Subquery in FROM clause (derived table) in SQL - Step-by-Step Execution

Concept Flow - Subquery in FROM clause (derived table)

Start Query

↓

Identify FROM clause

↓

Detect Subquery

↓

Execute Subquery

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Create Derived Table

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Use Derived Table in Outer Query

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Execute Outer Query

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Return Final Result

The database first runs the subquery inside the FROM clause, treats its result as a temporary table, then runs the outer query using this temporary table.

Execution Sample

SQL

SELECT dept, avg_salary
FROM (SELECT department AS dept, AVG(salary) AS avg_salary
      FROM employees
      GROUP BY department) AS dept_avg
WHERE avg_salary > 50000;

This query calculates average salaries per department using a subquery in FROM, then selects departments with average salary over 50000.

Execution Table

Step	Action	Subquery Result	Outer Query Action	Output Rows
1	Start query execution	N/A	N/A	N/A
2	Execute subquery: SELECT department AS dept, AVG(salary) AS avg_salary FROM employees GROUP BY department	[{dept: 'HR', avg_salary: 48000}, {dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]	Create derived table dept_avg	N/A
3	Apply outer query WHERE avg_salary > 50000 on derived table	N/A	Filter rows where avg_salary > 50000	[{dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]
4	Return final result	N/A	Output filtered rows	[{dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]

💡 Outer query finishes after filtering derived table rows by avg_salary > 50000

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
subquery_result	N/A	[{dept: 'HR', avg_salary: 48000}, {dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]	[{dept: 'HR', avg_salary: 48000}, {dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]	Same as After Step 2
derived_table	N/A	Created from subquery_result	Same	Same
filtered_rows	N/A	N/A	[{dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]	Same

Key Moments - 3 Insights

Why does the subquery run before the outer query?

Because the subquery in the FROM clause creates a temporary table (derived table) that the outer query needs to use. This is shown in execution_table step 2 where the subquery runs first.

Is the derived table stored permanently in the database?

No, the derived table exists only during the query execution as a temporary result from the subquery, as seen in variable_tracker where 'derived_table' is created after step 2 and used immediately.

Can the outer query filter on columns from the subquery?

Yes, the outer query can filter on any columns produced by the subquery, as shown in execution_table step 3 where the WHERE clause filters on avg_salary.

Visual Quiz - 3 Questions

Look at the execution_table, what is the subquery_result after step 2?

A [{dept: 'HR', avg_salary: 48000}, {dept: 'IT', avg_salary: 60000}, {dept: 'Sales', avg_salary: 52000}]

B [{dept: 'HR', avg_salary: 60000}, {dept: 'IT', avg_salary: 48000}]

C Empty set

D N/A

💡 Hint

Check the 'Subquery Result' column in execution_table row with Step 2

At which step does the outer query apply the filter avg_salary > 50000?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Outer Query Action' column in execution_table to find when filtering happens

If the subquery returned no rows, what would the final output be?

A All rows from employees table

B Empty result set

C Error because no rows

D Only departments with salary > 50000

💡 Hint

Derived table is empty means outer query filters nothing, see variable_tracker for filtered_rows

Concept Snapshot

Subquery in FROM clause creates a temporary table (derived table).
Database runs subquery first, then outer query uses its result.
Derived table can be filtered or joined in outer query.
Useful for complex aggregations or intermediate results.
Syntax: FROM (subquery) AS alias
Always alias the subquery result.

Full Transcript

This visual execution shows how a subquery inside the FROM clause works. First, the database runs the subquery to get a temporary table called a derived table. Then, the outer query uses this derived table to filter or select data. For example, the subquery calculates average salaries per department. The outer query then selects only departments with average salary above 50000. The execution table traces each step: running the subquery, creating the derived table, filtering rows, and returning the final result. Variables track the subquery result, derived table, and filtered rows. Key moments clarify why the subquery runs first, that the derived table is temporary, and that the outer query can filter on subquery columns. The quiz tests understanding of these steps and outcomes. This helps beginners see how subqueries in FROM clauses work step-by-step.