Understanding SQL Server Merge: A Complete Guide for Dev

Hey Dev, are you looking for a solution to merge two tables in SQL Server? If yes, then you’ve landed on the right page. SQL Server Merge is a powerful feature of SQL Server that allows you to merge two source tables into a single target table using a few simple commands. In this article, we’ll explore different aspects of SQL Server Merge with examples and FAQs. So, let’s get started.

What is SQL Server Merge?

SQL Server Merge is a T-SQL statement that allows you to merge two tables (source tables) and returns a single table (target table) that contains all the rows from both the source tables.

The Merge statement can perform four types of operations when it finds a match between the source and target tables:

Let’s dive deeper into the syntax and usage of SQL Server Merge.

Syntax of SQL Server Merge

The syntax of the SQL Server Merge statement is as follows:

MERGE target_table AS targetUSING source_table AS sourceON merge_conditionWHEN MATCHED [AND update_condition]THEN UPDATE SET update_statementWHEN NOT MATCHED [BY TARGET]THEN INSERT (column_list) VALUES (value_list)WHEN NOT MATCHED BY SOURCETHEN DELETE;

Explanation of the SQL Server Merge Syntax

The SQL Server Merge statement consists of three main clauses:

The first clause is the MERGE statement that specifies the target table and the source table.

The second clause is the USING statement, which specifies the source table to be merged into the target table.

The third clause is the ON statement, which specifies the join condition between the source and target tables.

The fourth clause is the WHEN MATCHED statement, which specifies the action to take when a match is found between the source and target tables.

The fifth clause is the WHEN NOT MATCHED BY TARGET statement, which specifies the action to take when a row exists in the source table but not in the target table.

Finally, the sixth clause is the WHEN NOT MATCHED BY SOURCE statement, which specifies the action to take when a row exists in the target table but not in the source table.

Usage of SQL Server Merge

Let’s understand the usage of SQL Server Merge through an example:

MERGE dbo.target_table AS TUSING (SELECT * FROM dbo.source_table) AS SON T.ID = S.IDWHEN MATCHED AND T.Value <> S.ValueTHEN UPDATE SET T.Value = S.ValueWHEN NOT MATCHED BY TARGETTHEN INSERT (ID, Value) VALUES (S.ID, S.Value)WHEN NOT MATCHED BY SOURCETHEN DELETE;

In this example, we are merging two tables: target_table and source_table. The join condition is on the ID column of both tables.

When a match is found between the source and target tables, the update statement is executed to update the Value column of the target table with the value from the source table.

When a row exists in the source table but not in the target table, it is inserted into the target table.

When a row exists in the target table but not in the source table, it is deleted from the target table.

Now that we have a basic understanding of SQL Server Merge, let’s move on to some FAQs.

FAQs about SQL Server Merge

1. Can I merge more than two tables using SQL Server Merge?

No, you cannot merge more than two tables using SQL Server Merge. However, you can use nested Merge statements to merge multiple tables.

2. How do I handle NULL values while merging tables?

To handle NULL values while merging tables, you can use the IS NULL or IS NOT NULL operator in the merge condition.

3. Can I use a subquery as the source table in SQL Server Merge?

Yes, you can use a subquery as the source table in SQL Server Merge.

4. How do I determine the number of updated, inserted, and deleted rows in SQL Server Merge?

You can use the @@ROWCOUNT function after executing the Merge statement to determine the number of updated, inserted, and deleted rows.

5. Can I use the Merge statement to merge tables across different databases?

Yes, you can use the Merge statement to merge tables across different databases by specifying the fully qualified name of the table in the target or source clause.

I hope these FAQs have cleared some of your doubts about SQL Server Merge.

Conclusion

SQL Server Merge is a powerful feature of SQL Server that allows you to merge two tables into a single table using a few simple commands. In this article, we explored the syntax and usage of SQL Server Merge with examples and FAQs.

By now, you should have a solid understanding of SQL Server Merge and how to use it effectively in your projects. Happy merging!