In SQL, the execution order of a query follows a logical sequence that defines how the data is processed before returning the final result. Below is the typical order of execution in an SQL SELECT statement, though this can vary slightly depending on the database engine:
- FROM Clause: This is where the data source (tables, views, subqueries, etc.) is specified. The system first retrieves the data from the specified tables or joins multiple tables if necessary.
- JOINs: If there are any joins, the system determines how to combine the rows from the different tables based on the join conditions.
- WHERE Clause: This step filters the rows based on the conditions specified in the WHERE clause. Only rows that meet the conditions will be passed on to the next steps.
- GROUP BY Clause: Once the rows are filtered, the system groups them based on the columns listed in the GROUP BY clause, which is typically used for aggregation.
- HAVING Clause: After grouping, the HAVING clause is applied to filter the groups (not rows like WHERE). Only groups that meet the specified conditions are retained.
- SELECT Clause: At this point, the system selects the columns or expressions specified in the SELECT clause. This step defines what will appear in the final result set.
- DISTINCT: If DISTINCT is used, duplicates are removed from the result set.
- ORDER BY Clause: The results are sorted according to the columns and directions specified in the ORDER BY clause.
- LIMIT/OFFSET: Finally, the system applies any limits on the number of rows to be returned (LIMIT) or skips a specific number of rows (OFFSET).
