92 CHAPTER 4 Types of rules 4.9.2.2.3 Equality rules An equality rule requires that the content of a data item be the same as or not the same as that of some other data item: for example, R111. The origin city of the outgoing flight specified in each flight booking confirmation must be the same as the origin city specified in the flight booking request that gives rise to that flight booking confirmation. R112. The destination city specified in each flight booking request must be different from the origin city specified in that flight booking request. 4.9.2.2.4 Uniqueness constraints A uniqueness constraint requires that the content of a data item (or combination or set 20 of data items) be different from that of the corresponding data item(s) in the same or other records or transactions: for example, R113. The record locator allocated to each flight booking confirmation must be different from the record locator allocated to any other flight booking confirmation. R114. The combination of departure date, flight number, departure city, and seat number specified in each seat allocation must be different from the combination of departure date, flight number, departure city, and seat number specified in any other seat allocation. 4.9.2.2.5 Data consistency rules A data consistency rule requires the content of multiple data items to be consistent with each other, other than as provided for by a value set rule, range rule, or equality rule: for example, R115. The combination of start date and end date specified in each leave application must be such that the end date is no earlier than the start date. R116. The sum of the shares held by the proprietors of each real property parcel must be equal to 1. 4.9.2.2.6 Temporal data constraints A temporal data constraint constrains one or more temporal data items (data items that represent time points or time periods). There are various subcategories of temporal constraint. A simple temporal data constraint requires that a particular date or time fall within a certain tem- poral range: for example, 20 The difference between a combination and a set of data items is discussed in Section 7.2.3.2.2 in Chapter 7.