Statement takes in two tables – a source and a target and compares the records based on a key column, often the index column, and then performs an operation on it. This MERGE statement has been introduced in the SQL Server 2008 which brought a great revolution in writing simpler and maintainable code in SQL. In this article, I have explained in detail about the SQL MERGE statement. It is mandatory that one of the MATCHED clauses is provided in order for the MERGE statement to operate You can use SELECT after writing the MERGE statement which will return the number of records that have been modified by the transaction If a semi-colon is not present at the end of the MERGE statement, then an error will be thrown Should keep in mind while preparing the scripts.Įvery MERGE statement must end with a semi-colon. Include logic for handling inserts, updates and deletes, there are also some other key important points that we You can use the following SQL script to create the database and tables on your machine.įigure 6 – Checking output actions by the merge statement Important things to remember while implementing SQL MERGEĪlthough we have now understood how to write the MERGE statement from scratch and how to modify the script to For the purpose of this tutorial, I am going to create a simple table and insert aįew records in it. Now that we have gathered enough information regarding how the MERGE statement works, lets us go ahead and try to implement the same practically. We should also try to filter the source table so that only necessary records are being fetched by the statement to do the necessary operations. Apart from indexes, it is also essential that the join conditions are optimized as well. From aĭata processing perspective, this is quite helpful as it reduces the I/O operations from the disk for each of the three statements individually and now data is being read from the source only once.Īlso, the performance of the MERGE statement greatly depends on the proper indexes being used to match both the source and the target tables. Having said that, it means now you can write all your DML statements (INSERT, UPDATE, and DELETE) combined in a single statement. There are a few aspects using which you can optimize the performance of your MERGE statements. Optimizing the performance of the SQL MERGE statement The SQL MERGE statement was introduced in the SQL Server 2008 edition which allowed great flexibility to the database programmers to simplify their messy code around the INSERT, UPDATE and DELETE statements while applying the logic to implement SCD in ETL. In such cases, you need to insert new records into the data warehouse, remove or flag records from the warehouse which are not in the source anymore, and update the values of those in the warehouse which have been updated in the source. Slowly Changing Dimensions (SCD) in the data warehouse. A most common use case is while trying to maintain In a typical SQL Data warehouse solution, it is often essential to maintain a history of data in the warehouse with a reference to the source data that is being fed to the ETL tool. Although the MERGE statement is a little complex than the simple INSERTs or UPDATEs, once you are able to master the underlying concept, you can easily use this SQL MERGE more often than using the individual INSERTs or UPDATEs. The MERGE statement actually combines the INSERT, UPDATE, and the DELETE operations altogether. The MERGE statement tries to compare the source table with the target table based on a key field and then do some of the processing. As you can see, there are two circles that represent two tables and can be considered as Source and a Target. The above illustration depicts how a SQL MERGE statement basically works. The MERGE statement basically modifies an existing table based on the result of comparison between the key fields with another table in the context. Using the MERGE statement in SQL gives you better flexibility in customizing your complex SQL scripts and also enhances the readability of your scripts. You can specify conditions on which you expect the MERGE statement to insert, update, or delete, etc. MERGE statement in SQL is a very popular clause that can handle inserts, updates, and deletes all in a single transaction without having to write separate logic for each of these. In this article, I am going to give a detailed explanation of how to use the SQL MERGE statement in SQL Server.
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