Fact Table
A fact table is the primary table in a dimensional model where the
numerical performance measurement of the business are stored. There
can be many performance measurements or facts in a fact table.
A row in a fact table corresponds to a measurement. The most useful facts
in a fact table are numeric and additive. Fact tables tend to be deep in
terms of the number of rows but narrow in terms of the number of columns.
Thus, fact tables usually make up 90 percent or more space of a
dimensional database.
All fact tables have two or more foreign keys that connect to the dimension
tables’ primary keys. When all the keys in the fact table match their
respective primary keys correctly in the corresponding dimension tables,
we say that the tables satisfy referential integrity. We access the fact table
via the dimension tables joined to it.
The fact table itself generally has its primary key made up of a subset of
the foreign keys. This key is called a composite or concatenated key. Every
fact table in a dimensional model has a composite key, and conversely,
every table that has a composite key is a fact table.
Another way to say this is that in a dimensional model, every table that
expresses a many-to-many relationship must be a fact table. All other
tables are dimension tables.
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