Understanding SQL Query Execution Order and Clause Functions

The article introduces the standard execution order of an SQL query, showing how the database processes each clause step by step to produce the final result set.

First, the FROM and JOIN clauses determine the tables and their relationships, forming an initial data set. Then WHERE applies basic row filters. Next, GROUP BY groups rows without filtering, followed by HAVING , which can filter groups using ordinary conditions or aggregate functions. After grouping, the SELECT clause retrieves the required columns and adds any aggregate results, with DISTINCT removing duplicate rows. Finally, ORDER BY sorts the result set, and LIMIT restricts the number of rows returned.

Data association is illustrated with a diagram of two tables and examples such as:

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

and a multi‑table form using WHERE for the join condition:

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

The article warns that omitting join conditions leads to a Cartesian product.

For GROUP BY , the article shows grouping by a simple condition (e.g., odd/even IDs) and provides a visual example.

It contrasts WHERE and HAVING , noting that HAVING can use aggregate functions while WHERE cannot. An illustrative arithmetic example demonstrates that applying WHERE before or after grouping does not change the final filtered rows, but HAVING can filter based on aggregates such as having salary<avg(salary) .

The SELECT example after grouping shows how aggregate fields are added and how duplicate column names must be disambiguated:

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

After HAVING , the grouped data are merged, and the final step is ORDER BY , e.g., sorting by id . The article emphasizes that LIMIT is applied last; applying it before sorting would return the first rows rather than the smallest values.

Throughout, the article includes several diagrams (displayed as tags) that visualize the data flow for each clause.