This tutorial is the second tutorial about oracle. If you miss first tutorial please read my first tutorial from https://steemit.com/utopian-io/@arif019/oracle-tutorial-1-oracle-ddl-and-dml-statements.
In this tutorial you know about Oracle joins statements and Constraints. These are very important to make a strong database.
Constraints :
Constraints are categorized as follows.
Not null.
Check.
Unique .
Primary key.
Foreign key.
Not null :
This is used to avoid null values. Syntax: create table ‘table-name’(column1 not null,column2,column3..); Ex : create table students_info (id number(20) , name varchar2(50),roll number(30) not null);
Check :
This is used to insert the values based on specified condition. syntax : create table ‘table-name’ ( column1,column2,column3,constraint ‘constraint-name’ check (condition)); Ex : create table marks (id number(20),name varchar2(50),roll number(30),bangla number(20),English number(20),math number(20),constraint ch check(math>50) );
Unique :
This is used to avoid duplicates but it allows nulls. syntax : create table ‘table-name’ (column1,column2,column3, constraint ‘constraint-name’ unique(column-name)); Ex : create table students_info (id number(20), roll number(20),name varchar2(50) ,constraint un unique(roll));
Primary key :
This is used to avoid duplicates and nulls. This will work as combination of unique and not null. syntax : create table ‘table-name’ (column1,column2,column3.. constraint ‘constraint-name’ primary key(‘column-name’));
Ex : create table students_info (id number(20), roll number(20),name varchar2(50) ,constraint pk primary key (id));
Foreign key :
This is used to reference the parent table primary key column which allows duplicates. syntax : create table ‘table-name’ (column1,column2,column3.. constraint ‘constraint-name’ foreign key(‘column-name’) references ‘table-name’(column-name)); Ex : create table marks (id number(20), roll number(20),name varchar2(50),bangle number(20),English number(20), math number(20) ,constraint fk foreign key (id) references students_info(id));
