Rimary key database PRIMARY KEY constraint
Database PRIMARY KEY constraint
The PRIMARY KEY constraint uniquely identifies each record in a database table.
The primary key must contain unique values.
Primary key columns cannot contain null values.
Each table should have a primary key, and each table can have only one primary key.
Database primary KEY constraint creation table
When the following SQL creates a "p_id" column on a primary key, "person" is creating a table:
MySQL's:
CREATE TABLE Persons
(
p_id int not NULL,
LastName varchar (255) Not NULL,
FirstName varchar (255),
Address varchar (255),
City varchar (255),
PRIMARY KEY (p_id)
)
SQL server/oracle/ms Access:
CREATE TABLE Persons
(
p_id int not NULL PRIMARY KEY,
LastName varchar (255) Not NULL,
FirstName varchar (255),
Address varchar (255),
City varchar (255)
)
To allow a named PRIMARY KEY constraint and to determine a primary KEY constraint for multiple columns, use the following SQL syntax: CREATE TABLE Persons
(
p_id int not NULL,
LastName varchar (255) Not NULL,
FirstName varchar (255),
Address varchar (255),
City varchar (255),
CONSTRAINT Pk_personid PRIMARY KEY (p_id,lastname)
)
Database PRIMARY KEY constraint Change table The table is already established when you want to create a PRIMARY KEY constraint "p_id" column, use the following sql:
SQL Server/oracle/ms Access:
ALTER TABLE Persons
ADD PRIMARY KEY (p_id)
To allow a named PRIMARY KEY constraint and to determine a primary KEY constraint for multiple columns, use the following SQL syntax:
SQL Server/oracle/ms Access:
ALTER TABLE Persons
ADD CONSTRAINT Pk_personid PRIMARY KEY (p_id,lastname)
Note: If you use ALTER TABLE to add a primary key to the declaration, the primary key column (Sunday) must have been declared to contain no null values (the table was first established). One of the key constraints of descent is to discard a primary key constraint, using the following sql:mysql: ALTER TABLE Persons
DROP PRIMARY KEY
SQL server/oracle/ms Access:
ALTER TABLE Persons
DROP CONSTRAINT Pk_personid