NULL in AND, OR, NOT logic in SQL - Time & Space Complexity

Time Complexity: NULL in AND, OR, NOT logic

O(n)

Understanding Time Complexity

We want to understand how SQL evaluates logical expressions involving NULL values.

How does the presence of NULL affect the number of checks the database performs?

Scenario Under Consideration

Analyze the time complexity of evaluating logical expressions with NULL.

SELECT * FROM table_name
WHERE (column1 = 10 AND column2 IS NULL)
   OR (column3 > 5 AND NOT (column4 = 20));
    

This query filters rows based on combined AND, OR, and NOT conditions that include NULL checks.

Identify Repeating Operations

Look at what happens for each row in the table.

• Primary operation: Evaluating logical conditions (AND, OR, NOT) including NULL checks.
• How many times: Once per row, for each condition in the WHERE clause.

How Execution Grows With Input

Each row requires checking all conditions once.

Input Size (n)	Approx. Operations
10	About 10 sets of condition checks
100	About 100 sets of condition checks
1000	About 1000 sets of condition checks

Pattern observation: The number of checks grows directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to evaluate the query grows linearly with the number of rows.

Common Mistake

[X] Wrong: "NULL in logical expressions causes the query to run slower exponentially."

[OK] Correct: Each row is checked once; NULL just affects the logic result, not the number of checks.

Interview Connect

Understanding how NULL affects logical checks helps you reason about query performance and correctness in real databases.

Self-Check

"What if the query had nested subqueries with NULL checks? How would that affect time complexity?"

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