Comparison operators in MySQL - Time & Space Complexity

Time Complexity: Comparison operators

O(n)

Understanding Time Complexity

When using comparison operators in SQL, it's important to know how the time to run queries changes as data grows.

We want to see how the number of rows affects the work done when comparing values.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT *
FROM employees
WHERE salary > 50000;
    

This query selects all employees whose salary is greater than 50,000 using a comparison operator.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking the salary value of each employee row against 50,000.
• How many times: Once for every row in the employees table.

How Execution Grows With Input

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

Input Size (n)	Approx. Operations
10	10 comparisons
100	100 comparisons
1000	1000 comparisons

Pattern observation: The work grows directly with the number of rows; doubling rows doubles comparisons.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows linearly with the number of rows in the table.

Common Mistake

[X] Wrong: "Comparison operators instantly find matching rows regardless of table size."

[OK] Correct: Without indexes, the database must check each row one by one, so more rows mean more work.

Interview Connect

Understanding how comparisons scale helps you explain query performance clearly and shows you know how databases handle data.

Self-Check

"What if we added an index on the salary column? How would the time complexity change?"

Practice

1. Which of the following comparison operators in MySQL means "not equal to"?

easy

A. =

B. <>

C. >=

D. <

Solution

1. Step 1: Understand the meaning of comparison operators

   The operator "<>" is used in SQL to mean "not equal to".

2. Step 2: Compare with other operators

   "=" means equal, ">=" means greater than or equal, and "<" means less than.

3. Final Answer:

   <> -> Option B

4. Quick Check:

   Not equal operator = <> [OK]

Hint: Remember: <> means not equal in SQL [OK]

Common Mistakes:

• Confusing = with not equal
• Using != which is valid but less common in MySQL
• Mixing up > and < operators

2. Which of the following is the correct syntax to check if column age is greater than 18 in a WHERE clause?

easy

A. WHERE age > 18

B. WHERE age => 18

C. WHERE age =< 18

D. WHERE age <> 18

Solution

1. Step 1: Identify the correct greater than operator

   The correct operator for "greater than" is ">". The operator ">=" means "greater than or equal to".

2. Step 2: Check syntax correctness

   "=>" and "=<" are invalid operators in SQL syntax. "<>" means not equal, which is not what we want.

3. Final Answer:

   WHERE age > 18 -> Option A

4. Quick Check:

   Greater than operator is > [OK]

Hint: Use > for greater than, not => or =< [OK]

Common Mistakes:

• Using => or =< which are invalid
• Confusing <> with greater than
• Using = instead of >

3. Given the table Employees with a column salary, what will the query SELECT * FROM Employees WHERE salary <= 50000; return?

medium

A. All employees with salary less than or equal to 50000

B. All employees with salary exactly 50000

C. All employees with salary less than 50000

D. All employees with salary greater than 50000

Solution

1. Step 1: Understand the meaning of <= operator

   The operator <= means "less than or equal to".

2. Step 2: Interpret the query condition

   The query filters employees whose salary is either less than 50000 or exactly 50000.

3. Final Answer:

   All employees with salary less than or equal to 50000 -> Option A

4. Quick Check:

   <= means less than or equal [OK]

Hint: <= means less than or equal, includes the number [OK]

Common Mistakes:

• Thinking <= means only less than
• Confusing <= with <
• Ignoring the equal part of <= operator

4. Identify the error in this MySQL query: SELECT * FROM Products WHERE price ! 100;

medium

A. Missing comparison operator after !

B. No error, query is correct

C. Missing quotes around 100

D. Incorrect use of ! for not equal, should be != or <>

Solution

1. Step 1: Analyze the use of ! operator

   In MySQL, "!" alone is not a valid comparison operator. For "not equal", use "!=" or "<>".

2. Step 2: Check the query syntax

   The query uses "price ! 100" which is invalid syntax and will cause an error.

3. Final Answer:

   Incorrect use of ! for not equal, should be != or <> -> Option D

4. Quick Check:

   Use != or <> for not equal [OK]

Hint: Use != or <> for not equal, not just ! [OK]

Common Mistakes:

• Using ! alone as not equal
• Forgetting to use != or <>
• Assuming ! means not equal in SQL

5. You want to select all customers whose age is between 20 and 30 inclusive. Which WHERE clause correctly uses comparison operators?

hard

A. WHERE age > 20 AND age < 30

B. WHERE age = 20 OR age = 30

C. WHERE age >= 20 AND age <= 30

D. WHERE age <> 20 AND age <> 30

Solution

1. Step 1: Understand the requirement for inclusive range

   Inclusive means including 20 and 30, so use >= and <= operators.

2. Step 2: Analyze each option

   WHERE age > 20 AND age < 30 excludes 20 and 30 (strictly greater and less). WHERE age = 20 OR age = 30 only selects exactly 20 or 30, not the range. WHERE age <> 20 AND age <> 30 excludes 20 and 30, opposite of requirement.

3. Final Answer:

   WHERE age >= 20 AND age <= 30 -> Option C

4. Quick Check:

   Use >= and <= for inclusive ranges [OK]

Hint: Use >= and <= for inclusive range checks [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using OR instead of AND for range
• Using <> which excludes values