Updating Data

Change existing rows with UPDATE ... SET, always limited by a WHERE that you've tested first with a SELECT.

Apprentice · T-SQL · SQL · Updated November 3, 2026

After this lesson you can

Change existing rows with UPDATE and SET, limited by a WHERE clause

Key takeaways

SET lists each column to change and its new value; WHERE says which rows change. Without a WHERE, every row in the table changes.
Before every UPDATE, run a SELECT with the same WHERE and check that it returns exactly the rows you mean to change.
The new value can use the row's current value, as in SET UnitPrice = UnitPrice * 1.10.
The rows-affected count should match the rows your SELECT returned. If it doesn't, stop and find out why.

UPDATE changes values in rows that already exist.

UPDATE <table>
SET <column1> = <value1>
    , <column2> = <value2>
WHERE <which rows>;

SET lists each column to change and its new value.
WHERE says which rows change. Without it, every row in the table changes.

One column of one row

UPDATE Customers
SET ContactName = 'Alfred Blauer'
WHERE CustomerId = 'ALFKI';

(1 row affected)

Filtering on the primary key is the safest WHERE there is: it matches exactly one row.

Several columns at once

UPDATE Products
SET UnitPrice = 19.99
    , UnitsInStock = 25
WHERE ProductId = 5;

Calculating the new value

The new value can use the row's current values. This raises every beverage's price by 10%:

UPDATE Products
SET UnitPrice = UnitPrice * 1.10
WHERE CategoryId = 1;

(12 rows affected)

For each matching row, UnitPrice on the right is the row's price before the change.

Forgetting the WHERE

Don't run this one; just read it.

UPDATE Orders
SET ShippedDate = '2026-04-01';

This sets the shipped date of all 830 orders. There's no undo button, and in a real company's database that's a very bad day. SSMS won't warn you.

Always SELECT first

Before every UPDATE, run a SELECT with the same WHERE, and check that it returns exactly the rows you mean to change:

SELECT ProductId, ProductName, UnitPrice
FROM Products
WHERE ProductId = 15;

When the rows are right, change SELECT ... FROM to UPDATE ... SET, and keep the WHERE exactly as it was:

UPDATE Products
SET UnitPrice = UnitPrice * 1.10
WHERE ProductId = 15;

The (1 row affected) message should match the number of rows your SELECT returned. If it doesn't, stop and find out why. The next lesson in this chapter shows how a transaction gives you a way back.

Check yourself

2 questions to try on the lesson page.

So what

UPDATE is only as safe as its WHERE. Test the WHERE with a SELECT, change only what you meant to change, and check the row count every time.

Key terms

Cheat sheets

Updating Data