Database Systems CSE 303. Outline. Lecture 06: SQL. What is Sub-query? Sub-query in WHERE clause Subquery

Transcription

1 Database Systems CSE 303 Lecture 06: SQL 2016 Subquery Outline What is a Subquery Subquery in WHERE clause >ALL, >ANY, >=ALL, <=ANY etc. Subquery in FROM clause 1 2 What is Sub-query? Sub-query in WHERE clause 2 1 Queries within query Query as a part of another query SELECT <attributes> FROM <one or more relations> WHERE <conditions> 3 4 1

2 Sub-query in WHERE clause Find the employee(s) with the highest paid salary First thought FROM WHERE salary = Sub-query in WHERE clause Find the employee(s) with the highest paid salary Suppose you are not allowed to use any aggregate function?? FROM WHERE salary = (SELECT MAX(salary) FROM ) 5 6 Sub-query in WHERE clause Find the employee with the highest paid salary > ALL >= ALL < ALL <= ALL > ANY >= ANY < ANY <= ANY FROM WHERE salary >= ALL (SELECT salary FROM ) Chain of sub queries in the WHERE clause Find the employee(s) with the highest paid salary FROM WHERE salary > ALL (SELECT salary FROM WHERE salary < ANY (SELECT salary FROM ) 7 ) 8 2

3 Chain of sub queries in the WHERE clause Find the highest paid employee(s) in each department, dept FROM WHERE (dept, salary) IN (SELECT dept, MAX(salary) FROM GROUP BY dept); Correlated sub queries in the WHERE clause Find the employees that earn above average SELECT * FROM WHERE salary > Correlated sub queries in the WHERE clause Find the employees that earn above average SELECT * FROM WHERE salary > (SELECT AVG(salary) FROM ) Correlated sub queries in the WHERE clause Find the employees that earn above average of the employees within respective department SELECT * FROM s WHERE salary > (SELECT AVG(salary) FROM t WHERE s.dept = t.dept);

4 EXISTS and NOT EXISTS Find all manufacturers that make both and Laptop SELECT <attributes> FROM <one or more relations> WHERE EXISTS (sub-query) 1001 A 1002 B Laptop 2001 A 2002 B Empty FALSE Not Empty TRUE C SELECT DISTINCT FROM WHERE EXISTS (SELECT * FROM Laptop WHERE. = Laptop.) 2003 D 14 A B Find all manufacturers that make s but not Laptops 1001 A 1002 B 1003 C SELECT DISTINCT FROM WHERE NOT EXISTS (SELECT * FROM Laptop WHERE. = Laptop.) Laptop 2001 A 2002 B 2003 D 15 C SELECT FROM WHERE speed = (SELECT MAX(speed) FROM ); Practice correlated sub queries Find the with maximum processing speed using and without using aggregate function SELECT FROM WHERE speed >= ALL (SELECT speed FROM ) MODEL SPEED RAM HD PRICE

5 Correlated sub queries in the FROM clause 17 Product Find manufacturers of at least two different computers ( s or laptops) with speeds of at least 2.80 Maker Model Type A 1001 pc A 1002 pc A 1003 pc A 2004 laptop A 2005 laptop A 2006 laptop B 1004 pc B 1005 pc B 1006 pc B 2007 laptop C 1007 pc. MODEL SPEED RAM HD PRICE MODEL SPEEDRAM HD SCREENPRICE Laptop Find manufacturer of at least two different s with speed of at least 2.0 Find manufacturer of at least two different Laptops with speed of at least 2.0 SELECT FROM Product, WHERE Product. =. AND speed >= 2.0 GROUP BY HAVING COUNT(.) >= 2 SELECT FROM Product, Laptop WHERE Product. = Laptop. AND speed >= 2.0 GROUP BY HAVING COUNT(Laptop.) >=

6 Find manufacturers of at least two different computers ( s or laptops) with speeds of at least 2.80 Think combined SELECT FROM Product, (SELECT, speed FROM UNION SELECT, speed FROM Laptop) T WHERE Product. = T. AND speed >= 2.0 GROUP BY HAVING COUNT(T.) >=