Why DELETE needs caution in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete all rows from the table named 'employees'.

SQL

DELETE FROM [1];

Drag options to blanks, or click blank then click option'

Aemp

Bemployee

Cemployees

Dstaff

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong table name causes an error or deletes nothing.

Misspelling the table name.

The correct table name is 'employees'. Using the exact table name is important to delete the intended data.

2fill in blank

medium

Complete the code to delete only employees with the job title 'Intern'.

SQL

DELETE FROM employees WHERE job_title = [1];

Drag options to blanks, or click blank then click option'

A'Intern'

B'Manager'

C'Engineer'

D'Clerk'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around text values.

Deleting the wrong job title.

We want to delete only rows where the job_title is 'Intern'. The value must be in quotes because it is text.

3fill in blank

hard

Fix the error in the DELETE statement to remove employees hired before 2020.

SQL

DELETE FROM employees WHERE hire_date [1] '2020-01-01';

Drag options to blanks, or click blank then click option'

A>

B<

C=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than deletes employees hired after 2020.

Using equal deletes only those hired exactly on 2020-01-01.

To delete employees hired before 2020, use the less than operator (<) comparing hire_date to '2020-01-01'.

4fill in blank

hard

Fill both blanks to delete employees from the 'sales' department hired after 2018.

SQL

DELETE FROM employees WHERE department = [1] AND hire_date [2] '2018-12-31';

Drag options to blanks, or click blank then click option'

A'sales'

B'marketing'

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong department name.

Using less than or equal operator deletes wrong rows.

We delete employees in the 'sales' department (text in quotes) hired after 2018 (use > operator).

5fill in blank

hard

Fill all three blanks to delete employees with salary less than 30000 and job title 'Assistant'.

SQL

DELETE FROM [1] WHERE salary [2] [3] AND job_title = 'Assistant';

Drag options to blanks, or click blank then click option'

Aemployees

B<

C30000

Dstaff

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong table name.

Using quotes around salary number.

Using wrong comparison operator.

We delete from 'employees' table where salary is less than 30000 and job title is 'Assistant'.

Practice

1. Why should you be careful when using the DELETE statement in SQL?

easy

A. Because it creates a backup automatically

B. Because it only hides rows temporarily

C. Because it permanently removes rows from the table

D. Because it only works on empty tables

Solution

1. Step 1: Understand what DELETE does

   The DELETE statement removes rows from a table permanently, not temporarily.

2. Step 2: Consider the impact of DELETE

   Since rows are permanently removed, careless use can cause data loss.

3. Final Answer:

   Because it permanently removes rows from the table -> Option C

4. Quick Check:

   DELETE = permanent removal [OK]

Hint: Remember DELETE removes data permanently, no undo [OK]

Common Mistakes:

• Thinking DELETE only hides data
• Assuming DELETE creates backups
• Believing DELETE works only on empty tables

2. Which of the following is the correct syntax to delete rows from a table named employees where the department is 'Sales'?

easy

A. DELETE * FROM employees WHERE department = 'Sales';

B. REMOVE FROM employees WHERE department = 'Sales';

C. DELETE employees WHERE department = 'Sales';

D. DELETE FROM employees WHERE department = 'Sales';

Solution

1. Step 1: Recall correct DELETE syntax

   The correct syntax is DELETE FROM table_name WHERE condition;

2. Step 2: Check each option

   DELETE FROM employees WHERE department = 'Sales'; matches the correct syntax exactly. The other options have syntax errors.

3. Final Answer:

   DELETE FROM employees WHERE department = 'Sales'; -> Option D

4. Quick Check:

   Correct DELETE syntax = DELETE FROM employees WHERE department = 'Sales'; [OK]

Hint: Use DELETE FROM table WHERE condition; [OK]

Common Mistakes:

• Using REMOVE instead of DELETE
• Omitting FROM keyword
• Using DELETE * which is invalid

3. Given the table products with 5 rows, what will be the result of this query?

DELETE FROM products;

medium

A. Deletes all 5 rows from the products table

B. Deletes no rows because no WHERE clause is given

C. Deletes only the first row

D. Causes a syntax error

Solution

1. Step 1: Understand DELETE without WHERE

   DELETE without a WHERE clause removes all rows from the table.

2. Step 2: Apply to products table

   Since products has 5 rows, all 5 will be deleted.

3. Final Answer:

   Deletes all 5 rows from the products table -> Option A

4. Quick Check:

   DELETE without WHERE = all rows deleted [OK]

Hint: No WHERE means delete all rows [OK]

Common Mistakes:

• Thinking DELETE without WHERE deletes no rows
• Assuming only one row is deleted
• Expecting a syntax error

4. Identify the error in this DELETE statement:

DELETE employees WHERE id = 10;

medium

A. id cannot be used in WHERE clause

B. Missing FROM keyword after DELETE

C. WHERE clause is not allowed in DELETE

D. Incorrect table name

Solution

1. Step 1: Review DELETE syntax

   The correct syntax requires FROM keyword: DELETE FROM table WHERE condition;

2. Step 2: Check the given statement

   The statement misses FROM after DELETE, causing a syntax error.

3. Final Answer:

   Missing FROM keyword after DELETE -> Option B

4. Quick Check:

   DELETE needs FROM keyword [OK]

Hint: Always use DELETE FROM, not DELETE alone [OK]

Common Mistakes:

• Omitting FROM keyword
• Thinking WHERE is not allowed
• Assuming id cannot be used in WHERE

5. You want to delete only inactive users from the users table, but accidentally run:

DELETE FROM users;

What is the best way to avoid this mistake in the future?

hard

A. Always run DELETE inside a transaction and review before commit

B. Use DELETE without WHERE to remove all users quickly

C. Avoid using DELETE and only use UPDATE to mark inactive

D. Delete the entire database to start fresh

Solution

1. Step 1: Understand the risk of DELETE without WHERE

   Running DELETE without WHERE removes all rows, causing data loss.

2. Step 2: Learn best practice to prevent mistakes

   Using transactions allows you to review changes and rollback if needed before final commit.

3. Final Answer:

   Always run DELETE inside a transaction and review before commit -> Option A

4. Quick Check:

   Use transactions to avoid accidental deletes [OK]

Hint: Use transactions to review DELETE before finalizing [OK]

Common Mistakes:

• Thinking DELETE without WHERE is safe
• Avoiding DELETE completely instead of careful use
• Deleting entire database as a solution