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Practice

Logical Operators

Logical operators allow you to combine multiple conditions in a SQL query using the WHERE clause. This helps you make your data filters more precise and powerful.


Main Logical Operators​

OperatorDescriptionExample
ANDAll conditions must be truemembership_type = 'student' AND has_overdue_books = 'no'
ORAt least one condition must be truemembership_type = 'guest' OR number_of_books > 3
NOTReverses or excludes a conditionNOT has_overdue_books = 'yes'

AND Operator Example​

The AND operator is used to filter rows where all conditions must be true.

Filter: Students with no overdue books
SELECT name, membership_type
FROM library_members
WHERE membership_type = 'student' AND has_overdue_books = 'no';

This query returns students who do not have any overdue books.


OR Operator Example​

The OR operator is used to filter rows where at least one condition must be true.

Filter: Guests or those with many books
SELECT name, number_of_books
FROM library_members
WHERE membership_type = 'guest' OR number_of_books > 3;

This query returns either guests or anyone with more than 3 books checked out.


NOT Operator Example​

The NOT operator is used to filter rows where the condition is false.

Filter: Exclude members with overdue books
SELECT name
FROM library_members
WHERE NOT has_overdue_books = 'yes';

This query returns only members who do not have overdue books.


Why logical operators matter​

Logical operators are critical when:

  • Applying multiple filters to a dataset
  • Handling exceptions and special cases
  • Writing queries that mirror real-world logic

They give you precise control over what data you retrieve.

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