Understanding the Execution Order of SQL Queries

In this tutorial, the author walks through the actual execution sequence of a typical SQL query, illustrating how the database engine processes each clause.

FROM & JOIN & WHERE – First, the engine resolves table relationships using FROM table1 JOIN table2 ON table1.id = table2.id or the comma‑separated form with a WHERE condition, producing the initial result set.

GROUP BY – The intermediate rows are then grouped according to the specified grouping keys, without yet filtering any data.

HAVING & WHERE – After grouping, HAVING can apply both ordinary predicates and aggregate functions, while WHERE can only use non‑aggregate conditions; often the WHERE filter can be moved into HAVING for a smoother query.

SELECT & DISTINCT – The SELECT clause extracts the required columns, adding any aggregate results as new fields; DISTINCT removes duplicate rows if needed.

ORDER BY – The result set is then sorted according to the ORDER BY clause, for example by id .

LIMIT – Finally, LIMIT truncates the sorted result to the desired number of rows; applying it earlier would yield incorrect data.

Code examples: from table1 join table2 on table1.id=table2.id from table1, table2 where table1.id=table2.id select employee.id, distinct name, salary, avg(salary)

Throughout the article, diagrams illustrate each phase of the execution process, helping readers visualize how data flows from the initial tables to the final ordered result.