Lesson 3: Filtering and Operators

⏱ ~35 min Lesson 3 of 10 💚 Free

What You'll Learn

Filter rows with WHERE using comparison operators
Combine conditions with AND, OR, and NOT
Match patterns with LIKE and ranges with BETWEEN
Check for NULL values and filter with IN

The WHERE Clause

WHERE filters which rows are included in the result. Only rows where the condition is TRUE are returned. WHERE comes after FROM but before ORDER BY and LIMIT.

Comparison operators: = equal, != or <> not equal, >, <, >=, <=

AND, OR, NOT

AND — both conditions must be true

OR — at least one condition must be true

NOT — inverts the condition. NOT (score > 80) is the same as score <= 80

Use parentheses to group conditions: WHERE grade = 10 AND (score > 80 OR city = 'Tulsa')

LIKE, BETWEEN, IN, IS NULL

WHERE name LIKE 'A%' — names starting with A. % matches any string, _ matches one character.

WHERE score BETWEEN 80 AND 90 — inclusive range (80 and 90 included).

WHERE city IN ('Tulsa', 'OKC') — matches any value in the list.

WHERE email IS NULL — checks for missing values. Never use = NULL.

Quick Check

1. What does WHERE score >= 90 return?

Rows where score equals 90 exactly

Rows where score is 90 or higher

Rows where score is above 90 only

All rows except score 90

2. What does the % wildcard mean in a LIKE pattern?

Exactly one character

Zero or more characters of any type

Only a digit

A literal percent sign only

3. How do you check if a column is empty/missing in SQL?

WHERE column = NULL

WHERE column IS NULL

WHERE column == ""

WHERE column EQUALS NOTHING

4. What does WHERE city IN ('Tulsa', 'OKC') do?

Returns only rows from Tulsa

Returns rows where city is either Tulsa or OKC

Excludes Tulsa and OKC

Creates a new column with those values