一意性（UNIQUE） → 同じ値を持つレコードは存在できない NOT NULL 制約付き → NULL 値は許可されない 複数カラムを主キーにする(複合主キー) CREATE TABLE EmployeeProjects ( EmployeeID INT, ProjectID INT, Role NVARCHAR(50), PRIMARY KEY (EmployeeID, ProjectID)- ...

-- Primary key -- Constraint -- parimary key constraint -- (value can not be null and cannot repeat) -- there can be only one primary key in a table drop table snapdeal_orders; create table ...

When you create a Microsoft Access table, Access will automatically create a primary key to your database table. Still, you can also specify the field you want as the primary key for your database ...

A primary key is a field used to uniquely identify every record in the database. Sometime primary keys are obvious, for example a car license plate could be used to uniquely identify cars. Often, ...