Why do we need temporal database? Explain different time dimensions in this database.

We need a temporal database because of the following reasons:-

(i) A temporal database efficiently stores a time series of data, typically by having some fixed timescale and then storing only changes in the measured data.

(ii) It prevents fragmentation of an object description.

(iii) It provides query algebra to deal with temporal data.

(iv) It is Compatible with the old databases without temporal data.

(v) It maintains the past or historical values and the time at which data was updated. 

The different time dimensions in this database that occur on the temporal database are: Valid time, Transaction time, bitemporal data

Valid time

Valid time is a time period during which a fact is true in the real world. Given a particular event or fact that is associated with a particular time point or time period in the database, the association may be interpreted to mean different things. The most natural interpretation is that the associated time is the time that the event occurred or the period during which the fact was considered to be true in the real world. If this interpretation is used, the associated time is often referred to as the valid time. A temporal database using this interpretation is called a valid time database.

For example, in a company, the Salary of the employees has a valid time and end time

Transaction time

Transaction time is the time period during which a fact stored in the database was known.

However, a different interpretation can be used, where the associated time refers to the time when the information was actually stored in the database; that is, it is the value of the system time clock when the information is valid in the system. In this case, the associated time is called the transaction time. A temporal database using this interpretation is called a transaction time database. Unlike valid time here we can roll back the database. For example, in a company, the Salary of the employees has a valid time and end time. 

Bitemporal data

It combines both valid ad transaction times. It stores data with respect to both valid time and transaction time. In some applications, only one of the dimensions is needed and in other cases, both time dimensions are required, in which case the temporal database is called a bitemporal database.