UNIQUE Constraint on ALTER TABLE

 UNIQUE Constraint on ALTER TABLE

Continuation of last blog…

To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD UNIQUE (ID);

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

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

DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

ALTER TABLE Persons
DROP INDEX UC_Person;

MySQL PRIMARY KEY Constraint

MySQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values.

A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

PRIMARY KEY on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:

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

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);