![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%’)](https://image.slidesharecdn.com/9subqueries-091111181352-phpapp02/85/Subqueries-6-320.jpg)
Subqueries
- 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)