SQL UPDATE Statement

Chapter 10 of 40 · Estimated read time: 7 min

The UPDATE statement changes the value of one or more columns in existing rows. It always works with a SET clause (what to change) and — almost always — a WHERE clause (which rows to change).

Syntax

Safe Update Workflow

Before every UPDATE, run a SELECT with the same WHERE clause to preview exactly which rows will change:

-- Step 1: Preview — which rows will be affected?
SELECT product_id, product_name, price
FROM products
WHERE product_id = 1;
-- Shows: Wireless Mouse, price = 899

-- Step 2: Run the UPDATE only after confirming
UPDATE products
SET price = 999
WHERE product_id = 1;

-- Step 3: Verify the change
SELECT product_id, product_name, price
FROM products
WHERE product_id = 1;
-- Shows: Wireless Mouse, price = 999

Example 1 — Update a Single Column

-- A customer moved cities — update their city
UPDATE customers
SET city = 'Gurugram'
WHERE customer_id = 'C04';

Example 2 — Update Multiple Columns at Once

-- Update both price AND stock for a product
UPDATE products
SET price = 1299,
    stock = 200
WHERE product_id = 7;   -- HDMI Cable

Example 3 — Update Using a Calculation

You can reference the current column value in the calculation — useful for applying percentage changes:

-- Apply a 10% price increase to all Furniture products
UPDATE products
SET price = price * 1.10
WHERE category = 'Furniture';

-- Verify: what are the new prices?
SELECT product_name, category, price
FROM products
WHERE category = 'Furniture';

Example 4 — Update Multiple Rows Matching a Condition

-- All Electronics products are out of stock — update stock to 0
UPDATE products
SET stock = 0
WHERE category = 'Electronics';

-- This updates 3 rows (Wireless Mouse, USB-C Hub, HDMI Cable) at once

Example 5 — Update to NULL (Clear a Value)

-- Remove a customer's email (they requested data deletion)
UPDATE customers
SET email = NULL
WHERE customer_id = 'C03';

Example 6 — Update Based on Another Table

Use a subquery in the WHERE clause to update rows based on data from another table:

-- Mark all orders from customers in Noida as "priority"
-- (assumes an order_flag column exists)
UPDATE orders
SET order_flag = 'priority'
WHERE customer_id IN (
  SELECT customer_id
  FROM customers
  WHERE city = 'Noida'
);

What Happens Without WHERE

-- ❌ DANGEROUS — updates EVERY row in the table
UPDATE products SET stock = 0;

-- This sets stock to 0 for Wireless Mouse, USB-C Hub, Notebook, Pens,
-- Desk Lamp, Ergonomic Chair, and HDMI Cable — all 7 products.
-- There is no automatic undo in most databases.

-- 1.
UPDATE products SET price = price + 10 WHERE category = 'Stationery';

-- 2.
UPDATE customers SET email = 'priya.new@example.com' WHERE customer_id = 'C02';

-- 3.
UPDATE products SET stock = 500 WHERE supplier_id = 'S02';