How do I enforce uniqueness in a table?

Question

For example, I have to program an enrolment table which has:

1. enrolment_id
2. academic_period
3. results
4. student_id
5. subject
<

Answer 1

Add a unique constraint. From here:

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

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
 (
 P_Id int NOT NULL,
 LastName varchar(255) NOT NULL,
 FirstName varchar(255),
 Address varchar(255),
 City varchar(255),
 CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
 )

(Modify the above for the column(s) you need to set as unique)