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Beginner Topic 9 of 51 Databases

WHERE and Filtering

Clauses restricting which rows are returned.

How it works

WHERE keeps rows where a predicate is true, using =, <, BETWEEN, IN, LIKE and IS NULL combined with AND/OR/NOT.

Where it is used

Finding orders from a date range.

Common pitfall

NULL comparisons: x = NULL is never true; use IS NULL. Functions on indexed columns can prevent index use.

Where it fits in the course

WHERE and Filtering is topic 9 of 51 in Databases, rated Beginner. It follows SELECT Queries. The next topic is ORDER BY and LIMIT.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Databases quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is WHERE and Filtering?
Clauses restricting which rows are returned. It belongs to Databases and is a beginner-level topic.
Where do people use WHERE and Filtering?
Finding orders from a date range.
What is the most common mistake with WHERE and Filtering?
NULL comparisons: x = NULL is never true; use IS NULL.
What should I learn before this topic?
Start with SELECT Queries, the topic just before it in Databases.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Databases quiz.
Is WHERE and Filtering hard to learn?
It is a Beginner topic, so it works as an entry point. If the terms are new, read the explanation twice and try the visualizer before the quiz.
How long does it take to learn WHERE and Filtering?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to WHERE and Filtering?
SELECT Queries, ORDER BY and LIMIT, SQL Basics are closely connected. The full list is in the related topics section of this page.
What should I study after WHERE and Filtering?
Continue with ORDER BY and LIMIT, the next topic in Databases.

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