SQL Query Execution Order
1️⃣ FROM
This is where it all begins. SQL starts by identifying the tables involved and establishing relationships through joins if needed.
Example: FROM employees
2️⃣ WHERE
Next, it filters rows based on specified conditions.
Example: WHERE salary > 50000
3️⃣ GROUP BY
After filtering, SQL groups rows that share a specified column’s value.
Example: GROUP BY department
4️⃣ HAVING
This is similar to WHERE but operates on groups created by GROUP BY, allowing for group-level filtering.
Example: HAVING COUNT(*) > 10
5️⃣ SELECT
Now, SQL identifies which columns or calculations to return.
Example: SELECT department, AVG(salary)
6️⃣ DISTINCT
This step removes duplicate rows from the result set.
Example: SELECT DISTINCT department
7️⃣ ORDER BY
Finally, SQL orders the result set based on specified columns.
Example: ORDER BY AVG(salary) DESC
8️⃣ LIMIT/OFFSET
If you need to restrict the number of rows returned or paginate results, this comes last.
Example: LIMIT 10 OFFSET 20
Putting It All Together: Consider this query:
“SELECT department, AVG(salary)
FROM employees
WHERE salary > 50000
GROUP BY department
HAVING COUNT(*) > 10
ORDER BY AVG(salary) DESC
LIMIT 10 OFFSET 20;
“
Step 1️⃣: Identify tables from employees.
Step 2️⃣: Filter rows where salary > 50000.
Step 3️⃣: Group rows by department.
Step 4️⃣: Filter groups having more than 10 rows.
Step 5️⃣: Select the department and the average salary for each group.
Step 6️⃣: No duplicates to remove here, as DISTINCT is not used.
Step 7️⃣: Order results by average salary in descending order.
Step 8️⃣: Return the 21st to 30th rows from the ordered result.
By understanding this order, you can write more efficient queries and troubleshoot why your query isn’t returning the expected results.
