Subqueries Lecture 9

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1 Subqueries Lecture 9 Robb T. Koether Hampden-Sydney College Mon, Feb 6, 2012 Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

2 1 Subqueries 2 Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

3 Outline 1 Subqueries 2 Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

4 Subqueries In MySQL queries may be nested. For example, suppose we have a table new_courses that contains tuples of new courses to be added to courses. The following query will insert the new courses into the courses table. Nested Queries INSERT INTO courses (SELECT * FROM new_courses); Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

5 Subqueries Similarly, if we have a table old_courses that contains tuples of old courses to be deleted from courses, then the following query will delete the old courses from the courses table. Nested Queries DELETE FROM courses WHERE (dept, number, title, credit) IN (SELECT * FROM old_courses); Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

6 The company Database For the rest of this lecture we will use the company database, adapted from the example in the textbook. The company database has the following tables. employees departments projects works_on dependents Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

7 Outline 1 Subqueries 2 Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

8 attribute IN relation (attribute_list) IN relation The IN operator may be used in the WHERE clause to test whether a value or set of values is in a relation. The expression is true if the attribute or attribute list matches any of the tuples in the relation. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

9 SELECT fname, lname FROM employee WHERE ssn IN (SELECT ssn FROM dependents); Find all employees who have dependents. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

10 SELECT fname, lname FROM employee WHERE ssn IN (SELECT ssn FROM dependents); Find all employees who have dependents. What is another way to write the query without using IN? Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

11 SELECT fname, lname FROM employee WHERE ssn IN (SELECT mgr_ssn FROM departments); Find all employees who are managers. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

12 SELECT fname, lname FROM employee WHERE ssn IN (SELECT mgr_ssn FROM departments); Find all employees who are managers. Write this another way without using IN. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

13 SELECT pname, sex FROM projects AS p NATURAL JOIN employees WHERE ssn IN (SELECT ssn FROM works_on AS w WHERE w.proj = p.proj) Create a table of project names and sexes of all employees working on that project. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

14 SELECT pname FROM projects AS p WHERE M IN (SELECT sex FROM employees NATURAL JOIN works_on AS w WHERE p.proj = w.proj GROUP BY sex) Find all projects, if any, on which at least one male is working. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

15 SELECT pname FROM projects AS p WHERE M IN (SELECT sex FROM employees NATURAL JOIN works_on AS w WHERE p.proj = w.proj GROUP BY sex) Find all projects, if any, on which at least one male is working. Find all projects, if any, on which no male is working. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

16 Using the NATURAL JOIN Operator SELECT pname FROM projects NATURAL JOIN employees NATURAL JOIN works_on WHERE sex = M GROUP BY proj HAVING COUNT(*) > 0; Any query that can be accomplished by using IN can also be accomplished by using joins. Robb T. Koether (Hampden-Sydney College) SubqueriesLecture 9 Mon, Feb 6, / 13

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