Ensuring unique primary keys

Ensuring unique primary keys

When I use autonumbers for primary keys, I need to prevent the entry of duplicate records. I have been told that there is a standard SQL algorithm to prevent the entry of duplicate records, but I do not know where to find it.

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Excellent question. One of the problems with using a surrogate key such as an autonumber for the table's primary key is that you can end up with duplicates in the "real" primary key.

Consider the following Product table:

prodID  prodCode  prodDescription

  1      XTBW     Widget size 27 
  2      XTCW     Widget size 52 
  3      XTDW     Widget one-size 
  4      YBWD     Widget size 27 
  5      YBWX     Widget size 52

In this scenario, assuming prodID is an autonumber column, you could easily run:

insert 
  into Product
     ( ProdCode, ProdDescription )
values 
     ( 'XTDW', 'Widget any size' )

This insert would succeed, and now you have two rows with the same prodCode.

The "standard algorithm" you're looking for is the unique constraint.

alter table Product
  add constraint uniqueprodcode 
       unique (prodCode)

Note that "duplicate records" would have to be equal in all columns. Usually this is not a concern. If it is necessary to check multiple columns for uniqueness of the combinations of values, just list those columns in the UNIQUE clause when defining the constraint. For example,

alter table Customer
  add constraint uniquecustomer 
       unique (LastName, MiddleInit, FirstName)

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This was first published in December 2002

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