT EXISTS in SQL Server is usually better than or equal to in performance the other options (NOT IN, OUTER JOIN ... NULL, EXCEPT)

SELECT *
FROM   table_1 t1
WHERE  NOT EXISTS (SELECT *
                   FROM   table_2 t2
                   WHERE  t1.Date = t2.Date
                          AND t1.OpId = t2.OpId) 

See Left outer join vs NOT EXISTS. You may well be missing a useful index though.