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Chapter 8 - Subqueries

Chapter 8 - Subqueries. A Subquery nests a SELECT statement in the WHERE clause of another SELECT, INSERT, UPDATE, or DELETE statement A Subquery’s Outer SELECT operates on rows returned from the Inner SELECT The Inner and Outer SELECT statements may refer to the same table.

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Chapter 8 - Subqueries

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  1. Chapter 8 - Subqueries • A Subquery nests a SELECT statement in the WHERE clause of another SELECT, INSERT, UPDATE, or DELETE statement • A Subquery’s Outer SELECT operates on rows returned from the Inner SELECT • The Inner and Outer SELECT statements may refer to the same table

  2. From the Inside Out • Most Subqueries are evaluated from the inner query to the outer query • A correlated Subquery, discussed later, works in reverse

  3. Simplified Subquery Syntax • SELECT select_listFROM table_listWHERE expression { [NOT] IN | operator | [NOT] EXISTS } (SELECT subquery_select_list FROM table_list WHERE search_conditions);

  4. Subquery Options • Produces an ‘IN’ list of values for evaluation by the outer query • Returns a single value for comparison • Returns a TRUE/FALSE value for an existence test

  5. Why Use a Subquery? • Subqueries can be difficult to read • Many can be replaced with an equivalent join • A subquery is sometimes the only alternative

  6. Example - without a Subquery • Find all the books that have the same price as Straight Talk About Computers • SELECT priceFROM titlesWHERE title = 'Straight Talk About Computers'; • SELECT title, priceFROM titlesWHERE price = $19.99;

  7. Example - Using a Subquery • SELECT title, priceFROM titlesWHERE price = (SELECT price FROM titles WHERE title = 'Straight Talk About Computers’);

  8. Joins vs. Subqueries • Join • Advantages: Can return results from all tables • Disadvantages: May need two statements to get desired results • Subquery • Advantages: Can compare aggregates to other values. Can get information in a single statement • Disadvantages: Can only return results from the Outer Select statement

  9. Two Ways • SELECT pub_name, au_lnameFROM publishers p, authors aWHERE p.city = a.city; • SELECT pub_nameFROM publishersWHERE city IN (SELECT city FROM authors);

  10. The IN operator • Use the ‘IN’ operator when the results of the inner query will have zero, one, or more values • Once the inner query returns results, the outer query will make use of them

  11. Example • SELECT pub_nameFROM publishersWHERE pub_id IN (SELECT DISTINCT pub_id FROM titles WHERE type = 'business');

  12. Conceptually... • Inner Query- • SELECT DISTINCT pub_idFROM titlesWHERE type = 'business';(2 rows returned) • Outer Query Evaluates the 2 rows - • SELECT pub_nameFROM publishersWHERE pub_id IN ('1389', '0736');

  13. Done as a Join • SELECT DISTINCT pub_name, titleFROM publishers p, titles tWHERE p.pub_id = t.pub_idAND type = 'business'; • With Joins you can list columns from both tables (see difference when title is left off the select list)

  14. Another Example as a Join • Find the names of all authors listed 2nd on a book who live in California and receive less than 30 percent of the royalties • SELECT au_lname, au_fnameFROM authors a, titleauthors taWHERE state = 'CA'AND a.au_id = ta.au_idAND royaltyshare < .30AND au_ord = 2;

  15. Example as a Subquery • SELECT au_lname, au_fnameFROM authorsWHERE state = 'CA'AND au_id IN (SELECT au_id FROM titleauthors WHERE royaltyshare < .30 AND au_ord = 2);

  16. Exercise - Use Subqueries • List the author's names for all books with more than one author

  17. Answer • SELECT au_lname, au_fnameFROM authorsWHERE au_id IN (SELECT au_id FROM titleauthors WHERE title_id IN (SELECT title_id FROM titleauthors WHERE au_ord > 1));

  18. Using NOT IN • SELECT pub_nameFROM publishersWHERE pub_id NOT IN (SELECT DISTINCT pub_id FROM titles WHERE type = 'business');

  19. NOT IN Does Not Equal < > • This is NOT an equivalent statement… • SELECT DISTINCT pub_nameFROM publishers p, titles tWHERE p.pub_id = t.pub_idAND type < > 'business'; • Why?

  20. NOT not equal < > • This last statement returns any publisher who is publishing a book that is not a ‘business’ type book. Every publisher has at least one book that is some other type than business, so all publishers are returned by the statement.

  21. Comparison Operator Subqueries • Comparison Operators (=, >, <, etc.) • Must resolve to a single value… unless they are modified with the ANY or ALL keyword

  22. Example • SELECT au_lname, au_fnameFROM authorsWHERE city = (SELECT city FROM publishers WHERE pub_name = 'New Age Books');

  23. Exercise • List the author’s name who appear as the third author on a book. (Note: there's is only one)

  24. Answer • SELECT au_lname, au_fnameFROM authorsWHERE au_id = (SELECT au_id FROM titleauthors WHERE au_ord = 3);

  25. Using Aggregate Functions • Aggregate Functions always return a single value for a set of rows • SELECT titleFROM titlesWHERE advance/ytd_sales > (SELECT MAX(price) FROM titles);

  26. GROUP BY and HAVING • Don't use GROUP or HAVING unless you know that they will return a single value! • SELECT title, typeFROM titlesWHERE price > (SELECT MIN(price) FROM titles GROUP BY type HAVING type = 'trad_cook');

  27. Exercise • List the titles and advances where the advance is equal to the average advance of the popular computing books.

  28. Answer • SELECT title, advanceFROM titlesWHERE advance = (SELECT AVG(advance) FROM titles WHERE type = ‘popular_comp’);

  29. Modified Comparison Operators • You may modify a comparison operator with the ANY or ALL keyword • Allows the Inner query to return more than one value

  30. Example • SELECT au_lnameFROM authorsWHERE city > ALL (SELECT city FROM publishers);

  31. ALL means All • > ALL means greater than all the values returned from the Inner query • SELECT priceFROM titlesWHERE price > ALL (1, 2, 3) • Means where price is greater than 3. • > ALL means greater than the maximum value

  32. ANY means Some • > ANY means greater than at least one value returned from the query • SELECT priceFROM titlesWHERE price > ANY (1, 2, 3) • Means where price is greater than 1. • ANY mean greater than the minimum value

  33. Confusing SQL with English • In English: • Which books commanded an advance greater than any book published by Algodata Infosystems? • In SQL: • Which books have an advance greater than the largest advance paid by Algodata Infosystems? • Must use the ALL keyword, not the ANY

  34. Example • SELECT au_lnameFROM authorsWHERE city > ANY (SELECT city FROM publishers);

  35. Comparing IN, ANY and ALL • The '= ANY' operator is equivalent to IN • SELECT au_lname, au_fnameFROM authorsWHERE city IN (SELECT city FROM publishers); • SELECT au_lname, au_fnameFROM authorsWHERE city = ANY (SELECT city FROM publishers);

  36. < > ANY is different than NOT IN • SELECT DISTINCT au_lname, au_fnameFROM authorsWHERE city < > ANY (SELECT city FROM publishers); • What does this find?

  37. Discussion • This returns all authors because every author does not in at least one of the citys that a publishers is headquartered in.

  38. Correctly Done • SELECT au_lname, au_fnameFROM authorsWHERE city NOT IN (SELECT city FROM publishers); • Also, the < > ALL would have worked

  39. Exercise • List the sales order number, store id, and sale date that have a sale date later than any shipped date for books that cost $19.99.

  40. Answer • SELECT sonum, stor_id, sdateFROM sales, salesdetailsWHERE sdate > date_shippedAND title_id IN (SELECT DISTINCT title_id FROM titles WHERE price = 19.99);

  41. Correlated Subqueries • Inner query references the outer query one row at a time for each row in the Outer query.

  42. Example • SELECT pub_nameFROM publishers pWHERE 'business' IN (SELECT type FROM titles WHERE pub_id = p.pub_id); • Inner query needs values from the Outer query, then passes result to the Outer query

  43. EXISTS Correlated Subquery • EXISTS tests for the presence or absence of the "empty set" of rows. • If Inner query returns at least one row… • EXISTS will succeed • NOT EXISTS will fail • If Inner query returns no row… • EXISTS will fail • NOT EXISTS will succeed

  44. Example • SELECT DISTINCT pub_nameFROM publishers pWHERE EXISTS (SELECT * FROM titles WHERE pub_id = p.pub_id AND type = 'business');

  45. Easier to use Join • SELECT DISTINCT pub_nameFROM publishers p, titles tWHERE t.pub_id = p.pub_idAND type = 'business';

  46. Example - Non-Existence • SELECT titleFROM titlesWHERE NOT EXISTS (SELECT title_id FROM salesdetails WHERE title_id = titles.title_id);

  47. Intersection & Difference • EXISTS and NOT EXISTS can be used for two set theory operation • Intersection: Results are returned for the elements that belong to both original sets • Difference: Results are returned for the elements that belong only to the first of the two sets.

  48. Example - Intersection • SELECT DISTINCT cityFROM authorsWHERE EXISTS (SELECT * FROM publishers WHERE authors.city = publishers.city); • SELECT DISTINCT city FROM authors WHERE city IN (SELECT city FROM publishers WHERE city = authors.city);

  49. Example - Difference • SELECT DISTINCT cityFROM authorsWHERE NOT EXISTS (SELECT * FROM publishers WHERE authors.city = publishers.city);

  50. Subquery Rules • The select of an Inner query using an 'IN' or a comparison operator can only have one expression or column name • Select list of an Inner query should be * when using EXISTS • Subqueries using unmodified operators must return a single value • Subqueries cannot use ORDER BY

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