UPDATE with WHERE clause in MySQL

Introduction

The UPDATE statement changes existing data in a table. The WHERE clause limits which rows get changed.

You want to fix a wrong phone number for a specific customer.

You need to mark only overdue invoices as 'late'.

You want to increase the price of a product only if it is in a certain category.

You want to update the status of orders placed before a certain date.

Syntax

MySQL

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

The WHERE clause is important to avoid changing all rows by mistake.

You can update one or many columns at the same time.

Examples

Updates salary only for the employee with ID 101.

MySQL

UPDATE employees
SET salary = 5000
WHERE employee_id = 101;

Increases price by 10% for all products in the 'Books' category.

MySQL

UPDATE products
SET price = price * 1.1
WHERE category = 'Books';

Sets status and shipped date for a specific order.

MySQL

UPDATE orders
SET status = 'shipped', shipped_date = '2024-06-01'
WHERE order_id = 2001;

Sample Program

This example creates a customers table, inserts three rows, then updates Bob's city to San Francisco. Finally, it shows all customers.

MySQL

CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  city VARCHAR(50)
);

INSERT INTO customers (id, name, city) VALUES
(1, 'Alice', 'New York'),
(2, 'Bob', 'Los Angeles'),
(3, 'Charlie', 'Chicago');

UPDATE customers
SET city = 'San Francisco'
WHERE name = 'Bob';

SELECT * FROM customers ORDER BY id;

Important Notes

Always double-check your WHERE clause to avoid updating unintended rows.

If you omit the WHERE clause, all rows in the table will be updated.

You can use multiple conditions in WHERE with AND/OR to be more specific.

Summary

UPDATE changes data in existing rows.

WHERE clause controls which rows get updated.

Be careful: no WHERE means update all rows.

Practice

1. What does the WHERE clause do in an UPDATE statement in MySQL?

easy

A. It specifies which rows should be updated.

B. It creates a new table.

C. It deletes rows from the table.

D. It selects rows to display.

Solution

1. Step 1: Understand the purpose of UPDATE

   The UPDATE command changes data in existing rows of a table.

2. Step 2: Role of WHERE clause

   The WHERE clause limits which rows get changed. Without it, all rows update.

3. Final Answer:

   It specifies which rows should be updated. -> Option A

4. Quick Check:

   WHERE limits rows to update [OK]

Hint: WHERE filters rows to update; no WHERE updates all [OK]

Common Mistakes:

• Thinking WHERE creates or deletes rows
• Confusing UPDATE with SELECT
• Ignoring WHERE causes all rows to update

2. Which of the following is the correct syntax to update the salary to 5000 for employees with id 10 in MySQL?

easy

A. UPDATE employees SET salary = 5000 AND id = 10;

B. UPDATE employees SET salary = 5000 WHERE id = 10;

C. UPDATE employees SET salary WHERE id = 10 = 5000;

D. UPDATE employees WHERE id = 10 SET salary = 5000;

Solution

1. Step 1: Identify correct UPDATE syntax

   The correct order is: UPDATE table SET column = value WHERE condition.

2. Step 2: Check each option

   UPDATE employees SET salary = 5000 WHERE id = 10; follows correct syntax. Others have misplaced clauses or wrong operators.

3. Final Answer:

   UPDATE employees SET salary = 5000 WHERE id = 10; -> Option B

4. Quick Check:

   UPDATE SET column=value WHERE condition [OK]

Hint: SET comes before WHERE in UPDATE statements [OK]

Common Mistakes:

• Placing WHERE before SET
• Using AND instead of WHERE
• Incorrect assignment syntax

3. Given the table products with columns id, name, and price, what will be the price of product with id = 3 after running this query?

UPDATE products SET price = price + 10 WHERE id = 3;

Assume the original price for id = 3 was 50.

medium

A. 10

B. 50

C. 60

D. NULL

Solution

1. Step 1: Understand the UPDATE operation

   The query adds 10 to the current price for the product where id = 3.

2. Step 2: Calculate new price

   Original price is 50, so 50 + 10 = 60.

3. Final Answer:

   60 -> Option C

4. Quick Check:

   50 + 10 = 60 [OK]

Hint: Add to existing value only for rows matching WHERE [OK]

Common Mistakes:

• Ignoring WHERE and updating all rows
• Setting price to 10 instead of adding
• Confusing original and updated values

4. Identify the error in this MySQL query:

UPDATE users SET age = 30 WHERE;

medium

A. SET keyword is misplaced.

B. Table name is missing.

C. UPDATE keyword is misspelled.

D. Missing condition after WHERE clause.

Solution

1. Step 1: Analyze the query structure

   The query has UPDATE, SET, and WHERE keywords but WHERE has no condition.

2. Step 2: Identify the error

   WHERE must be followed by a condition to specify which rows to update.

3. Final Answer:

   Missing condition after WHERE clause. -> Option D

4. Quick Check:

   WHERE requires condition [OK]

Hint: WHERE must always have a condition [OK]

Common Mistakes:

• Leaving WHERE empty
• Misplacing SET and WHERE
• Forgetting semicolon

5. You have a table orders with columns order_id, status, and amount. You want to increase the amount by 20% only for orders with status 'pending' and amount less than 100. Which query correctly does this?

hard

A. UPDATE orders SET amount = amount * 1.2 WHERE status = 'pending' AND amount < 100;

B. UPDATE orders SET amount = amount + 20 WHERE status = 'pending' OR amount < 100;

C. UPDATE orders SET amount = amount * 1.2 WHERE status = 'pending' OR amount < 100;

D. UPDATE orders SET amount = amount * 0.2 WHERE status = 'pending' AND amount < 100;

Solution

1. Step 1: Understand the condition

   We want to update only rows where status is 'pending' AND amount is less than 100.

2. Step 2: Calculate new amount

   Increase by 20% means multiply amount by 1.2.

3. Step 3: Check options

   UPDATE orders SET amount = amount * 1.2 WHERE status = 'pending' AND amount < 100; correctly uses AND and multiplies amount by 1.2. Others use OR or wrong calculations.

4. Final Answer:

   UPDATE orders SET amount = amount * 1.2 WHERE status = 'pending' AND amount < 100; -> Option A

5. Quick Check:

   AND condition + *1.2 [OK]

Hint: Use AND for multiple conditions in WHERE [OK]

Common Mistakes:

• Using OR instead of AND
• Adding fixed 20 instead of 20%
• Multiplying by 0.2 instead of 1.2