SQL PRIMARY KEY Constraint
The PRIMARY KEY Constraint is a special single field that used to uniquely defines all records in a table.
PRIMARY KEY Constraint
- A PRIMARY KEY must contain a unique values for each row of data in a SQL table.
- The PRIMARY KEY can not contain NULL values.
PRIMARY KEY - CREATE TABLE [MySQL]
CREATE TABLE STUDENTS (
ID INT NOT NULL,
FULL_NAME VARCHAR (200) NOT NULL,
ADDRESS VARCHAR (200),
CITY VARCHAR (200),
COUNTRY VARCHAR (200) NOT NULL,
PRIMARY KEY (ID)
);
PRIMARY KEY - CREATE TABLE [SQL/Oracle/MS Access]
CREATE TABLE STUDENTS (
ID INT NOT NULL PRIMARY KEY,
FULL_NAME VARCHAR (200) NOT NULL,
ADDRESS VARCHAR (200),
CITY VARCHAR (200),
COUNTRY VARCHAR (200) NOT NULL
);
ALTER TABLE - PRIMARY KEY
ALTER TABLE STUDENTS
ADD PRIMARY KEY (ID);
PRIMARY KEY - Multiple Columns
A table can have only one PRIMARY KEY but this Primary Key can consist of single or multiple columns.
CREATE TABLE STUDENTS (
ID INT NOT NULL,
FULL_NAME VARCHAR (200) NOT NULL,
ADDRESS VARCHAR (200),
CITY VARCHAR (200),
COUNTRY VARCHAR (200) NOT NULL,
CONSTRAINT PK_Student PRIMARY KEY (ID, FULL_NAME)
);
ALTER TABLE - [PK] Multiple Column
ALTER TABLE STUDENTS
ADD CONSTRAINT PK_Student PRIMARY KEY (ID, FULL_NAME);