Your first SQL query, explained line by line

Suppose a table called courses stores a title, a level and a duration in hours for each course. Here is a query that lists beginner courses, shortest first:

SELECT title, duration_hours
FROM courses
WHERE level = 'beginner'
ORDER BY duration_hours ASC;

What each line does

• SELECT title, duration_hours chooses which columns appear in the result.
• FROM courses says which table to read.
• WHERE level = 'beginner' keeps only the rows where the condition is true.
• ORDER BY duration_hours ASC sorts the result, smallest first. Without ORDER BY, the database may return rows in any order — never rely on it.

The order the database runs it in

You write SELECT first, but the database works roughly like this:

1. FROM — find the table.
2. WHERE — filter rows.
3. SELECT — pick and compute columns.
4. ORDER BY — sort what is left.

This explains a common surprise: you cannot use a column alias created in SELECT inside WHERE, because WHERE runs first.

A note on NULL

If some courses have no duration recorded, duration_hours is NULL — meaning unknown, not zero. WHERE duration_hours > 5 silently leaves those rows out, because "unknown is greater than 5" is not true. To find them, use WHERE duration_hours IS NULL.

Try it yourself

Change the query to list advanced courses, longest first. Then add LIMIT 3 at the end to see only the top three.