SQL UPDATE – Modify Existing Records in Tables

Introduction – Why Use SQL UPDATE?

The UPDATE statement in SQL is used to modify existing data in a table. It allows you to change one or more column values for rows that match a specific condition.

Without UPDATE, you’d have to delete and re-insert records — a far less efficient method.

In this guide, you’ll learn how to:

• Update a single row or multiple rows
• Use WHERE to control which records are updated
• Update based on calculations or subqueries
• Use UPDATE with joins

1. Basic UPDATE Syntax

UPDATE table_name
SET column1 = value1, column2 = value2
WHERE condition;

Always include WHERE to avoid changing all rows.

2. Update a Single Record

UPDATE employees
SET salary = 60000
WHERE id = 101;

Modifies the salary for the employee with ID 101.

3. Update Multiple Records

UPDATE products
SET status = 'inactive'
WHERE quantity = 0;

Updates all rows where the condition matches.

4. Update with Expressions

UPDATE employees
SET salary = salary * 1.05
WHERE department = 'Sales';

Increases salary by 5% for all Sales employees.

5. Update Using a Subquery

UPDATE employees
SET department = 'Operations'
WHERE id IN (SELECT employee_id FROM transfers);

Uses data from another table to filter rows.

6. Update with JOIN (varies by SQL dialect)

-- MySQL / PostgreSQL
UPDATE employees
SET employees.salary = new_salaries.salary
FROM new_salaries
WHERE employees.id = new_salaries.id;

Useful for updating based on related table data.

7. Update All Rows (Use Carefully)

UPDATE products
SET discount = 10;

Applies the same change to every row — use only if intended.

Best Practices

Summary – Recap & Next Steps

The UPDATE command is essential for modifying existing records safely and efficiently. From adjusting salaries to bulk status changes, UPDATE gives you precise control.

Key Takeaways:

• Use UPDATE with SET to change values
• Always add a WHERE clause to scope the update
• Can use expressions, subqueries, and joins
• Use transactions when modifying large datasets

Real-World Relevance:
UPDATE is used in inventory management, account maintenance, audit logs, and operational workflows.

Next: Learn how to remove data using SQL DELETE.

FAQ – SQL UPDATE

Can I update multiple columns at once?

Yes. Separate them with commas in the SET clause.

What if I forget the WHERE clause?

All rows in the table will be updated. Always double-check.

Can I use a subquery in an UPDATE?

Yes, subqueries are often used to filter which rows are updated.

Can I undo an UPDATE?

Only if you’ve used a TRANSACTION and haven’t committed yet. Otherwise, no.

Can I update data from another table?

Yes, using joins or subqueries depending on your SQL dialect.