This document discusses subqueries, which are SELECT statements nested inside other SELECT statements. Subqueries can be used in the SELECT, FROM, WHERE, or HAVING clauses. They can return scalar values, lists, or correlated results depending on how they are used. Examples are provided to illustrate scalar, list, and correlated subqueries as well as common uses of subqueries to retrieve related data from different tables.

1. Subqueries
CIS-182

2. Subqueries
• A subquery is one SELECT statement
inside a second SELECT statement
– May be used throughout the main query, in
SELECT, FROM, WHERE or HAVING
– Parentheses control order of execution

3. Sample Subqueries
• Display the price and average price of all
books:
SELECT Price, (SELECT Avg(Price) FROM
titles) AS AveragePrice FROM titles
• Display all books with a higher than
average price:
SELECT title FROM titles WHERE
Price>(SELECT Avg(Price) FROM titles)

4. Subquery Results
• Scalar values: Subqueries may return a
single value
• Lists: Subqueries may return one or more
rows
– Some situations require a single column list
– Typically use IN or EXISTS to test

5. Scalar Subqueries Example
• Display titles that have the highest price:
SELECT title, price
FROM titles
WHERE Price = (SELECT Max(price) FROM
Titles)

6. List Subqueries Example
• Display the publishers who have
published cook books:
SELECT pub_name
FROM publishers
WHERE pub_id IN
(SELECT pub_id FROM titles WHERE [type]
LIKE ‘%cook%’)

7. Correlated Subqueries
• A correlated subquery uses a value from
the main query as part of the inner query
– Data from each row in the main query is
“passed” to the subquery for processing
• Typically a processing-intensive operation
– Subquery must be re-run with new value(s)
for each row

8. Correlated Subquery Example
• Display all books with a higher than
average price for that type of book
SELECT title
FROM titles t1
WHERE price >
(SELECT Avg(price) FROM titles t2 WHERE
t1.type = t2.type)

9. Using Subqueries - 1
• If data that’s known is from one table and
data to return is in a second table
• Display authors who have written books
(titleauthors represents what’s known):
SELECT au_fname, au_lname
FROM authors
WHERE au_id IN
(SELECT au_id FROM titleauthors)

10. Using Subqueries – 2
• If data to return depends on a calculation
from a related set
• Display books where actual sales have
exceeded projected sales:
SELECT title
FROM titles t
WHERE projected_sales <
(SELECT sum(qty_shipped) FROM salesdetails
sd WHERE t.title_id=sd.title_id)