Lesson Beginner

WHERE

WHERE decides which rows are eligible for everything that comes after.
If a filter is wrong, every downstream number is wrong, even when the SQL parses cleanly. That is why getting WHERE right matters more than the rest of the query combined.

Syntax #

WHERE goes right after FROM. The condition must evaluate to true for a row to be kept.

sql

SELECT id, status, total
FROM orders
WHERE status = 'paid';

Comparison Operators #

The condition in WHERE is almost always a comparison:

Operator	Meaning
`=`	Equal
`<>` or `!=`	Not equal
`<`, `<=`	Less than / at most
`>`, `>=`	Greater than / at least

String equality is case-sensitive in most SQL dialects. status = 'Paid' and status = 'paid' are different filters. For pattern-based text matching, see LIKE later in this section.

Warning

WHERE col = NULL returns zero rows, even for rows where col really is NULL.
NULL is not a value you can compare with =. Use IS NULL / IS NOT NULL instead. This is the single most common filtering mistake in SQL. Full treatment in NULL handling.

Try It #

Run the query, then change the filter and rerun.

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Practice #

Return id, status, and total for orders that are paid and have a total strictly greater than 100. No ordering requirement.

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Mistakes to Watch For #

Using = NULL instead of IS NULL.
Wrong case or stray whitespace in string comparisons ('Paid ' ≠ 'paid').
Trying to reuse a SELECT alias in WHERE; aliases don't exist yet at filter time.
Placing an aggregate filter like COUNT(*) > 5 in WHERE. Those belong in HAVING (covered in the Aggregation section).

Knowledge check #

3 questions

0 / 3 answered

When does WHERE run, relative to GROUP BY?
WHERE col = NULL returns which rows?
You want every row where status is not 'cancelled', including rows where status is NULL. What works?

Next Step #

Continue to AND / OR / NOT to combine multiple conditions in a single filter.