UPDATE

Explanation

UPDATE changes values in existing rows. You name the table, set one or more columns to new values (which can be expressions), and — critically — filter with WHERE to limit which rows change.

Without a WHERE, every row is updated, which is rarely what you want. A safe habit: write the SELECT version of your filter first, confirm the rows, then convert it to an UPDATE.

Syntax

UPDATE products
SET price = price * 1.10,
    stock = stock + 10
WHERE category = 'Electronics';
 
-- Update from another table:
UPDATE employees e
SET salary = e.salary + 5000
FROM departments d
WHERE e.department_id = d.id AND d.name = 'Engineering';

Interactive Example

Preview what a 10% price increase would look like for Electronics products before you'd ever run the update.

Common Mistakes

• Omitting WHERE. The update hits every row — a classic, painful mistake.
• Updating the wrong column. Double-check column names; a typo can zero out or null out data.
• Not wrapping in a transaction. For anything important, BEGIN first so you can ROLLBACK if the result is wrong.

Best Practices

• Always specify a WHERE; if you truly want all rows, say so deliberately.
• Preview with SELECT using the same filter before committing.
• Use expressions (price * 1.10) to compute new values from old ones.
• Run updates inside a transaction and verify before COMMIT.

Practice Question

Write an UPDATE that reduces the stock of every 'Stationery' product by 25 (use stock - 25), and describe the WHERE clause you'd use to limit it.

Summary

UPDATE modifies existing rows via SET column = value and a WHERE filter. Never skip the WHERE, preview with SELECT, and prefer a transaction for reversible, safe changes.

FAQ