Temporal Database
A temporal database is a collection of transactions ordered by their timestamps. A sample temporal database generated from the set of items, I={a,b,c,d,e,f}, is shown in below table:
TID
Timestamp
Transactions
1
1
a, b, c
2
2
d, e
3
4
a, e, f
4
7
d, f, g
Types of temporal databases:
Regular temporal database: Uniform time gap exists between any two transactions.
Irregular temporal database: Non-uniform time gap exists between any two transactions.
Type-1 irregular temporal database: Every transaction will have a distinct timestamp.
Type-2 irregular temporal database: Multiple transactions can have a common timestamp.
Rules to create a temporal database:
Since TID of a transaction implicitly represents the row number, this information can be ignored to save space.
The first column in the database must represent a timestamp.
The timestamp of the first transaction must always start from 1. The timestamps of remaining transactions follow thereafter. In other words, the timestamps in a temporal database must be relative to each other, rather than being absolute timestamps.
Irregular time gaps can exist between the transactions.
Multiple transactions can have a same timestamp. In other words, multiple transactions can occur at a particular timestamp. (Please note that some pattern mining algorithms, especially variants of ECLAT, may not work properly if multiple transactions share a common timestamp.)
All items in a transaction must be seperated with a separator.
The items in a temporal database can be integers or strings.
‘ Tab space ’ is the default seperator. However, temporal databases can be constructed using other seperators, such as comma and space.
Format of a temporal database:
>>> timestamp<sep>item1<sep>item2<sep>...<sep>itemNExamples:
Regular temporal database: Uniform time gap exists between the transactions.
1 a b c
2 d e
4 a e f
7 d f g
Irregular temporal database (Type-1): Non-uniform time gap exists between the transactions. More important, every transaction contains a unique timestamp.
1 a b c
2 d e
4 a e f
7 d f g
Irregular temporal database (Type-2): Non-uniform time gap exists between the transactions. More important, multiple transactions can have same timestamps.
1 a b c
1 d e
4 a e f
8 d f g