Free SQL Help for Devs, DBAs, and the Curious

The MERGE statement, introduced in SQL Server 2008, is a powerful tool that simplifies the process of performing insert, update, and delete operations in a single statement. It is particularly useful in ETL (Extract, Transform, Load) processes, where data from various sources need to be transformed and loaded into a data warehouse. In this blog post, we will demonstrate how to use the MERGE statement in T-SQL for ETL loads.

The MERGE statement in T-SQL allows for the combination of insert, update, and delete operations into one statement. This is done by comparing the source and target tables based on a given condition. If a match is found, an update or delete operation can be performed. If no match is found, an insert operation can be executed.

-- T-SQL code goes here
MERGE TargetTable AS T
USING SourceTable AS S
ON (T.ID = S.ID)
WHEN MATCHED THEN 
   UPDATE SET T.Name = S.Name
WHEN NOT MATCHED BY TARGET THEN
   INSERT (ID, Name) VALUES (S.ID, S.Name)
WHEN NOT MATCHED BY SOURCE THEN 
   DELETE;
-- Add 
 after each line to simulate line breaks

MySQL 5.7 does not directly support the MERGE statement. However, you can achieve similar functionality using a combination of insert, update, and delete statements. A common approach is to use the ON DUPLICATE KEY UPDATE clause in the INSERT statement. This clause allows you to update a record if it already exists, or to insert it if it does not.

-- MySQL code goes here
INSERT INTO TargetTable (ID, Name)
SELECT ID, Name FROM SourceTable
ON DUPLICATE KEY UPDATE Name = VALUES(Name);
-- Add 
 after each line to simulate line breaks

In DB2 11.1, you can use the MERGE INTO statement to perform similar operations as in T-SQL. The syntax is quite similar. However, DB2 does not support the DELETE operation in the MERGE statement.

-- DB2 code goes here
MERGE INTO TargetTable AS T
USING (SELECT ID, Name FROM SourceTable) AS S
ON T.ID = S.ID
WHEN MATCHED THEN 
   UPDATE SET Name = S.Name
WHEN NOT MATCHED THEN
   INSERT (ID, Name) VALUES (S.ID, S.Name);
-- Add 
 after each line to simulate line breaks

Lastly, Azure SQL, being a cloud-based variant of SQL Server, also supports the MERGE statement. Its syntax and usage are identical to that of SQL Server's T-SQL. This makes Azure SQL an attractive choice for cloud-based ETL processes that require the flexibility and power of the MERGE statement.

In conclusion, the MERGE statement is a powerful tool for combining insert, update, and delete operations into a single statement. Its usage simplifies and optimizes ETL processes, making it an essential tool for any ETL developer. Whether you're using SQL Server, DB2, or Azure SQL, understanding and utilizing the MERGE statement can significantly improve your ETL processes.