# Uniqueness

A record's [identifying fields]() must be **unique** amongst all other current
records of the same [record type]().

Deleted records are not considered; if a record is given a `DELETE` modification
then its identifying fields may be reused. Note that once a record exists it
isn't possible to `UPDATE` its identifying fields.

This rule ensures that the correct record to modify can be unambiguously
identified when changes to a record are communicated, because records being
updated in the [transaction stream](Data-structures;-Transactions.md) are
identified by their identifying fields.

## Examples

An additional code has type `A`, code `123` and start date `2021-01-01`. If
another additional code is created with type `A`, code `123` and start date
`2021-01-01`, then the creation will fail this rule.

If another additional code is created with a different start date (e.g. created
with type `A`, code `123` and start date `2021-01-02`) then it will not fail
this rule (although it may fail another rule about overlapping dates.)

If the first original code is deleted, then creating a new additional code with
type `A`, code `123` and start date `2021-01-01` will then not fail the rule.