NULL in AND, OR, NOT logic in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where the condition is TRUE or UNKNOWN using OR.

SQL

SELECT * FROM employees WHERE active = TRUE [1] salary > 50000;

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

ANOT

B=

COR

DAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND instead of OR causes rows with NULL to be excluded.

Using = instead of logical operators.

Using OR returns TRUE if either condition is TRUE or UNKNOWN (NULL treated as UNKNOWN).

2fill in blank

medium

Complete the code to find rows where NOT condition is TRUE.

SQL

SELECT * FROM orders WHERE NOT (shipped_date IS NULL) [1] status = 'pending';

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

ANOT

B=

COR

DAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR causes rows to be included even if NOT condition is FALSE.

Using NOT again causes double negation.

Use AND to combine NOT condition with another condition that must also be TRUE.

3fill in blank

hard

Fix the error in the WHERE clause to correctly handle NULL with AND.

SQL

SELECT * FROM products WHERE price > 100 [1] discount IS NULL;

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

AAND

BOR

CNOT

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR includes rows where price > 100 but discount is not NULL.

Using = causes syntax error with IS NULL.

Use AND to ensure both conditions must be TRUE, including NULL check.

4fill in blank

hard

Fill both blanks to select rows where either condition is TRUE or the other is NOT TRUE.

SQL

SELECT * FROM sales WHERE region = 'East' [1] amount > 1000 [2] NOT (discount IS NULL);

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

AOR

BAND

CAND NOT

DNOT

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND in first blank excludes rows unnecessarily.

Using NOT alone causes syntax errors.

Use OR to allow either condition, and AND to combine with NOT condition properly.

5fill in blank

hard

Fill all three blanks to create a condition that excludes NULLs and selects rows with amount > 500 or status 'active'.

SQL

SELECT * FROM transactions WHERE amount IS NOT [1] AND (amount [2] 500 [3] status = 'active');

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

ANULL

B>

COR

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = NULL instead of IS NOT NULL.

Using AND instead of OR in the last blank.

Use IS NOT NULL to exclude NULLs, > to compare amount, and OR to allow status 'active'.

Practice

1. What is the result of the SQL expression TRUE AND NULL?

easy

A. TRUE

B. NULL

C. FALSE

D. Error

Solution

1. Step 1: Understand AND with NULL

   In SQL, AND returns FALSE if any side is FALSE. If one side is TRUE and the other is NULL, the result is NULL because the unknown value affects the outcome.

2. Step 2: Apply to TRUE AND NULL

   Since the first operand is TRUE and the second is NULL, the result is NULL.

3. Final Answer:

   NULL -> Option B

4. Quick Check:

   TRUE AND NULL = NULL [OK]

Hint: AND with NULL returns NULL unless FALSE present [OK]

Common Mistakes:

• Assuming TRUE AND NULL is TRUE
• Confusing NULL with FALSE
• Thinking NULL causes error

2. Which of the following is the correct SQL syntax to check if a column col is NOT NULL?

easy

A. col != NULL

B. col <> NULL

C. col IS NOT NULL

D. NOT col = NULL

Solution

1. Step 1: Recall NULL comparison rules

   In SQL, you cannot use = or != to compare with NULL. Instead, use IS NULL or IS NOT NULL.

2. Step 2: Identify correct syntax

   The correct way to check if col is not null is col IS NOT NULL.

3. Final Answer:

   col IS NOT NULL -> Option C

4. Quick Check:

   Use IS NOT NULL for null checks [OK]

Hint: Use IS NOT NULL, not != or <> [OK]

Common Mistakes:

• Using != or <> to compare with NULL
• Using NOT col = NULL which is invalid
• Confusing NULL with empty string

3. Given a table Test with a column flag that can be TRUE, FALSE, or NULL, what is the result of this query?

SELECT * FROM Test WHERE flag OR FALSE;

medium

A. Rows where flag is TRUE

B. Rows where flag is TRUE or NULL

C. Rows where flag is TRUE or FALSE

D. No rows returned

Solution

1. Step 1: Understand OR with FALSE

   In SQL, OR FALSE returns the value of the first operand because FALSE does not affect the OR result.

2. Step 2: Evaluate flag OR FALSE

   If flag is TRUE, the expression is TRUE. If flag is FALSE, expression is FALSE. If flag is NULL, expression is NULL (unknown), which is treated as FALSE in WHERE filtering.

3. Final Answer:

   Rows where flag is TRUE -> Option A

4. Quick Check:

   flag OR FALSE = flag; WHERE filters TRUE only [OK]

Hint: OR FALSE returns first operand value [OK]

Common Mistakes:

• Including NULL rows in result
• Thinking OR FALSE always returns TRUE
• Confusing OR with AND behavior

4. Consider this SQL WHERE clause:

WHERE NOT (col = NULL)

What is the problem with this condition?

medium

A. It always returns NULL (no rows match)

B. It always returns TRUE

C. It always returns FALSE

D. It causes a syntax error

Solution

1. Step 1: Understand comparison with NULL

   In SQL, col = NULL always returns NULL (unknown), never TRUE or FALSE.

2. Step 2: Apply NOT to NULL

   NOT NULL is still NULL, so the condition never evaluates to TRUE, filtering out all rows.

3. Final Answer:

   It always returns NULL (no rows match) -> Option A

4. Quick Check:

   NOT (col = NULL) = NULL, no matches [OK]

Hint: NOT NULL remains NULL, no rows pass filter [OK]

Common Mistakes:

• Expecting NOT NULL to be TRUE
• Using = NULL instead of IS NULL
• Assuming syntax error occurs

5. You want to select rows from a table Employees where the ManagerID is either NULL or the employee is active (Active = TRUE). Which WHERE clause correctly handles NULL logic?

hard

A. WHERE ManagerID IS NOT NULL OR Active = TRUE

B. WHERE ManagerID = NULL OR Active = TRUE

C. WHERE NOT ManagerID = NULL AND Active = TRUE

D. WHERE ManagerID IS NULL OR Active = TRUE

Solution

1. Step 1: Correctly check for NULL

   Use IS NULL to check if ManagerID is NULL. Using = NULL is invalid and returns NULL.

2. Step 2: Combine conditions with OR

   The condition requires rows where ManagerID is NULL or Active is TRUE, so use OR between these checks.

3. Final Answer:

   WHERE ManagerID IS NULL OR Active = TRUE -> Option D

4. Quick Check:

   Use IS NULL and OR for correct logic [OK]

Hint: Use IS NULL, combine with OR for either condition [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using AND instead of OR
• Using IS NOT NULL which is opposite