SQL WHERE Clause: Row Filtering & Comparison Operators

In production databases holding millions of records, you rarely want to retrieve all rows at once. The WHERE clause specifies the search condition that rows must satisfy to be selected for reading, updating, or deleting. The database evaluates the condition for each row: if the condition evaluates to TRUE, the row is returned; if FALSE or UNKNOWN (NULL), it is discarded.

SQL Comparison Operators Reference

Hands-on Filtering Examples

-- 1. Filter by numeric threshold
SELECT student_id, first_name, gpa
FROM students
WHERE gpa >= 3.80;

-- 2. Filter by exact string match
SELECT student_id, first_name, course
FROM students
WHERE course = 'BCA';

-- 3. Filter by date comparison
SELECT student_id, first_name, enrollment_date
FROM students
WHERE enrollment_date >= '2026-01-01';

The Critical Role of Indexes in WHERE Filtering

When an indexed column is evaluated in a WHERE clause (e.g., WHERE student_id = 105), the database traverses a B+ Tree index in (O(\log N)) time, reading only 2 or 3 disk blocks. Without an index on that column, the engine must perform a Full Table Scan (FTS), inspecting every single row on disk!

Next, let us combine multiple filter criteria using SQL AND, OR, NOT Logical Operators!