How do I UPDATE from a SELECT in SQL Server?

If you need to update a table in SQL Server using values from another table, you can do this efficiently using the UPDATE statement with a JOIN or a FROM clause. Here’s how you can do it!

1. Using UPDATE with JOIN (Recommended Approach)

This method is commonly used when updating a table with data from another table:

UPDATE t1  

SET t1.column_name = t2.column_name  

FROM Table1 t1  

JOIN Table2 t2 ON t1.id = t2.id  

WHERE t2.some_column = 'some_value';

• Efficient for large datasets
  
• Allows filtering with WHERE to update specific rows
  

2. Using UPDATE with a Subquery

If you don’t need a join, you can update using a SELECT subquery:

UPDATE Table1  

SET column_name = (SELECT column_name FROM Table2 WHERE Table2.id = Table1.id)  

WHERE EXISTS (SELECT 1 FROM Table2 WHERE Table2.id = Table1.id);

• Useful when updating from a single column result
  
• Make sure the subquery returns only one row per update
  

3. Using MERGE (For Complex Updates)

If you need to update or insert data based on conditions, use MERGE:

MERGE INTO Table1 AS t1  

USING Table2 AS t2  

ON t1.id = t2.id  

WHEN MATCHED THEN  

    UPDATE SET t1.column_name = t2.column_name;

• Ideal for conditional updates
  
• Can also handle inserts (WHEN NOT MATCHED THEN INSERT)
  

Final Tips:

• ✅ Always backup your data before running an update!
  
•  ✅ Use WHERE conditions to avoid updating unintended rows.
  
•  ✅ Test your query with a SELECT before applying the update.