lesson 28 Dates, strings & subqueries · intermediate

Subqueries

Use the result of one query inside another.

the idea

A subquery is a query inside another query. A scalar subquery produces one value, which can be used in a comparison. Parentheses separate the inner query.

tables in this lesson

The challenges use this data. Each column shows its type underneath; → marks a link to another table.

products4 columns · 6 rows

idinteger · key	nametext	categorytext	pricereal
1	Notebook	Stationery	12
2	Desk lamp	Home	45
3	Keyboard	Tech	80
4	Pen set	Stationery	8
5	Headphones	Tech	120

showing 5 of 6 rows

orders6 columns · 12 rows

idinteger · key	customer_idinteger	product_idinteger	quantityinteger	statustext	ordered_attext
101	1	1	2	paid	2025-01-01
102	2	3	1	paid	2025-01-02
103	1	2	1	paid	2025-01-05
104	3	5	2	pending	2025-01-08
105	4	4	5	paid	2025-02-01

showing 5 of 12 rows

example

The idea in one query. Press Run to see what it returns; running examples never affects your progress.

example.sql

SELECT name, price
FROM products
WHERE price > (
  SELECT AVG(price)
  FROM products
);

Watch for: The inner average summarizes the entire table; it is not a per-row calculation.

build it step by step

Each step changes one thing. Run them in order and watch the result change.

1
Compute the comparison value. A subquery can produce the average price once. The outer query will compare each product to that single number.
step-1.sql
sql›
```
SELECT AVG(price) AS avg_price
FROM products;
```
2
Filter rows against the subquery. The parenthesized SELECT runs as a scalar subquery. Rows with price above that average remain.
step-2.sql
sql›
```
SELECT name, price
FROM products
WHERE price > (
  SELECT AVG(price)
  FROM products
);
```