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SQL-Unique key constraint|What is unique key constraint in MYSQL|Netezza

#CTTUTORIAL #SQL #UNIQUEKEY
In this video we will learn
1. What is unique key constraint
2. When do you use unique key constraint over primary key constraint
3. Difference between unique key constraint and primary key constraint

code:
Create Table Constraints:
SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);

MySQL:

CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT UC_Person UNIQUE (ID,LastName)
);

For Alter Table:
MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD UNIQUE (ID);

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);

MySQL:

ALTER TABLE Persons
DROP INDEX UC_Person;
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT UC_Person;

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