UPDATE single column in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL UPDATE statement do?

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

beginner

How do you update a single column in SQL?

Use UPDATE table_name SET column_name = new_value WHERE condition; to change one column's value in rows that meet the 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 an example SQL query to update the salary column to 5000 for employees with id 3.

Example:
UPDATE employees SET salary = 5000 WHERE id = 3;

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.

Which SQL keyword is used to change data in a table?

A SELECT

B UPDATE

C INSERT

D DELETE

The UPDATE keyword modifies existing data in a table.

What does the WHERE clause do in an UPDATE statement?

A Specifies which rows to update

B Specifies which columns to update

C Creates a new table

D Deletes rows

The WHERE clause limits the rows affected by the update.

How do you update only the price column in a table named products?

A UPDATE products SET price = new_value;

B UPDATE products WHERE price = new_value;

C SELECT price FROM products;

D UPDATE products SET price = new_value WHERE condition;

You must use SET to specify the column and WHERE to limit rows.

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

A All rows are updated

B Only one row is updated

C No rows are updated

D The query fails

Without WHERE, all rows in the table are updated.

Which of these is a valid SQL update statement for a single column?

A UPDATE table SET (column = value);

B UPDATE table WHERE id = 1 SET column = value;

C UPDATE table SET column = value WHERE id = 1;

D UPDATE table (column) VALUES (value);

The correct syntax is UPDATE table SET column = value WHERE condition;.

Explain how to update a single column in a SQL table safely.

Think about how to change only certain rows without affecting the whole table.

What could happen if you forget the WHERE clause in an UPDATE statement?

Consider the impact on the entire table.

Practice

1. What does the SQL statement UPDATE employees SET salary = 5000 WHERE id = 3; do?

easy

A. It changes the salary to 5000 for the employee with id 3.

B. It adds a new employee with salary 5000 and id 3.

C. It deletes the employee with id 3.

D. It shows the salary of employee with id 3.

Solution

1. Step 1: Understand the UPDATE statement

   The UPDATE command changes existing data in a table.

2. Step 2: Analyze the SET and WHERE clauses

   SET salary = 5000 changes the salary column, WHERE id = 3 limits the change to the employee with id 3.

3. Final Answer:

   It changes the salary to 5000 for the employee with id 3. -> Option A

4. Quick Check:

   UPDATE with SET and WHERE changes specified rows [OK]

Hint: UPDATE changes data; SET picks column; WHERE picks rows [OK]

Common Mistakes:

• Thinking UPDATE adds new rows
• Forgetting WHERE updates all rows
• Confusing UPDATE with SELECT

2. Which of the following is the correct syntax to update the price column to 20 in the products table for product with id 10?

easy

A. UPDATE products SET price = 20 WHERE id = 10;

B. UPDATE products price = 20 WHERE id = 10;

C. UPDATE products SET price WHERE id = 10 = 20;

D. UPDATE products SET price = 20;

Solution

1. Step 1: Check the UPDATE syntax

   The correct syntax is UPDATE table_name SET column = value WHERE condition;

2. Step 2: Validate each option

   UPDATE products SET price = 20 WHERE id = 10; matches the correct syntax with SET and WHERE clauses properly used.

3. Final Answer:

   UPDATE products SET price = 20 WHERE id = 10; -> Option A

4. Quick Check:

   Correct UPDATE syntax uses SET column = value and WHERE [OK]

Hint: Always use SET column = value and WHERE condition [OK]

Common Mistakes:

• Omitting SET keyword
• Placing WHERE before SET
• Missing semicolon at end

3. Given the table students with columns id, name, and grade, what will be the result of this query?

UPDATE students SET grade = 90 WHERE name = 'Alice';

Assuming the table before update:

id | name  | grade
1  | Alice | 85
2  | Bob   | 88
3  | Alice | 92

medium

A. No rows will be updated because name is not unique.

B. Only the first row with name 'Alice' will be updated.

C. Both rows with name 'Alice' will have grade updated to 90.

D. All rows in the table will have grade set to 90.

Solution

1. Step 1: Understand the WHERE clause effect

   WHERE name = 'Alice' matches all rows where name is exactly 'Alice'.

2. Step 2: Apply UPDATE to matching rows

   Both rows with name 'Alice' (id 1 and 3) will have their grade set to 90.

3. Final Answer:

   Both rows with name 'Alice' will have grade updated to 90. -> Option C

4. Quick Check:

   UPDATE affects all rows matching WHERE condition [OK]

Hint: WHERE filters all matching rows, not just one [OK]

Common Mistakes:

• Assuming only one row updates with non-unique WHERE
• Thinking UPDATE without WHERE updates one row
• Confusing UPDATE with SELECT

4. Identify the error in this SQL statement:

UPDATE orders SET status 'shipped' WHERE order_id = 101;

medium

A. Incorrect table name.

B. Missing equal sign (=) between column and value.

C. Using single quotes around column name.

D. Missing WHERE clause.

Solution

1. Step 1: Check SET clause syntax

   SET clause must have column = value format. Here, equal sign is missing.

2. Step 2: Verify other parts

   WHERE clause is present, table name looks correct, and no quotes around column name.

3. Final Answer:

   Missing equal sign (=) between column and value. -> Option B

4. Quick Check:

   SET requires column = value syntax [OK]

Hint: SET needs column = value, not just column value [OK]

Common Mistakes:

• Omitting equal sign in SET
• Forgetting WHERE causes all rows update
• Using quotes incorrectly around column names

5. You have a table inventory with columns item_id, quantity, and price. You want to increase the price by 10% only for items with quantity less than 5. Which SQL statement correctly updates the price column?

hard

A. UPDATE inventory SET price = price + 10 WHERE quantity < 5;

B. UPDATE inventory SET price = price + 0.10 WHERE quantity < 5;

C. UPDATE inventory SET price = price * 0.10 WHERE quantity < 5;

D. UPDATE inventory SET price = price * 1.10 WHERE quantity < 5;

Solution

1. Step 1: Understand the requirement

   Increase price by 10% means multiply price by 1.10.

2. Step 2: Check the WHERE condition

   Only update rows where quantity is less than 5.

3. Step 3: Evaluate each option

   UPDATE inventory SET price = price * 1.10 WHERE quantity < 5; correctly multiplies price by 1.10 with proper WHERE clause.

4. Final Answer:

   UPDATE inventory SET price = price * 1.10 WHERE quantity < 5; -> Option D

5. Quick Check:

   Multiply price by 1.10 to increase by 10% [OK]

Hint: Multiply price by 1.10 to increase by 10% [OK]

Common Mistakes:

• Adding 10 instead of 10%
• Multiplying by 0.10 reduces price
• Adding 0.10 increases price by 0.10 units only