Introduction to SQL Example: CREATE TABLE Students (Sid CHAR(9), Name VARCHAR(20), Login CHAR(8), Age INTEGER, Gpa REAL);

CS 5614: Basic Data Definition and Modification in SQL 67

Introduction to SQL

Structured Query Language (‘Sequel’) Serves as DDL as well as DML

Declarative Say what you want without specifying how to do it One of the main reasons for commercial success of DBMSs

Many standards and implementations ANSI SQL SQL-92/SQL-2 (Null operations, Outerjoins etc.) SQL3 (Recursion, Triggers, Objects) Vendor specific implementations

“Bag Semantics” instead of “Set Semantics” Used in commercial RDBMSs

CS 5614: Basic Data Definition and Modification in SQL 68

Example:

Create a Relation/Table in SQL

CREATE TABLE Students (sid CHAR(9), name VARCHAR(20), login CHAR(8), age INTEGER, gpa REAL);

Support for Basic Data Types CHAR(n) VARCHAR(n) BIT(n) BIT VARYING(n) INT/INTEGER FLOAT REAL, DOUBLE PRECISION DECIMAL(p,d) DATE, TIME etc.

CS 5614: Basic Data Definition and Modification in SQL 69

More Examples

And one for Courses

CREATE TABLE Courses (courseid CHAR(6), department CHAR(20));

And one for their relationship!

CREATE TABLE takes (sid CHAR(9), courseid CHAR(6)); Why?

Can also provide default values

CREATE TABLE Students (sid CHAR(9), .... age INTEGER DEFAULT 21, gpa REAL);

CS 5614: Basic Data Definition and Modification in SQL 70

Examples Contd.

DATE and TIME Implementations vary widely Typically treated as strings of a special form Allows comparisons of an ordinal nature (<, > etc.)

DATE Example ‘1999-03-03’ (No Y2K problems)

TIME Examples ‘15:30:29’ ‘15:30:29.3875’

Deleting a Relation/Table in SQL

DROP TABLE Students;

CS 5614: Basic Data Definition and Modification in SQL 71

Modifying Relation Schemas

‘Drop’ an attribute (column)

ALTER TABLE Students DROP login;

‘Add’ an attribute (column)

ALTER TABLE Students ADD phone CHAR(7);

What happens to the new entry for the old records? Default is ‘NULL’ or say

ALTER TABLE Students ADD phone CHAR(7) DEFAULT ‘unknown’;

Always begin with ‘ALTER TABLE ’

Can use DEFAULT even with regular definition (as in Slide 69)

CS 5614: Basic Data Definition and Modification in SQL 72

How do you enter/modify data?

INSERT command

INSERT INTO Students VALUES (‘53688’,’Mark’,’mark2345’,23,3.9)

Cumbersome (use bulk loading; described later)

DELETE command

DELETE FROM Students S WHERE S.name = ‘Smith’

UPDATE command

UPDATE Students S SET S.age=S.age+1, S.gpa=S.gpa-1 WHERE S.sid = ‘53688’

CS 5614: Basic Data Definition and Modification in SQL 73

Domains

Domains: Similar to Structs and other user-defined types

CREATE DOMAIN Email AS CHAR(8) DEFAULT ‘unknown’; .... login Email // instead of login CHAR(8) DEFAULT ‘unknown’

Advantages: can be reused

junkaddress Email, fromaddress Email, toaddress Email, ....

Can DROP DOMAINS too!

DROP DOMAIN Email; Affects only future declarations

CS 5614: Basic Data Definition and Modification in SQL 74

Keys

To Specify Keys Use PRIMARY KEY or UNIQUE Declare alongside attribute For multiattribute keys, declare as a separate line

CREATE TABLE takes ( sid CHAR(9), courseid CHAR(6), PRIMARY KEY (sid,courseid) );

Whats the difference between PRIMARY KEY and UNIQUE? Typically only one PRIMARY KEY but any number of UNIQUE keys Implementor allowed to attach special significance

CS 5614: Basic Data Definition and Modification in SQL 75

Creating Indices/Indexes

Why? Speeds up query processing time

For Students

CREATE INDEX indexone ON Students(sid); CREATE INDEX indextwo ON Students(login);

How to decide attributes to place indices on? One is (typically) created by default on PRIMARY KEY Creation of indices on UNIQUE attributes is implementation-dependent In general, physical database design/tuning is very difficult! Use Tools: Microsoft SQLServer has an index selection Wizard

Why not place indices on all attributes? Too cumbersome for insertions/deletions/updates

Like all things in computer science, there is a tradeoff! :-)

CS 5614: Basic Data Definition and Modification in SQL 76

Other Properties

‘NOT NULL’ instead of DEFAULT

CREATE TABLE Students (sid CHAR(9), name VARCHAR(20), login CHAR(8), age INTEGER, gpa REAL);

Can insert a tuple without a value for gpa NULL will be inserted

If we had specified

gpa REAL NOT NULL);

insert cannot be made!

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5 CS 5614: Misc. SQL Stuff, Safety in Queries 81

What is still to be covered (and will be)

Declaring constraints Domain Constraints Referential Integrity (Foreign Keys)

More SQL Stuff Subqueries Aggregation

SQL Peculiarities Strange Phenomena More on Bag Semantics Ifs and Buts

Embedding SQL in a Programming Environment Accessing DBs from within a PL (will be covered in Module 3)

CS 5614: Misc. SQL Stuff, Safety in Queries 82

What will be mentioned (but not covered in detail)

Triggers Read Cow Book or Boat Book

More SQL Gory Details

Recursive Queries (SQL3) Why do we need these?

Security

Authorization and Privacy

Trends towards Object Oriented DBMSs

CS 5614: Misc. SQL Stuff, Safety in Queries 83

Tuple-Based Domain Constraints

Already Seen NOT NULL UNIQUE, PRIMARY KEY etc.

In General

CREATE TABLE Students (sid CHAR(9), name VARCHAR(20), login CHAR(8), age INTEGER, gpa REAL, CHECK (gpa >= 0.0) );

Note: Implementations vary, but this is the general idea

Other Complicated Forms Constraints on whole relations, Assertions

CS 5614: Misc. SQL Stuff, Safety in Queries 84

Referential Integrity Constraints

Foreign Keys An attribute a of R1 is a foreign key if it “references” the primary key (say b) of another relation R2 In addition, there is a ref. integrity constraint from R1 to R2.

Example login is a FOREIGN KEY for Students

CREATE TABLE Students (sid CHAR(9) PRIMARY KEY, name VARCHAR(20), login CHAR(8) REFERENCES Accounts(acct), age INTEGER, gpa REAL );

CREATE TABLE Accounts ( acct CHAR(8) PRIMARY KEY );

CS 5614: Misc. SQL Stuff, Safety in Queries 85

Alternatively

Can use “FOREIGN KEY” construct

CREATE TABLE Students (sid CHAR(9) PRIMARY KEY, name VARCHAR(20), login CHAR(8), age INTEGER, gpa REAL, FOREIGN KEY login REFERENCES Accounts(acct) );

CREATE TABLE Accounts ( acct CHAR(8) PRIMARY KEY );

Note: acct should be declared as PRIMARY KEY for Accounts! in both cases

CS 5614: Misc. SQL Stuff, Safety in Queries 86

SQL Subqueries

Given Students(sid,name,login,age,gpa) HasCar(sid,carname) Find the car of the student with login=”mark”

Traditional Way

SELECT carname FROM Students, HasCar WHERE Students.login=’mark’ AND Students.sid=HasCar.sid;

The ‘Subway’

SELECT carname FROM HasCar WHERE sid= (SELECT sid FROM Students WHERE login=’mark’);

CS 5614: Misc. SQL Stuff, Safety in Queries 87

Aggregation

Given Students(sid,name,login,age,gpa)

Find the average of the ages of all the students

Solution

SELECT AVG(age) FROM Students;

Other Operations SUM (summation of all the values in a column) MIN (least value) MAX (highest value) COUNT (the number of values), e.g.

SELECT COUNT(*) FROM Students;

COUNTs the number of Students!

CS 5614: Misc. SQL Stuff, Safety in Queries 88

Ordering

Given Students(sid,name,login,age,gpa)

List the students in (ascending) alphabetical order of name

Solution

SELECT * FROM Students ORDER BY name;

and that for DESCending ORDER is

SELECT * FROM Students ORDER BY name DESC;

Default is ASC

CS 5614: Misc. SQL Stuff, Safety in Queries 89

Grouping

Given Students(sid,name,login,age,gpa)

Find the names of students with gpa=4.0 and group people with like ages together

Solution

SELECT name FROM Students WHERE gpa=4.0 GROUP BY name;

CS 5614: Misc. SQL Stuff, Safety in Queries 90