Understanding SQL Execution Order: From FROM to LIMIT Explained

SQL Execution Order Explained

This article describes the actual order in which a database processes a SQL query, which differs from the textual order of the statement.

Execution steps

1. FROM and JOIN are evaluated first to determine table relationships and produce an initial row set.

2. WHERE applies simple filters to that row set.

3. GROUP BY groups rows according to the specified columns.

4. HAVING filters each group; it can use aggregate functions, unlike WHERE.

5. SELECT computes the final columns, adding any aggregate results; DISTINCT removes duplicate rows.

6. ORDER BY sorts the result set.

7. LIMIT truncates the sorted set to the requested number of rows.

FROM, JOIN and WHERE

These clauses define which tables are involved and how they are related.

from table1 join table2 on table1.id=table2.id

from table1,table2 where table1.id=table2.id

Without a join condition the query produces a Cartesian product.

GROUP BY

Groups rows based on the specified columns but does not filter them.

HAVING vs WHERE

HAVING can contain aggregate functions and filters groups after aggregation; WHERE can only use simple conditions before grouping.

Example: having salary < avg(salary) removes rows whose salary is below the group’s average.

SELECT and DISTINCT

After grouping, SELECT retrieves the desired columns; DISTINCT removes duplicate rows.

select employee.id, distinct name, salary, avg(salary)

ORDER BY and LIMIT

ORDER BY arranges the final rows; LIMIT stops processing after the required number of rows are retrieved, which is why it must be applied last.