Database Design lecture 3_1 1 Database Design Lecture 3_1 Data definition in SQL.

Database Design lecture 3_1 1

Database Design

Lecture 3_1Data definition in SQL


SQL Overview

SQL (Structured Query Language) has 3 major components:

– A DDL for defining database structure.– A DML for retrieving and updating data.– A DCL for data control, granting of rights etc.

Official pronunciation is ‘S-Q-L'. Can be used by range of users including DBAs,

management, application developers, and other types of end users.

Easy to learn:

– Consists of standard English words, case insensitive


SQL

An ISO standard now exists for SQL, making it both the formal and de facto standard language for relational databases (Latest version SQL-99).

Supported by many vendors– There are vendor-specific variations

Data types vary from vendor to vendor Standard SQL data types: see text 8.1

Different conformity to SQL standards SQL terminology table, row, column SQL uses bag-semantics-- a table may contain

duplicate rows We will use Oracle 10g SQL, but omitting most of the

complex Oracle-specific details.


Oracle Data Types

Basic built-in data types include– Numbers

Number– Character Strings

Charvarchar2

– Date and timeDate

– large strings and BLOBs (binary large objects)


Number

Syntax for specifying numbers– Number (precision, scale)

Precision is the maximum digits of numbersScale specifies the position of decimal point.Eg

Number(5) 5 digit integer, 12345 Number(6,2) 6 digit (not including decimal

point) decimal number with 2 digits after the decimal point ,

1234.56 Can store 1—38 digits precision


String

To store strings, you can choose from – Char

stores fixed-length character strings of up to 2000 characters. Eg. char(10)

should use it to store short strings or strings with ‘neat’ lengths

– Varchar2 Stores variable-length strings of up to 4000

characters long. Eg. Varchar2(50) Preferred for most string types

– String values are quoted with single quotesEg ‘12234’, ‘abcd’, ‘a12’


Date and time

Oracle uses the date data type to store both date and time– Always uses 7 bytes for date-time data.– Oracle date has rich formats, you need to specify it

SS second 0-59

MI Minute 0-59

HH Hour 1-12

HH24 Military hour 1-24

DD day of month 1-31 (depends on month)

DAY day of the week Sunday-Saturday

D day of the week 1-7

MM month number 1-12

MON month abbreviated Jan—Dec

Month Month spelled out January-December

YY last 2 digits of year eg, 98

YYYY full year value eg, 1998


Date and time

Example of date format:– ‘dd-mon-yyyy’ 01-dec-2001– ‘dd/mm/yyyy’ 01/12/2001– ‘mm-dd-yy hh:mi:ss’ 12-01-01 12:30:59

Default format: ‘dd-mon-yyyy’ Current date and time:

– Sysdate Oracle has built-in functions that converts between data

types: eg, date to string, string to date, string to number and number to string.


Data Definition

Creating a table

create table table-name ( column-name1 datatype [not null], …….. column-nameN datatype [not null]);Table name – can not exceed 30 characters long, – Must begin with an alphabetic character– May contain letters, numbers, $, # and _– Should not be an oracle reserved word – Should be descriptive


Identifying Primary Key

create table table-name (

column-name1 datatype [not null],

……..

column-nameN datatype [not null],

[constraint constraint-name]

Primary key (clolumn-nameA,…,column-nameX) ); or


column-name1 datatype [not null]

[constraint constraint-name] primary key,

……..

column-nameN datatype [not null])


Example

create table Department (

Dept_no char(4)PRIMARY KEY,

Dept_name varchar2(25));

or

create table Department (

dept_no char(4),

Dept_name varchar2(25),

CONSTRAINT dept_PK PRIMARY KEY(dept_no));


Constraints

Primary key is an example of constraints. Other constraints exist, eg. foreign key, check constraints, null-value constraints.

If you don’t give the constraint name, the system will generate a name automatically, but the name is hard for human understanding.

As seen, primary key constraints (in fact other constraints too) can be specified as row constraints and table constraints. Tables constraints can, for example, identify several columns as the primary key.

Viewing constraints – User constraints are stored in the table

user_constraints;– Can use select command to view the constraints.


Identifying Foreign Keys


column-name1 datatype [not null] [unique],

……..

column-nameN datatype [not null] [unique],

[constraint constraint-name1]

Primary key (clolumnA,…,columnX),

[constraint constraint-name2]

Foreign key (Column_list) references referenced_table

(column_list_in_referenced_table) );

Create table Staff(

staff_no char(3),

staff_name varchar2(20),

dept_no char(4),

Constraint staff_fk foreign key (dept_no) references department (dept_no));


More about Foreign Keys

When you delete or update a value of the columns referenced by other tables, the referential integrity constraints may be violated.

You can specify what action to take in such cases– No action or restrict: reject the delete or update

(default)– Cascade: cascading the deletion – Set null – Set default


Check Constraints

You can use check constraints to limit the values of a column or some columns

Example:Create table staff(

Staff_no char(3),

Staff_name varchar2(20) not null,

Staff_gender char(1) check (staff_gender in (‘M’, ‘F’)),

Staff_salary number(8,2) not null,

Dept_no char(4),

Constraint staff_pk Primary key (staff_no),

Constraint staff_fk Foreign key (dept_no) references department (dept_no),

Constraint staff_sal check (staff_salary >10000.00));


Unique Constraints and Default Values

Create table staff(

Staff_no char(3),

Staff_name varchar2(20) not null,

DateofBirth date,

Staff_nationality char(10) default ‘Australia’,

Staff_salary number(8,2) not null,

Dept_no char(4),

Constraint staff_pk Primary key (staff_no),

Constraint staff_fk Foreign key (dept_no) references department (dept_no) on delete set null,

Constraint staff_sal check (staff_salary >10000.00),

UNIQUE(staff_name, DateofBirth));

The table can not have two rows having the same values for staff_name and DateofBirth

Unlike primary key columns, unique columns are not automatically NOT NULL


Modifying Table Definitions

Alter table table_nameadd (column_specification | constraint,..., column_specification| constraint);

Alter table table_namemodify (column_specification | constraint,...,

Column_specification | constraint);Alter table table_name

drop column column_name | drop (column_list); Alter table table_name

drop primary key;Alter table table_name

drop constraint constraint_name; Compare with standard SQL in text


Examples

alter table orders

add (quantity number (3) not null);

alter table orders

modify (quantity number(5));

alter table orders

drop (quantity);

Be careful if the table contains data.


Dropping Tables

Drop table table_name [Cascade Constraints]; Pay attention to referential integrity constraints when

dropping tables. If Cascade Constraints is used, the constraints will be

dropped first. example

Drop table Staff;

Drop table Department;

Drop table Department cascade constraints;


Viewing/enabling/disabling/dropping Constraints

To view the constraints defined for table Department, type SELECT constraint_name, constraint_type, search_condition

FROM user_constraints

WHERE table_name ='DEPARTMENT'; To disable/enable a constraint, use

ALTER TABLE Table_name DISABLE CONSTRAINT constraint_name;

ALTER TABLE Table_name ENABLE CONSTRAINT constraint_name;

To drop a constraint, use

ALTER TABLE Table_name

DROP PRIMARY KEY| UNIQUE (column_name) | CONSTRAINT constraint_name;


Viewing Tables and Table Structures

To see what tables are there in SQL*plus, type

select * from cat;

To see the structure of a table, type

describe table_name;

or

desc table _name;

Database Design lecture 3_1 1 Database Design Lecture 3_1 Data definition in SQL.

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