CS 464/564 Introduction to Database Management System Instructor: Abdullah Mueen

Transcription

1 CS 464/564 Introduction to Database Management System Instructor: Abdullah Mueen LECTURE 10: INTRODUCTION TO SQL FULL RELATIONAL OPERATIONS MODIFICATION LANGUAGE

2 Union, Intersection, Differences (select emp_no from employees) union (select emp_no from dept_manager); There is no EXCEPT or INTERSECT in MySQL.

3 Joins Inner Join Use ON clause to specify condition. Use NATURAL to go with the attributes present in both operators. 3

4 Example SELECT * FROM dept_manager NATURAL JOIN departments; SELECT * FROM dept_manager JOIN departments ON dept_manager.dept_no=departments.dept_no; SELECT * FROM dept_manager, departments WHERE dept_manager.dept_no=departments.dept_no;

5 Another Example SELECT dept_manager.emp_no, dept_manager.from_date, employees.hire_date FROM dept_manager, employees WHERE employees.emp_no=dept_manager.emp_no;

6 Outerjoins R [LEFT RIGHT] OUTER JOIN S is the core of an outerjoin expression. It is modified by: 1. Optional NATURAL in front of OUTER. 2. Optional ON <condition> after JOIN. 3. Optional LEFT, RIGHT, or FULL before OUTER. LEFT = pad dangling tuples of R only. RIGHT = pad dangling tuples of S only. FULL = pad both; this choice is the default. 6

7 Examples SELECT * FROM dept_manager LEFT OUTER JOIN employees ON dept_manager.emp_no=employees.emp_no; SELECT count(*) FROM dept_manager RIGHT OUTER JOIN employees ON dept_manager.emp_no=employees.emp_no; SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id UNION SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id

8 Aggregations SUM, AVG, COUNT, MIN, and MAX can be applied to a column in a SELECT clause to produce that aggregation on the column. Also, COUNT(*) counts the number of tuples. 8

9 Example: Aggregation SELECT max(hire_date) FROM employees WHERE gender= M ; 9

10 Eliminating Duplicates in an Aggregation Use DISTINCT inside an aggregation. Example: find the number of different titles : SELECT COUNT(DISTINCT title) FROM titles; 10

11 NULL s Ignored in Aggregation NULL never contributes to a sum, average, or count, and can never be the minimum or maximum of a column. But if there are no non-null values in a column, then the result of the aggregation is NULL. Exception: COUNT of an empty set is 0. Sells(store, candy, price), 11

12 Example: Effect of NULL s SELECT count(*) FROM Sells WHERE candy = Snickers ; The number of stores that sell Snickers. SELECT count(price) FROM Sells WHERE candy = Snickers ; The number of stores that sell Snickers at a known price. 12

13 Grouping We may follow a SELECT-FROM-WHERE expression by GROUP BY and a list of attributes. The relation that results from the SELECT-FROM-WHERE is grouped according to the values of all those attributes, and any aggregation is applied only within each group. 13

14 Example: Grouping Find the number of employees in each department: SELECT dept_no, COUNT(emp_no) FROM dept_emp GROUP BY dept_no; Find the number of managers in each department: SELECT dept_no, COUNT(emp_no) FROM dept_manager GROUP BY dept_no; 14

15 Restriction on SELECT Lists With Aggregation If any aggregation is used, then each element of the SELECT list must be either: 1. Aggregated, or 2. An attribute on the GROUP BY list. 15

16 Illegal Query Example You might think you could find the first managers of every department: SELECT from_date, MIN(emp_no) FROM dept_manager GROUP BY dept_no; 16

17 HAVING Clauses HAVING <condition> may follow a GROUP BY clause. If so, the condition applies to each group, and groups not satisfying the condition are eliminated. 17

18 Example: HAVING From dept_emp, find the departments that have more than employees. 18

19 Solution SELECT dept_no, count(emp_no) FROM dept_emp GROUP BY dept_no HAVING count(emp_no) > 50000;

20 Requirements on HAVING Conditions Anything goes in a subquery. Outside subqueries, they may refer to attributes only if they are either: 1. A grouping attribute, or 2. Aggregated (same condition as for SELECT clauses with aggregation). 20

21 Creating Tables create table mysalary(name varchar(30), salary int); create table if not exists mysalary(name varchar(90), salary int);

22 Database Modifications A modification command does not return a result (as a query does), but changes the database in some way. Three kinds of modifications: 1. Insert a tuple or tuples. 2. Delete a tuple or tuples. 3. Update the value(s) of an existing tuple or tuples. 22

23 Insertion To insert a single tuple: INSERT INTO <relation> VALUES ( <list of values> ); Example: add to mysalary(name, salary) the fact that steve s salary is 50K. INSERT INTO mysalary VALUES( Steve, 50000); 23

24 Specifying Attributes in INSERT We may add to the relation name a list of attributes. Two reasons to do so: 1. We forget the standard order of attributes for the relation. 2. We don t have values for all attributes, and we want the system to fill in missing components with NULL or a default value. 24

25 Example: Specifying Attributes INSERT INTO mysalary(salary,name) VALUES(80000, Sally ); 25

26 Adding Default Values In a CREATE TABLE statement, we can follow an attribute by DEFAULT and a value. When an inserted tuple has no value for that attribute, the default will be used. 26

27 Example: Default Values DROP TABLE IF EXISTS mysalary; CREATE TABLE mysalary ( name CHAR(30) PRIMARY KEY, salary INT DEFAULT 10000, phone CHAR(16) ); 27

28 Example: Default Values INSERT INTO mysalary(name) VALUES( Sally ); Resulting tuple: name salary phone Sally NULL 28

29 Inserting Many Tuples We may insert the entire result of a query into a relation, using the form: INSERT INTO <relation> ( <subquery> ); 29

30 Example: Insert a Subquery Using the employees table, enter into the new relation mysalary(name,salary,phone) all of the distinct employees last name who were hired on or after 1995; 30

31 Solution insert into mysalary(name) (select distinct last_name from employees where hire_date >= ' '); 31

32 Deletion To delete tuples satisfying a condition from some relation: DELETE FROM <relation> WHERE <condition>; 32

33 Example: Deletion Delete from mysalary(name,salary,phone) every employee having a name started with M. DELETE FROM mysalary WHERE name LIKE M% ; 33

34 Example: Delete all Tuples Make the relation Likes empty: DELETE FROM mysalary; Note no WHERE clause needed. 34

35 Example: Delete Some Tuples Delete from Candies(name, manf) all candies for which there is another candy by the same manufacturer. DELETE FROM Candies c WHERE EXISTS ( SELECT name FROM Candies WHERE manf = c.manf AND name <> c.name); Correlated Subquery 35

36 Semantics of Deletion --- (1) Suppose Mars makes only Snickers and Twix. Suppose we come to the tuple c for Snickers first. The subquery is nonempty, because of the Twix tuple, so we delete Snickers. Now, when c is the tuple for Twix, the subquery becomes empty and do we delete Twix tuple too? 36

37 Semantics of Deletion --- (2) Answer: we do delete Twix as well. The reason is that deletion proceeds in two stages: 1. Mark all tuples for which the WHERE condition is satisfied. 2. Delete the marked tuples. 37

38 Updates To change certain attributes in certain tuples of a relation: UPDATE <relation> SET <list of attribute assignments> WHERE <condition on tuples>; 38

39 Example: Update Change Pollock s salary to ; UPDATE mysalary SET salary = WHERE name = Pollock ; 39

40 Example: Update Several Tuples UPDATE mysalary SET salary = WHERE name like M% ; 40