WHERE with comparison operators in SQL - Time & Space Complexity

Time Complexity: WHERE with comparison operators

O(n)

Understanding Time Complexity

When we use WHERE with comparison operators in SQL, the database checks each row to see if it matches the condition.

We want to understand how the time to run the query grows as the table gets bigger.

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.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

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

How Execution Grows With Input

As the number of rows grows, the database must check more salaries.

Input Size (n)	Approx. Operations
10	10 salary checks
100	100 salary checks
1000	1000 salary 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: "The database only looks at some rows, so the time stays the same no matter how big the table is."

[OK] Correct: Without special help like indexes, the database must check every row to be sure which ones match.

Interview Connect

Understanding how filtering with WHERE works helps you explain how databases handle data efficiently in real projects.

Self-Check

"What if we add an index on the Salary column? How would the time complexity change?"

Practice

1. What does the WHERE clause do in an SQL query?

easy

A. Filters rows based on a condition

B. Sorts rows in ascending order

C. Joins two tables together

D. Creates a new table

Solution

1. Step 1: Understand the purpose of WHERE

   The WHERE clause is used to select only those rows that meet a specific condition.

2. Step 2: Compare with other SQL clauses

   Sorting is done by ORDER BY, joining by JOIN, and creating tables by CREATE TABLE, so these are not correct.

3. Final Answer:

   Filters rows based on a condition -> Option A

4. Quick Check:

   WHERE filters rows = A [OK]

Hint: WHERE filters rows; ORDER BY sorts rows [OK]

Common Mistakes:

• Confusing WHERE with ORDER BY
• Thinking WHERE creates or joins tables
• Assuming WHERE changes table structure

2. Which of the following is the correct syntax to select rows where the column age is greater than 30?

easy

A. SELECT * FROM table WHERE age >= 30;

B. SELECT * FROM table WHERE age > 30;

C. SELECT * FROM table WHERE age >> 30;

D. SELECT * FROM table WHERE age = > 30;

Solution

1. Step 1: Identify correct comparison operator

   The operator for 'greater than' is >, so age > 30 is correct.

2. Step 2: Check syntax correctness

   The option with SELECT * FROM table WHERE age = > 30; has invalid operator '= >', the one with '>>' uses bitwise shift, and the one with '>=' means 'greater or equal to', not strictly greater.

3. Final Answer:

   SELECT * FROM table WHERE age > 30; -> Option B

4. Quick Check:

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

Hint: Use > for greater than, >= for greater or equal [OK]

Common Mistakes:

• Using '= >' instead of '>'
• Confusing '>' with '>>'
• Using '>=' when only '>' is needed

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

SELECT * FROM Employees WHERE salary <= 50000;

medium

A. All employees with salary greater than 50000

B. All employees with salary exactly 50000 only

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

D. No employees, syntax error

Solution

1. Step 1: Understand the comparison operator used

   The operator '<=' means 'less than or equal to', so it selects salaries 50000 or less.

2. Step 2: Interpret the query result

   The query returns all rows where salary is 50000 or below, not just exactly 50000 or above 50000.

3. Final Answer:

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

4. Quick Check:

   salary <= 50000 means salary ≤ 50000 [OK]

Hint: <= means less than or equal to [OK]

Common Mistakes:

• Thinking <= means only equal
• Confusing <= with <
• Assuming syntax error in query

4. Identify the error in this SQL query:

SELECT * FROM Products WHERE price <>;

medium

A. Wrong comparison operator used

B. Incorrect table name

C. Missing SELECT keyword

D. Missing value after comparison operator

Solution

1. Step 1: Analyze the WHERE clause

   The operator '<>' means 'not equal', but it must be followed by a value to compare.

2. Step 2: Identify syntax error

   Here, '<>' is not followed by any value, causing a syntax error.

3. Final Answer:

   Missing value after comparison operator -> Option D

4. Quick Check:

   Comparison operators need a value after them [OK]

Hint: Comparison operators must have a value after them [OK]

Common Mistakes:

• Leaving out value after operator
• Confusing <> with != (both valid but must have value)
• Assuming table or SELECT keyword is missing

5. You want to select all orders from the Orders table where the order_date is after January 1, 2023, and the total_amount is at least 100. Which query correctly uses WHERE with comparison operators?

hard

A. SELECT * FROM Orders WHERE order_date > '2023-01-01' AND total_amount >= 100;

B. SELECT * FROM Orders WHERE order_date >= '2023-01-01' OR total_amount > 100;

C. SELECT * FROM Orders WHERE order_date < '2023-01-01' AND total_amount > 100;

D. SELECT * FROM Orders WHERE order_date = '2023-01-01' AND total_amount = 100;

Solution

1. Step 1: Understand the date condition

   We want orders after January 1, 2023, so use 'order_date > '2023-01-01''.

2. Step 2: Understand the amount condition

   We want total_amount at least 100, so use 'total_amount >= 100'. Both conditions must be true, so use AND.

3. Step 3: Check each option

   SELECT * FROM Orders WHERE order_date > '2023-01-01' AND total_amount >= 100; matches both conditions correctly. SELECT * FROM Orders WHERE order_date >= '2023-01-01' OR total_amount > 100; uses OR and >= for date, which is incorrect. SELECT * FROM Orders WHERE order_date < '2023-01-01' AND total_amount > 100; uses < for date, wrong direction. SELECT * FROM Orders WHERE order_date = '2023-01-01' AND total_amount = 100; uses = which is too strict.

4. Final Answer:

   SELECT * FROM Orders WHERE order_date > '2023-01-01' AND total_amount >= 100; -> Option A

5. Quick Check:

   Use > for after date, >= for at least amount, AND to combine [OK]

Hint: Use AND to combine conditions, > for after date, >= for minimum amount [OK]

Common Mistakes:

• Using OR instead of AND
• Using >= for date when strictly after is needed
• Using = instead of comparison operators