SQL UPDATE from Another Table: JOIN-Based Updates Explained

The Problem

You need to update columns in one table using values from a related table. The syntax varies significantly between databases — this is one of the most common SQL portability headaches.

PostgreSQL: UPDATE ... FROM

UPDATE orders SET customer_name = c.name FROM customers c WHERE orders.customer_id = c.id;

PostgreSQL uses a FROM clause to join the source table. The WHERE clause links the two tables.

MySQL: UPDATE ... JOIN

UPDATE orders o INNER JOIN customers c ON o.customer_id = c.id SET o.customer_name = c.name;

MySQL puts the JOIN before SET. You can use INNER JOIN, LEFT JOIN, or multi-table joins.

SQL Server: UPDATE ... FROM (same as PostgreSQL)

UPDATE o SET o.customer_name = c.name FROM orders o INNER JOIN customers c ON o.customer_id = c.id;

SQL Server uses FROM with an alias on the target table. Very similar to PostgreSQL but requires the alias in UPDATE.

Safe Update Pattern

Always test with SELECT first: SELECT o.id, o.customer_name, c.name AS new_name FROM orders o JOIN customers c ON o.customer_id = c.id WHERE o.customer_name IS DISTINCT FROM c.name; Then wrap in a transaction and run the UPDATE.