📊 Investigate: Trace SQL Query Execution

Understand how SQL processes queries by working through them step by step!

id	name	department	salary	years
1	Anna	IT	45000	3
2	Ben	Sales	38000	5
3	Cara	IT	52000	7
4	Dan	HR	35000	2
5	Emma	Sales	42000	4

How SQL Processes a Query

Let's trace through this query step by step:

SQL Query

SELECT name, salary
FROM employees
WHERE department = 'IT' AND salary > 40000;

Step 1

SQL starts by looking at the employees table with all 5 records.

id	name	department	salary	years
1	Anna	IT	45000	3
2	Ben	Sales	38000	5
3	Cara	IT	52000	7
4	Dan	HR	35000	2
5	Emma	Sales	42000	4

Step 2

Filter to only IT department employees:

id	name	department	salary	years	Matches?
1	Anna	IT	45000	3	YES
2	Ben	Sales	38000	5	NO
3	Cara	IT	52000	7	YES
4	Dan	HR	35000	2	NO
5	Emma	Sales	42000	4	NO

Remaining: Anna, Cara (2 records)

Step 3

From the IT employees, now filter by salary > 40000:

name	department	salary	salary > 40000?
Anna	IT	45000	YES (45000 > 40000)
Cara	IT	52000	YES (52000 > 40000)

Both Anna and Cara pass this filter too!

Step 4

Finally, select only the name and salary columns:

name	salary
Anna	45000
Cara	52000

Final result: 2 rows, 2 columns

Work through this query on paper, then check your answer:

SQL Query

SELECT name, years
FROM employees
WHERE years >= 4 OR department = 'HR';

Click to reveal answer

Condition 1 (years >= 4): Ben (5), Cara (7), Emma (4)

Condition 2 (department = 'HR'): Dan

OR means either condition: Ben, Cara, Dan, Emma

SELECT name, years:

Note: Anna is excluded because she has 3 years (< 4) AND is in IT (not HR).