UPDATE with WHERE condition in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the UPDATE statement do in SQL?

The UPDATE statement changes existing data in a table by modifying one or more columns for rows that match a condition.

beginner

Why is the WHERE clause important in an UPDATE statement?

The WHERE clause limits which rows get updated. Without it, all rows in the table will be changed.

beginner

Write the basic syntax of an UPDATE statement with a WHERE condition.

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

intermediate

What happens if you run UPDATE employees SET salary = 5000; without a WHERE clause?

All rows in the employees table will have their salary set to 5000, which might not be what you want.

beginner

How can you update only the rows where the employee's department is 'Sales'?

Use a WHERE clause to specify the condition:

UPDATE employees SET salary = 6000 WHERE department = 'Sales';

What does the WHERE clause do in an UPDATE statement?

A Deletes rows from the table

B Specifies new column names

C Creates a new table

D Limits which rows are updated

The WHERE clause filters rows so only matching rows get updated.

What happens if you omit the WHERE clause in an UPDATE statement?

A All rows in the table are updated

B No rows are updated

C Only one row is updated

D The table is deleted

Without WHERE, the update applies to every row in the table.

Which SQL keyword is used to specify the new values in an UPDATE statement?

A WHERE

B FROM

C SET

D SELECT

The SET keyword defines the columns and their new values.

How would you update the 'status' column to 'active' only for users with 'id' 10?

A SELECT status FROM users WHERE id = 10;

B UPDATE users SET status = 'active' WHERE id = 10;

C UPDATE users SET status = 'active';

D UPDATE users WHERE id = 10 SET status = 'active';

The correct syntax places SET before WHERE.

Which of these is a valid UPDATE statement?

A UPDATE products SET price = 20 WHERE id = 5;

B UPDATE products price = 20 WHERE id = 5;

C UPDATE products SET price 20 WHERE id = 5;

D UPDATE products WHERE id = 5 SET price = 20;

Option A uses correct syntax: UPDATE table SET column = value WHERE condition;

Explain how the UPDATE statement works with a WHERE condition in SQL.

Think about how you tell SQL which rows to change and what to change.

Describe what could happen if you forget the WHERE clause in an UPDATE statement.

Imagine changing every item in a list by mistake.

Practice

1.

What does the WHERE clause do in an UPDATE statement?

easy

A. It adds new rows to the table.

B. It changes the table structure.

C. It limits which rows get updated.

D. It deletes rows from the table.

Solution

1. Step 1: Understand the purpose of UPDATE

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

2. Step 2: Role of WHERE clause in UPDATE

   The WHERE clause specifies which rows should be changed, so only those rows are updated.

3. Final Answer:

   It limits which rows get updated. -> Option C

4. Quick Check:

   WHERE clause = row filter [OK]

Hint: WHERE filters rows to update only those needed [OK]

Common Mistakes:

• Thinking WHERE adds or deletes rows
• Believing UPDATE changes table structure
• Ignoring WHERE updates all rows

2.

Which of the following is the correct syntax to update the salary to 5000 for employees with id 3?

UPDATE employees SET salary = 5000 WHERE id = 3;

easy

A. UPDATE employees SET salary 5000 WHERE id = 3;

B. UPDATE employees salary = 5000 WHERE id = 3;

C. UPDATE employees SET salary = 5000 id = 3;

D. UPDATE employees SET salary = 5000 WHERE id = 3;

Solution

1. Step 1: Check UPDATE syntax

   The correct syntax requires SET before column assignment and WHERE for condition.

2. Step 2: Validate each option

   UPDATE employees SET salary = 5000 WHERE id = 3; uses SET and WHERE correctly; others miss SET or WHERE or have wrong syntax.

3. Final Answer:

   UPDATE employees SET salary = 5000 WHERE id = 3; -> Option D

4. Quick Check:

   SET + WHERE = correct UPDATE syntax [OK]

Hint: Always use SET before column assignments in UPDATE [OK]

Common Mistakes:

• Omitting SET keyword
• Forgetting WHERE clause
• Missing equal sign (=) in assignment

3.

Given the table products:

id | name    | price
---+---------+-------
1  | Pen     | 10
2  | Pencil  | 5
3  | Eraser  | 3

What will be the result of this query?

UPDATE products SET price = price + 2 WHERE price < 10;

medium

A. All products will have their price increased by 2.

B. Only products with price less than 10 will have their price increased by 2.

C. No products will be updated because price is not changed.

D. Syntax error due to missing WHERE clause.

Solution

1. Step 1: Identify rows matching WHERE condition

   Prices less than 10 are 5 and 3 (Pencil and Eraser).

2. Step 2: Apply UPDATE only to matching rows

   Only Pencil and Eraser prices increase by 2, Pen remains 10.

3. Final Answer:

   Only products with price less than 10 will have their price increased by 2. -> Option B

4. Quick Check:

   WHERE filters rows updated [OK]

Hint: WHERE limits which rows get updated, others stay same [OK]

Common Mistakes:

• Assuming all rows update without WHERE
• Thinking price must be reset, not incremented
• Confusing syntax error with correct query

4.

Find the error in this query:

UPDATE employees SET salary = 6000 WHERE;

medium

A. Missing condition after WHERE clause.

B. Missing SET keyword.

C. Incorrect table name.

D. No error, query is correct.

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause is incomplete; it must have a condition after it.

2. Step 2: Check other parts of query

   SET keyword and table name are correct; only WHERE is incomplete.

3. Final Answer:

   Missing condition after WHERE clause. -> Option A

4. Quick Check:

   WHERE needs a condition [OK]

Hint: WHERE must always have a condition after it [OK]

Common Mistakes:

• Leaving WHERE empty
• Confusing missing SET with WHERE error
• Assuming query runs without WHERE condition

5.

You have a table students with columns id, name, and grade. You want to increase the grade by 5 for all students who scored less than 50, but not exceed 50 after update. Which query correctly does this?

hard

A. UPDATE students SET grade = CASE WHEN grade + 5 > 50 THEN 50 ELSE grade + 5 END WHERE grade < 50;

B. UPDATE students SET grade = grade + 5 WHERE grade < 50 AND grade + 5 <= 50;

C. UPDATE students SET grade = grade + 5 WHERE grade < 50;

D. UPDATE students SET grade = 50 WHERE grade < 50;

Solution

1. Step 1: Understand the requirement

   Increase grade by 5 only for grades less than 50, but cap max grade at 50.

2. Step 2: Analyze each option

   UPDATE students SET grade = 50 WHERE grade < 50; sets all below 50 to 50 directly, no increase by 5. UPDATE students SET grade = grade + 5 WHERE grade < 50 AND grade + 5 <= 50; increases by 5 only if result ≤ 50, skipping some rows. UPDATE students SET grade = grade + 5 WHERE grade < 50; increases by 5 without capping at 50. UPDATE students SET grade = CASE WHEN grade + 5 > 50 THEN 50 ELSE grade + 5 END WHERE grade < 50; uses CASE to increase by 5 but caps at 50 correctly.

3. Final Answer:

   UPDATE students SET grade = CASE WHEN grade + 5 > 50 THEN 50 ELSE grade + 5 END WHERE grade < 50; -> Option A

4. Quick Check:

   Use CASE to cap values in UPDATE [OK]

Hint: Use CASE in UPDATE to handle conditional value changes [OK]

Common Mistakes:

• Not capping grade at 50
• Updating all rows without WHERE
• Skipping rows that would exceed 50