Comparison operators in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the '=' operator do in SQL?

The '=' operator checks if two values are equal. It returns true if they are the same, false otherwise.

beginner

What is the difference between '<>' and '!=' in MySQL?

Both '<>' and '!=' mean 'not equal to' in MySQL. They work the same way and can be used interchangeably.

beginner

How does the '>' operator work in SQL?

The '>' operator checks if the value on the left is greater than the value on the right. It returns true if yes, false otherwise.

beginner

What does the '<=' operator mean in SQL?

The '<=' operator means 'less than or equal to'. It returns true if the left value is smaller or equal to the right value.

intermediate

Can you use comparison operators with text (strings) in MySQL?

Yes, you can compare strings using comparison operators like '=', '<>', '>', '<'. MySQL compares strings based on alphabetical order.

Which operator checks if two values are NOT equal in MySQL?

A =

B <>

C >=

D <

The '<>' operator means 'not equal to' in MySQL.

What will the expression '5 > 3' return in SQL?

A True

B False

C Error

D Null

5 is greater than 3, so '5 > 3' returns true.

Which operator means 'less than or equal to'?

A >=

B <=

C !=

D =

'<=' means less than or equal to.

If you want to check if two values are exactly equal, which operator do you use?

A <>

B !=

C =

D >

'=' checks if two values are equal.

Can you use '>' to compare two text strings in MySQL?

A Yes, based on alphabetical order

B No, only numbers

C Only dates

D Only boolean values

MySQL compares strings alphabetically using comparison operators.

Explain how the '=' and '<>' operators work in MySQL with examples.

Think about checking if two values are the same or different.

Describe how comparison operators like '>', '<=', and '!=' can be used in SQL queries.

Imagine filtering data based on conditions.

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