WHERE with OR operator in SQL - Time & Space Complexity

Time Complexity: WHERE with OR operator

O(n)

Understanding Time Complexity

We want to understand how the time to run a SQL query changes when using the WHERE clause with an OR operator.

Specifically, how does the query time grow as the table gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following SQL query.

SELECT *
FROM employees
WHERE department = 'Sales'
   OR department = 'Marketing';

This query selects all employees who work in either Sales or Marketing departments.

Identify Repeating Operations

Look for repeated checks or scans in the query.

• Primary operation: Scanning each row in the employees table to check the department.
• How many times: Once for each row in the table.

How Execution Grows With Input

As the number of employees grows, the database checks more rows.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 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 run the query grows in a straight line as the table gets bigger.

Common Mistake

[X] Wrong: "Using OR makes the query twice as slow because it checks two conditions separately."

[OK] Correct: The database still scans each row once and checks both conditions together, so it does not double the work.

Interview Connect

Understanding how conditions like OR affect query time helps you explain your reasoning clearly in interviews.

Self-Check

What if we changed OR to AND in the WHERE clause? How would the time complexity change?

Practice

1. What does the SQL WHERE clause with the OR operator do?

easy

A. It deletes rows that meet any condition.

B. It selects rows that meet at least one of the conditions.

C. It selects rows that meet all conditions simultaneously.

D. It updates rows that meet none of the conditions.

Solution

1. Step 1: Understand the WHERE clause

   The WHERE clause filters rows based on conditions.

2. Step 2: Understand the OR operator

   OR means if any condition is true, the row is selected.

3. Final Answer:

   It selects rows that meet at least one of the conditions. -> Option B

4. Quick Check:

   WHERE with OR = any condition true [OK]

Hint: OR means any condition true selects the row [OK]

Common Mistakes:

• Thinking OR requires all conditions true
• Confusing OR with AND
• Assuming OR deletes or updates rows

2. Which of the following SQL queries correctly uses the WHERE clause with the OR operator to find employees in department 10 or 20?

easy

A. SELECT * FROM employees WHERE department_id = 10 AND 20;

B. SELECT * FROM employees WHERE department_id = (10 OR 20);

C. SELECT * FROM employees WHERE department_id = 10 OR department_id = 20;

D. SELECT * FROM employees WHERE department_id IN 10 OR 20;

Solution

1. Step 1: Check correct OR syntax

   OR must connect full conditions: column = value OR column = value.

2. Step 2: Analyze each option

   A uses AND incorrectly; C uses invalid syntax; D misuses IN without parentheses.

3. Final Answer:

   SELECT * FROM employees WHERE department_id = 10 OR department_id = 20; -> Option C

4. Quick Check:

   Correct OR syntax = SELECT * FROM employees WHERE department_id = 10 OR department_id = 20; [OK]

Hint: Use OR between full conditions, not partial expressions [OK]

Common Mistakes:

• Using AND instead of OR
• Writing OR inside parentheses incorrectly
• Misusing IN without parentheses

3. Given the table products with columns id, name, and category, what rows will this query return?

SELECT * FROM products WHERE category = 'Books' OR category = 'Toys';

medium

A. Products in either 'Books' or 'Toys' categories.

B. Only products in both 'Books' and 'Toys' categories.

C. All products except those in 'Books' or 'Toys'.

D. No products because OR is invalid here.

Solution

1. Step 1: Understand the OR condition

   The query selects rows where category is 'Books' or 'Toys'.

2. Step 2: Determine matching rows

   Rows with category 'Books' or 'Toys' will be included, others excluded.

3. Final Answer:

   Products in either 'Books' or 'Toys' categories. -> Option A

4. Quick Check:

   OR selects rows matching any condition [OK]

Hint: OR includes rows matching any listed condition [OK]

Common Mistakes:

• Thinking OR means both conditions must be true
• Assuming OR excludes matching rows
• Believing OR causes syntax error

4. Identify the error in this SQL query:

SELECT * FROM orders WHERE status = 'Pending' OR;

medium

A. Missing condition after OR operator.

B. Incorrect use of WHERE clause.

C. OR operator should be AND.

D. No error, query is valid.

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause ends with OR but no condition follows it.

2. Step 2: Understand OR syntax

   OR must connect two conditions; missing second condition causes syntax error.

3. Final Answer:

   Missing condition after OR operator. -> Option A

4. Quick Check:

   OR needs two conditions [OK]

Hint: OR must connect two conditions; don't leave it hanging [OK]

Common Mistakes:

• Leaving OR at end without condition
• Confusing OR with AND syntax
• Ignoring syntax errors in WHERE clause

5. You have a table students with columns id, grade, and city. Write a query to select students who have grade 'A' or live in 'New York' or 'Los Angeles'. Which query is correct?

hard

A. SELECT * FROM students WHERE grade = 'A' OR city = 'New York' OR 'Los Angeles';

B. SELECT * FROM students WHERE grade = 'A' AND city = 'New York' OR 'Los Angeles';

C. SELECT * FROM students WHERE grade = 'A' OR city = 'New York' AND city = 'Los Angeles';

D. SELECT * FROM students WHERE grade = 'A' OR city IN ('New York', 'Los Angeles');

Solution

1. Step 1: Understand the conditions

   Select students with grade 'A' OR city 'New York' OR city 'Los Angeles'.

2. Step 2: Analyze each query

   A has syntax error (missing column in last condition); B has syntax error and wrong logic; C has incorrect logic due to AND; D is correct and concise using IN.

3. Final Answer:

   SELECT * FROM students WHERE grade = 'A' OR city IN ('New York', 'Los Angeles'); -> Option D

4. Quick Check:

   Use OR with IN for multiple values [OK]

Hint: Use OR with IN for multiple city values [OK]

Common Mistakes:

• Using AND between city conditions
• Omitting column name after OR
• Syntax errors with OR and IN