SQL UNIQUE Constraint Instance Tutorial

Unique constraints of the SQL
Unique identification of unique constraints per record in a database table.

Both the uniqueness and primary constraints provide the guarantee of a single column or set of columns.

A PRIMARY KEY constraint automatically has a unique constraint that defines it.

Please note that you can have many unique restrictions on each table, but only one primary KEY constraint per seat.


Unique SQL constraints Create a table
When the following SQL creates a "p_id" column with a unique constraint, "person" is creating a table:

CREATE TABLE Persons
(
p_id int not null,
LastName varchar (255) is not null 
 , FirstName varchar (255), Address
varchar (255), City
varchar (255),
UNIQUE (p_id
)

SQL Server/oracle/ms Access:
CREATE TABLE Persons
(
p_id int not NULL UNIQUE,
LastName varchar (255) Not null,< c17>
FirstName varchar (255), Address
varchar (255), City
varchar (255)
)

   
   

    
    

    
To make naming a unique constraint and to identify multiple columns on the unique constraints, use the following SQL syntax:
    

    
    

    
Mysql/sql Server/oracle/ms Access:
    

    
    

    

    

    
    
 CREATE TABLE Persons   (p_id int not null,   LastName varchar (255) NOT NULL,   FirstName varcha R (255), address varchar (255), City varchar (255), CONSTRAINT Uc_personid UNIQUE (p_id,lastname)) 

    
    
 
      
      

       
        Database Unique constraints Change table 
       
       
 When you create a "p_id" column for a unique constraint, the table is already established, use the following sql: 
       
       
 
       
       
 alter TAB LE Persons ADD Unique (p_id) 
 
       
       
   
 
       
       
 
        
        
 
         
         
 To make naming a unique constraint, and determined that multiple columns are on a unique constraint, use the following SQL syntax: 
 
         
         
   
 
         
         
 ALTER TABLE Persons ADD CONSTRAINT uc_personid Unique (p_id,lastna Me) 
 
         
         
   
 
         
         
 
          
          
 
           
           
 Drop uniqueness a unique constraint 
 abort a unique about Bundle, use the following sql: 
 
           
           
   
 
           
           
 ALTER TABLE Persons DROP INDEX uc_personid 
 
           
           
   
 
           
           
 
  SQL server/oracle/ms Access:  
 
 ALTER TABLE Persons DROP CONSTRAINT uc_personid< /pre>