SQL MERGE Statement - Complete Beginner Tutorial

Understanding the MERGE Statement

MERGE is a conditional DML statement that compares rows between a source and a target table using a specified join condition.

Based on whether rows match or not, MERGE can insert new rows, update existing rows, or delete rows from the target table.

• Source table: The dataset providing new or updated data.
• Target table: The table to be modified.
• Join condition: Defines how rows from source and target are matched.
• WHEN MATCHED clause: Defines actions for matched rows (update or delete).
• WHEN NOT MATCHED clause: Defines actions for unmatched rows (insert).

MERGE Statement Syntax

The basic syntax of the MERGE statement includes specifying the target table, the source dataset, the join condition, and the actions to perform when rows match or do not match.

Example: Synchronizing Customer Data

Consider two tables: Customers (target) and NewCustomers (source). We want to update existing customers, insert new ones, and delete customers no longer present.