How to Find Differences Between Two SQL Server Tables?

Comparing SQL Server Tables: A Comprehensive Guide to Finding Differences

Identifying data inconsistencies between two SQL Server tables requires a method to pinpoint rows unique to either table. The following query efficiently achieves this:

(SELECT * FROM table1 EXCEPT SELECT * FROM table2) UNION ALL (SELECT * FROM table2 EXCEPT SELECT * FROM table1)

Query Breakdown:

• The EXCEPT operator isolates rows present in table1 but absent in table2.
• UNION ALL merges the results of the first EXCEPT operation with a mirrored EXCEPT operation on table2, revealing all discrepancies.

Illustrative Example:

Let's assume two tables, TempTestData and RealData, each with columns FirstName, LastName, and Product. To uncover rows with differing values across these columns, use this query:

(SELECT * FROM TempTestData EXCEPT SELECT * FROM RealData) UNION ALL (SELECT * FROM RealData EXCEPT SELECT * FROM TempTestData)

This query will return all rows exhibiting variations in any of the specified columns across both tables.

Important Considerations:

• This approach assumes identical column names and data types in both tables.
• For tables containing NULL values, LEFT JOIN or FULL OUTER JOIN offer more robust NULL comparison handling compared to EXCEPT.

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