Creating a database

Creating a Database Chapter 13

DDL - D ata D efinition L anguage Define or create a new table Remove a table Change definition/structure of an existing table Define a virtual table(view) Make index Control physical storage of data

DDL consists of Create – Defining or creating database objects Drop – Removing an existing database object Alter – Changing definition of a database object

Create Table Define a new table Prepare to accept data Creator becomes owner Table must have legal SQL name Name not already existing

Create Table (contd.)-Column Definitions Column name(mandatory)- unique for the table but can be repeated in other tables Data Type(mandatory)- also require size, if not given –default…(domain can be used) Required data- NOT NULL if required Default value- optional default value Constraints:Primary key,Foreign key,Unique,Check

Create table example create table classmaster ( classid varchar2(4) primary key, classname varchar2(15) not null unique, intake number(3));

Create table example Create table classmaster2 ( classid varchar2(4) , classname varchar2(15) not null , intake number(3), doa date default '01-JUL-10', primary key(classid), unique(classname), check (intake between 0 and 100), Constraint fk_clm_clname foreign key(classname) references classmaster(classname) ) ;

Create table example Create table studentmaster ( regno varchar2(4) primary key, classid varchar2(14) constraint fk_sm_clsmstr references classmaster(classid) on delete set null, sfname varchar2(20) not null, slname varchar2(20) not null, DOB date, DOA date default ’01-JUL-10’, AdmStatus char(1) check (admstatus in ('A','C','P')), freeship char(1) check (freeship in ('Y','N')), check( doa>dob)); );

Creating table from another table Create table emp_copy as select * from emp; This command will create a table named emp_copy with the same structure and all records of the table emp. To create a table with the same structure without any records: Create table emp_str_copy as select * from emp where 1=2; To create table with selected columns and rows: Create table emp_fewcopy as select empno,ename,sal from emp where sal>1000;

Constraints Details of constraints can be obtained from the table user_constraints If constraint name is not declared dbms provides a name on its own

Removing a table DROP TABLE <table_name>; drop table classmaster2; Example:

Changing table definition Add a column definition to the table Drop a column Change the default value for a column Add or drop a primary key for a table Add or drop a foreign key for a table Add or drop a unique constraint for a table Add or drop a check constraint for a table Add or drop a not null constraint for a table ALTER TABLE statement can:

Adding a column Alter table studentmaster add contact_no varchar2(20); Usually not null not given or if given should be with a default value. If data already present, adding a column with not null will violate not null constraint

Dropping a column Alter table classmaster drop intake; This statement drops the column named intake from the table classmaster.

Change the default value for a column alter table girls modify locality default 'BORIVALI'; alter table classmaster modify classname default ‘TYBScIT’; alter table classmaster modify classname default NULL;

Add or drop primary key for a column alter table girls add primary key (name); alter table girls drop primary key ; (Column_name) (Table_name)

Add or drop foreign key for a column alter table emp drop constraint FK_DEPTNO ; Table altered alter table emp add constraint fk_deptno foreign key(deptno) references dept(deptno); Table altered

How to know constraint name select constraint_name,constraint_type from user_constraints where table_name='STUDENTMASTER'; CONSTRAINT_NAME C ------------------------------ - SYS_C002715 C SYS_C002716 C SYS_C002717 C SYS_C002718 C SYS_C002719 P fk_sm_clsmstr R

Add or drop UNIQUE constraint for a column alter table girls add constraint un_name unique(name); Table altered alter table girls drop constraint un_name ; Table altered

Add or drop CHECK constraint for a column alter table girls add constraint chk_loc check(locality in ('BORIVALI',‘KANDIVALI'); Table altered alter table girls drop constraint chk_loc; Table altered

Add or drop NOT NULL constraint for a column alter table girls modify locality not null; Table altered Get the constraint name from user_constraints alter table girls drop constraint SYS_C002763 ; Table altered

Creating assertions and domains Create assertion <assertion_name> Check (check_condition) Create domain <domain_name> <data type> check (check_condition) Drop assertion <assertion_name> Drop domain <domain_name> To alter the definitions of assertions or domains they must be first dropped and then recreated.

Creating and Dropping Alias/Synonyms A synonym or alias is a name defined by the user that stands for the name of some other table. Create synonym employee for emp; Synonym created Drop synonym employee; Synonym dropped

Indexes Provides rapid access to rows of a table based on values of one or more columns Stores data values and pointers to the rows where those values occur Data values arranged in ascending or descending order Presence of index transparent to user

Advantages- Speeds the execution of statements with search conditions referring to indexed columns Good for tables with more queries and less frequent change

Disadvantages X Consumes additional disk space X must be updated when a row is added or indexed column is updated-additional overhead X Not worth when a column contains wide range of values or large number of null values DBMS automatically creates index on columns with unique constraint

Example Indexes For creating indexes create unique index ind_stm_sfname on studentmaster(sfname); create unique index ind_stm_slname_desc on studentmaster(slname desc); For removing indexes Drop index ind_stm_slname_desc ; To view details of indexes - user_indexes

Types of Indexes Normal Function based B-tree Hash Bit map

Database structure Single database architecture Multi database architecture Multi location architecture Databases on multiple servers

Single database architecture DBMS supports one system wide database Tables in various applications can reference one another No choice is to be made as only one database-easy access Database grows huge as more and more applications are added Managing databases becomes difficult Examples- Oracle,DB2

Multi database architecture Each database assigned unique name Each database dedicated to particular application Divides data management tasks into smaller , manageable chunks Requires no overall coordination New application added as a new database,no need to disturb existing db Users remember structure of their db easily

Multi database architecture(contd.) Individual db become unconnected Table in one db cannot contain a foreign key reference to table in another db Cross db queries usually not supported Where ever allowed extended notation- db_name.owner.table_name Commands used to connect to other db Connect db_name Use db_name

Multi Location Architecture Supports multiple dbs by using system’s directory structure to organize them Each application has own database Each db in same directory has unique name Dbs may have same name in different directories Flexible-suited for applications where user needs his/her own applications in separate dbs

Multi Location Architecture(contd.) Disadvantages same as that of multi db. No master db to keep track of all dbs Gaining access to db objects difficult as directory name is to be specified

Databases on multiple servers Databases on a network Databases associated with named servers on a network Within one server db’s with different names Mapping of server names to physical server locations done by network software Mapping of db names to physical files on a server done by dbms

Creating a database

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