- Tags: Keyword, MySQL, SQL Server
- Categories: Reference, SQL Keywords, SQL Reference
Quick Reference
The PRIMARY KEY keywords uniquely identifies each record in a table, which can have only one primary key consisting of a single field or multiple fields.
PRIMARY KEY on CREATE TABLE
The following creates a PRIMARY KEY on the customer_id column when the Customers table is created.
MySQL:
CREATE TABLE Customers (
customer_id int NOT NULL,
last_name varchar(255) NOT NULL,
first_name varchar(255),
PRIMARY KEY (customer_id)
);
SQL Server:
CREATE TABLE Customers (
customer_id int NOT NULL PRIMARY KEY,
last_name varchar(255) NOT NULL,
first_name varchar(255)
);
The following creates a PRIMARY KEY on the customer_id and last_name columns when the Customers table is created. It is one PRIMARY KEY made up of two columns.
MySQL and SQL Server:
CREATE TABLE Customers (
customer_id int NOT NULL,
last_name varchar(255) NOT NULL,
first_name varchar(255),
CONSTRAINT pk_customer PRIMARY KEY (customer_id, last_name)
);
PRIMARY KEY on ALTER TABLE
The following creates a PRIMARY KEY on the customer_id column when the Customers table already exists.
MySQL and SQL: Server:
ALTER TABLE Customers
ADD PRIMARY KEY (customer_id);
The following creates a PRIMARY KEY on the customer_id and last_name columns when the Customers table already exists. It is one PRIMARY KEY made up of two columns.
MySQL and SQL Server:
ALTER TABLE Customers
ADD CONSTRAINT pk_customers PRIMARY KEY (customer_id, last_name);
DROP a PRIMARY KEY CONSTRAINT
MySQL:
ALTER TABLE Customers
DROP PRIMARY KEY;
SQL Server:
ALTER TABLE Customers
DROP CONSTRAINT pk_customer;
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