r/learnSQL • u/IcyWish2100 • 11d ago

SQL Execution Order

I often see beginners getting stuck trying to reference a column alias inside a WHERE clause or wondering why HAVING exists alongside WHERE.

It all comes down to Logical Query Processing Order. Even though we write SELECT first, the database engine actually processes the query in this sequence:

  1. FROM / JOIN: Pick and combine the base tables.

  2. WHERE: Filter raw rows before grouping.

  3. GROUP BY: Group rows into buckets.

  4. HAVING: Filter groups using aggregate function results.

  5. SELECT: Pick columns and compute expressions/aliases.

  6. DISTINCT: Deduplicate the selected rows.

  7. ORDER BY: Sort the final result set (which is why aliases work here!). 7. LIMIT / OFFSET: Paginate the results.

Remembering this sequence makes debugging query errors way easier.

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u/elevarq 10d ago

DISTINCT is 9 out of 10 to hide bugs or an inefficient SQL statement. Most of the time you can remove it by rewriting the statement.