Introduction to SQL: Intro to Querying

William Hendrix

EECS 317, Spring 2014

Lecture 16

Today

• Modifying tables: ALTER TABLE, DROP TABLE

• Adding data: INSERT

• Querying: the SELECT statement

– Basic queries

– NULL values

– Quantifiers

– Set operations

– Joins

2

CREATE TABLE exercise

Create tables for the following schema in SQL.

• BoughtBy(CustName, PubCode)

– Foreign keys: CustName (Customer), PubCode (Publication)

• Customer(Name, Street, City, State, Zip)

• Department(ID, Name, Manager)

– Foreign key: Manager (Employee)

• Employee(ID, Name, Salary, DeptID)

– Foreign key: DeptID (Department)

• Publication(Code, Title, DeptID, StartDate)

– Foreign key: DeptID (Department, not NULL)

• Topic-Area(PubCode, TopicCode, Name)

– Foreign key: PubCode (Publication, deletes)

In what order should the tables be created? 3

Changing your mind

• Deleting or renaming a table

– DROP TABLE TableName [CASCADE];

– CASCADE drops foreign keys pointing to this table (error otherwise)

• Changing a table specification

– ALTER TABLE TableName <Changes...>;

– Renaming table

• RENAME TO NewName

– Adding columns

• ADD ColName Domain ...

– Removing columns

• DROP COLUMN ColName

– Adding constraints

• ADD (PRIMARY KEY(Att1, ...))

– Removing constraints

• DROP FOREIGN KEY(Att1) REFERENCES TableName

• DROP CONSTRAINT Cname

– MODIFY: Many ways to use

4

Table modification example

• Suppose that we incorrectly constructed a Mayors table:

• CREATE TABLE Mayors(City VARCHAR(20), Election INTEGER,

Mayor VARCHAR(50), CONSTRAINT pk PRIMARY KEY(City,

Election))

• CREATE TABLE Cities(City VARCHAR(20) PRIMARY KEY

REFERENCES Mayors, State CHAR(2))

Problem

• Same city in different states (Springfield, IL and Springfield, MA)

5

Solution

1. Drop the Cities table: DROP TABLE Cities;

2. Add State to the Mayors table:

– ALTER TABLE Mayors ADD State CHAR(2);

3. Fix the primary key to include city and state:

– ALTER TABLE Mayors DROP CONSTRAINT pk;

– ALTER TABLE Mayors ADD CONSTRAINT pk PRIMARY

KEY (City, State, Election));

Populating tables

• Most database management software (DBMS) have utilities for importing information from text files, Excel, etc.

• Accomplished in SQL with INSERT command

• Syntax: INSERT INTO Table (Attributes) VALUES (Values);

– Attributes can be specified in any order

– Unspecified attributes become NULL or default value

– Not needed if specifying all attributes in order

• INSERT can also add data from other tables

• Syntax: INSERT INTO Table (Attributes) (SELECT ...);

– Useful for creating copies of data

– Analogous to renaming operation in relational algebra

6

Querying tables

• SELECT statement

• Usage: SELECT Attrib(s) FROM Table WHERE Condition(s);

– WHERE clause is optional (retrieves all tuples if omitted)

– Use * to retrieve all attributes

• SELECT * FROM Employees;

– Separate multiple attributes with commas

• SELECT Fname, Lname FROM Employees;

• Criteria construction

– Basic relational operators: =, <>, <, <=, >, >=

• Compares strings alphabetically, dates temporally

• Oracle: punctuate strings and dates with single quotes

• SELECT * FROM Employees WHERE Employees.Salary >=

60000;

– Logical connectives: NOT, AND, OR

• SELECT * FROM Employees WHERE NOT (Employees.Lname =

‘Smith’);

7

Comparing NULL values

• NULL values in the database represent data with unknown values

• Induces 3-valued logic: TRUE, FALSE, UNKNOWN (NULL)

– NULL = ‘James’? NULL >= ‘01-01-1900’?

• We don’t know: NULL

–

• Also unknown (NULL)

– Most comparisons with NULL are NULL except:

•

• Use Attrib IS NULL to identify NULL values

• NVL(Attrib, Value) can replace NULL with Value

– Sometimes called IFNULL or ISNULL

8

T F N

T T T T

F T F N

N T N N

T F N

T T F N

F F F F

N N F N

Table and column aliasing

• Aliasing

– Analogous to renaming in relational algebra

– Can alias table and attribute names

– Performed with AS keyword

• SELECT E.Lname AS Family FROM Employees AS E WHERE

E.Lname < ‘Johnson’;

– Renaming tables is very useful with multi-table queries

• Attributes must be specified by their table if query involves multiple tables with same attribute

• Not necessary if unambiguous

– Note: renaming tables happens before WHERE clause, renaming

attributes happens afterwards

• Wrong: SELECT Lname AS Family FROM Employees WHERE Family < `Johnson’;

– AS keyword can be omitted

• SELECT Lname Family FROM Employees WHERE Lname <

‘Johnson’; 9

Advanced criteria constructions

• Between: BETWEEN, NOT BETWEEN

– SELECT * FROM Employees WHERE Fname BETWEEN

‘George’ AND ‘Martha’;

– Different DBMSs include or exclude boundaries

• Oracle BETWEEN is inclusive

• String similarity: LIKE, NOT LIKE

– Use % to represent “anything”

– First names that start with J: Fname LIKE ‘J%’

– Last names that end with son: Lname LIKE ‘%son’

– Names that do not have a t in them: Name NOT LIKE ‘%t%’

• Set comparison: IN, NOT IN

– SELECT * FROM Musicians WHERE Fname NOT IN

(‘John’, ‘Paul’, ‘George’, ‘Ringo’);

– SELECT * FROM Musicians M WHERE Lname IN

(SELECT Fname FROM M);

10

Quantifiers

• Existential Quantifier

– Usage: EXISTS (Set or SELECT query)

– Example

• SELECT * FROM Musicians M WHERE EXISTS (SELECT

Fname FROM Musicians WHERE Fname = M.Lname);

– Alternative: ANY, SOME (Set or SELECT query)

– Example

• SELECT * FROM Musicians WHERE Fname <> ANY

(‘John’, ‘Paul’, ‘George’, ‘Ringo’);

• Universal quantifiers

– Usage: ALL (Set or SELECT query)

– Can be converted to NOT EXISTS expression (DeMorgan’s Law)

– Example

• SELECT * FROM Employees WHERE Salary >= ALL

(SELECT Salary FROM Employees);

11

Set operations and quantifiers

• Cartesian product

– Use commas to separate tables

• SELECT * FROM Employees, Departments;

– Attributes from different tables not required to be distinct

– Can be used to perform theta-joins

• Set union, intersection, and difference

– Combine results from two queries

– Attributes do not need to be identical, just compatible

– UNION, INTERSECT, MINUS (or EXCEPT)

– Examples

Emp(SSN, Name, Salary), Mgr(SSN, Benefits)

• SELECT Salary FROM Emp UNION SELECT Benefits FROM Mgr;

• SELECT Name FROM Emp MINUS SELECT Name FROM Emp, Mgr

WHERE Emp.SSN = Mgr.SSN;

12

Table joins

• Joins

– Use JOIN keyword, with NATURAL, FULL, LEFT, or RIGHT

• May use INNER JOIN if not using FULL, LEFT, or RIGHT

• FULL, LEFT, and RIGHT may be followed by OUTER

– Specify join as table value • SELECT * FROM Employee NATURAL JOIN Dept;

• SELECT * FROM Employee NATURAL FULL OUTER JOIN Dept;

– Need to specify ON Condition if join is not NATURAL

• SELECT * FROM Students S JOIN HasTaken H ON

S.NetID = H.Student;

• Equivalent to: SELECT * FROM Students S, HasTaken H WHERE S.NetID = H.Student;

• SELECT * FROM STUDENTS S LEFT JOIN HasTaken H ON

S.NetID = H.Student;

• Equivalent to: SELECT * FROM Students S, HasTaken H WHERE S.NetID = H.Student OR S.NetID IS NULL;

13

Coming up

• UPDATE, DELETE statements

• Aggregation

• Hardware considerations and indexing

• Recommended readings

– Sections 7.1-3

14