SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table. The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:
SQL Server / Oracle / MS Access:                                                                                  CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
MySQL / SQL Server / Oracle / MS Access:                                                                      CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
MySQL                                                                                                                              CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
SQL UNIQUE Constraint on ALTER TABLE
For single Col-    ALTER TABLE Persons ADD UNIQUE (P_Id)
For multi Col- ALTER TABLE Persons ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
For Drop:
  MySQL- ALTER TABLE Persons DROP INDEX uc_PersonID
  SQL Server / Oracle / MS Access- ALTER TABLE Persons DROP CONSTRAINT uc_PersonID