WHERE: Filter Rows

A table can have millions of rows, but you rarely want them all. "Who scored over 500?", "Show only Indian players" — these questions need a filter. The WHERE clause is SQL's filter for rows.

Definition

The WHERE clause filters rows before they reach the result. It tests a condition (a true/false expression) against every row and keeps only the rows where the condition is TRUE. Rows where it is FALSE or UNKNOWN (NULL) are discarded.

Syntax

SELECT column1, column2
FROM table_name
WHERE condition;

(Chapter 5 covers AND, OR, NOT, IN, BETWEEN, LIKE and IS NULL in depth.)

Example — numeric filter

SELECT *
FROM players
WHERE runs > 500;

Execution order: 1️⃣ FROM players → 2️⃣ WHERE runs > 500 checks each row (Rohit 500 ✗ — 500 is not greater than 500, Dhoni 350 ✗, Bumrah 20 ✗) → 3️⃣ SELECT *.

Example — text filter

SELECT name, team
FROM players
WHERE country = 'India';

Execution order: 1️⃣ FROM → 2️⃣ WHERE country = 'India' → 3️⃣ SELECT name, team. Notice that country is used in WHERE even though it is not in the SELECT list — that is perfectly fine, because WHERE runs before SELECT and can see every column.

Example — not equal

SELECT *
FROM players
WHERE team <> 'MI';

Common mistakes

• ❌ WHERE country = India → SQL thinks India is a column. ✅ WHERE country = 'India'.
• ❌ WHERE runs = '750' — comparing a number to text forces a conversion. ✅ WHERE runs = 750.
• ❌ Using a SELECT alias in WHERE: SELECT runs AS r … WHERE r > 500 → Invalid column name 'r' in SQL Server, because WHERE runs before SELECT creates the alias. ✅ Repeat the column: WHERE runs > 500.
• ❌ Expecting > 500 to include 500. ✅ Use >= 500 to include it.

SQL Server vs MySQL / PostgreSQL

Note: The browser playground compares text case-sensitively, so type values exactly as stored ('India', not 'india').

Interview tip

Q: Can you use a column in WHERE that is not in the SELECT list? A: Yes. WHERE is evaluated before SELECT, on the full rows from FROM, so it can use any column of the table. The reverse is also why a SELECT alias can't be used in WHERE.