How to set more than one primary key for a table?

Is it possible set more than one开发者_C百科 primary key for a table?

If it is possible as composite key - so how it can be used with application??

ADVANTAGES .?

No, that's not possible.

However:

You can have more than one field in the primary key.

You can also add unique indexes to fields that are not in the primary key.

You can't. There can only be a single primary key on a table (that's why it's called primary..). The only thing you can do is to specify a PRIMARY KEY on multiple columns (but that depends on the database layout:

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)
)

No, you cannot have more than one primary key in a table. Yes, a primary key can consist of multiple fields, but, considering your question, I don't think that is what you need. I'm not sure what it is that you need exactly, but a primary key has a few capabilities that you are probably looking for in another field:

Unicity: If you are in need of another field that has a unique value throughout all records, you can apply a UNIQUE constraint:

ALTER TABLE <table identifier> 
  ADD [ CONSTRAINT <constraint identifier> ] 
  UNIQUE ( <column expression> {, <column expression>}... )

Speed: If you have a field that is searched for very often, and need to speed up your query, you can add an index to that field.

CREATE INDEX <index identifier>
   ON <table identifier> (<field name>)