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In a typical data warehousing application, quite often during the ETL cycle you need to perform INSERT, UPDATE and DELETE operations on a TARGET table by matching the records from the SOURCE table.
You would need to write separate INSERT, UPDATE and DELETE statements to refresh the target table with an updated product list or do lookups. Though it seems to be straight forward at first glance, but it becomes cumbersome when you have do it very often or on multiple tables, even the performance degrades significantly with this approach.
In this tip we will walk through how to use the MERGE statement and do this in one pass.
Beginning with SQL Server 2008, now you can use MERGE SQL command to perform these operations in a single statement. This new command is similar to the UPSERT (fusion of the words UPDATE and INSERT.) command of Oracle; it inserts rows that don't exist and updates the rows that do exist. With the introduction of the MERGE SQL command, developers can more effectively handle common data warehousing scenarios, like checking whether a row exists, and then executing an insert or update or delete.
The MERGE statement basically merges data from a source result set to a target table based on a condition that you specify and if the data from the source already exists in the target or not. The new SQL command combines the sequence of conditional INSERT, UPDATE and DELETE commands in a single atomic statement, depending on the existence of a record. The new MERGE SQL command looks like as below:
Next use the MERGE SQL command to synchronize the target table with the refreshed data coming from the source table.
Merge Command Key Points
- The MERGE SQL statement requires a semicolon (;) as a statement terminator. Otherwise Error 10713 is raised when a MERGE statement is executed without the statement terminator.
- When used after MERGE, @@ROWCOUNT returns the total number of rows inserted, updated, and deleted to the client.
- At least one of the three MATCHED clauses must be specified when using MERGE statement; the MATCHED clauses can be specified in any order. However a variable cannot be updated more than once in the same MATCHED clause.
- Of course, it's obvious, but just to mention, the person executing the MERGE statement should have SELECT Permission on the SOURCE Table and INSERT, UPDATE and DELETE Permission on the TARGET Table.
- MERGE SQL statement improves the performance as all the data is read and processed only once whereas in previous versions three different statements have to be written to process three different activities (INSERT, UPDATE or DELETE) in which case the data in both the source and target tables are evaluated and processed multiple times; at least once for each statement.
- MERGE SQL statement takes same kind of locks minus one Intent Shared (IS) Lock that was due to the select statement in the ‘IF EXISTS' as we did in previous version of SQL Server.
- For every insert, update, or delete action specified in the MERGE statement, SQL Server fires any corresponding AFTER triggers defined on the target table, but does not guarantee on which action to fire triggers first or last. Triggers defined for the same action honor the order you specify.