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Primary key generation

You will encounter situations where the database should automatically generate a unique number. For example, if you are building a table to store sales invoices you might prefer that the database assign unique invoice numbers automatically, rather than require sales staff to pick them.

There are many methods for generating such numbers.

For example, invoice numbers could be obtained by adding 1 to the previous invoice number. This method will not work when there is more than one person adding invoices to the database. Two people may decide to use the same invoice number.

There is more than one solution to the problem:

• Assign a range of invoice numbers to each person who adds new invoices.

  You could implement this scheme by creating a table with the columns user name and invoice number. The table would have one row for each user that adds invoices. Each time a user adds an invoice, the number in the table would be incremented and used for the new invoice. To handle all tables in the database, the table should have three columns: table name, user name, and last key value. You should periodically check that each person still has a sufficient supply of numbers.

• Create a table with the columns table name and last key value.

  One row in this table would contain the last invoice number used. Each time someone adds an invoice, establish a new connection, increment the number in the table, and commit the change immediately. The incremented number can be used for the new invoice. Other users will be able to grab invoice numbers because you updated the row with a separate transaction that only lasted an instant.

• Use a column with a default value of NEWID in conjunction with the UNIQUEIDENTIFIER binary data type to generate a universally unique identifier.

  UUID and GUID values can be used to uniquely identify rows in a table. The values are generated such that a value produced on one computer will not match that produced on another. They can therefore be used as keys in replication and synchronization environments.

  For more information about generating unique identifiers, see The NEWID default.

• Use a column with a default value of AUTOINCREMENT.

  For example,

  CREATE TABLE Orders (
     OrderID INTEGER NOT NULL DEFAULT AUTOINCREMENT,
     OrderDate DATE,
     primary key( OrderID )
  )

  On inserts into the table, if a value is not specified for the autoincrement column, a unique value is generated. If a value is specified, it will be used. If the value is larger than the current maximum value for the column, that value will be used as a starting point for subsequent inserts. The value of the most recently inserted row in an autoincrement column is available as the global variable @@identity.