Mastering SQL: The 6-Step Process Behind Every SELECT Query

Use a simple example to explain the execution order of a SQL query.

Introduction

This article step‑by‑step explains how a relational database processes a SELECT statement.

Two sample tables are used: Citizen (citizen name and city_id) and City (city name and id).

The query aims to list the names of cities other than “San Bruno” that have at least two citizens, ordered alphabetically.

SELECT city.city_name AS City
FROM citizen
JOIN city ON citizen.city_id = city.city_id
WHERE city.city_name != 'San Bruno'
GROUP BY city.city_name
HAVING COUNT(*) >= 2
ORDER BY city.city_name ASC
LIMIT 2;

Query processing steps

From / Join : Combine the two tables according to the ON condition.

Where : Filter rows that do not satisfy the condition (city name not “San Bruno”).

Group by : Group the remaining rows by city name.

Having : Keep only groups whose count is ≥ 2.

Select : Choose the columns to return (city_name as “City”).

Order by / Limit : Sort the result alphabetically and limit the output to two rows.

Step 1: From / Join

FROM citizen
JOIN city

The FROM and JOIN clauses create the initial record set by pairing rows from the two tables.

ON citizen.city_id = city.city_id

Step 2: Where

WHERE city.city_name != 'San Bruno'

The WHERE clause removes rows that do not meet the condition.

Step 3: Group by

GROUP BY city.city_name

Rows with the same city_name are grouped together.

Step 4: Having

HAVING COUNT(*) >= 2

The HAVING clause filters groups, keeping only those with at least two citizens.

Step 5: Select

The SELECT clause outputs the city_name column with the alias “City”.

Step 6: Order by & Limit

The final step orders the result alphabetically and limits the number of rows to two.

Conclusion

The six stages—From/Join, Where, Group By, Having, Select, Order By/Limit—represent the full execution pipeline of a SQL SELECT statement.