What is a PRIMARY KEY ?
PRIMARY KEY CONSTRAINT: 1. Identified the columns or set of columns which uniquely identify each row of a table and ensures that no duplicate rows exist in the table. 2. Implicitly creates a uniqu index for the column(S) and specifies the column(s) as being NOT NULL. 3. The name of the index is the same as the constraint name. 4. Limited to one per table. Example : CREATE TABLE loans( account NUMBER(6), loan_number NUMBER(6), ……
A primary key is a table column that can be used to uniquely identify every row of the table. Any column that has this property will do — these columns are called candidate keys. A table can have many candidate keys but only one primary key. The primary key cannot be null. Consider the following table — FooNumber FirstName LastName BarTab 21 Fred Jones 47 32 Bill Smith 23 87 Wendy Jones – 32 Bob Stikino 943 In this example, only FirstName is a single-column candidate key, because it is the only column that is unique and not null. A composite primary key is a primary key consisting of more than one column. In the above example, the combinations (RecordNo,FirstName), (RecordNo,Lastname), (RecordNo,FirstName,Lastname), and (FirstName,LastName) are all candidate keys. Any combination including Age is not a candidate key because it contains a null. Often, database designers add an extra column to their table designs, a column defined as an integer, which will hold a number. In Microsoft A
