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Intermediate Topic 33 of 51 Databases

Window Functions

Calculations across related rows without collapsing them.

How it works

Window functions compute a value for each row using a set of related rows, called a window, defined with OVER, optionally with PARTITION BY, ORDER BY and a frame such as ROWS BETWEEN. Unlike GROUP BY, they do not collapse rows. Examples include ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, FIRST_VALUE, and running aggregates like SUM() OVER.

Step-by-step walkthrough

  1. A window function computes over related rows without collapsing them.
  2. RANK() OVER (ORDER BY score DESC) numbers the rows.
  3. Ana gets rank 1.
  4. Ben and Cy tie at rank 2.
  5. Di gets rank 4, and every input row is still in the output.

Where it is used

Ranking top products per category, running totals, moving averages, comparing a row with the previous one, calculating percent of total and deduplicating rows use window functions.

Common pitfall

They are evaluated after WHERE and GROUP BY, so you cannot filter on them directly in WHERE; wrap the query in a subquery or CTE. The default frame with ORDER BY may be RANGE and not ROWS, causing subtle differences with ties. Large windows can be memory intensive. Be explicit about partitioning and ordering to get deterministic results.

Where it fits in the course

Window Functions is topic 33 of 51 in Databases, rated Intermediate. It follows Query Execution Plans. The next topic is Common Table Expressions.

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 Window Functions?
Calculations across related rows without collapsing them. It belongs to Databases and is an intermediate-level topic.
Where do people use Window Functions?
Ranking top products per category, running totals, moving averages, comparing a row with the previous one, calculating percent of total and deduplicating rows use window functions.
What is the most common mistake with Window Functions?
They are evaluated after WHERE and GROUP BY, so you cannot filter on them directly in WHERE; wrap the query in a subquery or CTE.
What should I learn before this topic?
Start with Query Execution Plans, 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 Window Functions hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Window Functions?
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 Window Functions?
Query Execution Plans, Common Table Expressions, Query Optimization are closely connected. The full list is in the related topics section of this page.
What should I study after Window Functions?
Continue with Common Table Expressions, the next topic in Databases.

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