Friday, May 20, 2016

Solving the unique name on delete issue

A common requirement for data management is to have a table that contains a name that must be unique within a table. If rows cannot be physically deleted from the table, a status field is usually used to indicate that the record has been deleted. If another row is added with the same name, an error will be generated because the new row fails the unique constraint on the index.

To solve this issue, add another field called (say) DeletionId that defaults to zero and populate that field with the id of the record when the record is flagged as deleted. Include this field in the unique index.

For example, assume the following table

create table MyTable

   MyTableId int identity(1,1) not null
  ,MyTableName varchar(100) not null 

  ,StatusId smallint not null 
  ,DeletionId int not null constraint DF_MyTable_DeletionId default 0 
  ,constraint PK_MyTable primary key (MyTableId) 
  ,constraint UK_MyTable_MyTableName Unique (MyTableName, DeletionId) 

To flag as deleted (if 2 means deleted and 5 is the id of the record):

update MyTableName set StatusId = 2, DeletionFlag = MyTableId where MyTableId = 5

By using the DeletionId it is possible to have many deleted records in the table with the same name but only one record in the table that is not deleted with the name. That is, the undeleted name is unique.

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