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Does a database table need a foreign key?
A primary key is required to define a relational database table. A foreign key, is one or more columns for another table that references a table’s primary key. In some DBMS’s define foreign key constraints to protect this relationship.
What is the purpose of a foreign key in a database table?
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in the foreign key table.
What is the difference between dim and fact tables?
The main difference between fact table or reality table and the Dimension table is that dimension table contains attributes on that measures are taken actually table. 1. Fact table contains the measuring on the attributes of a dimension table. While in dimension table, There is less records than fact table.
Can a primary key exist in two tables?
Yes. You can have same column name as primary key in multiple tables. Column names should be unique within a table. A table can have only one primary key, as it defines the Entity integrity.
Can a table have two foreign keys and no primary key?
There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a “good” table.
What is a foreign key in data warehouse?
This constraint is implemented via a unique index and is available to be referenced by a Foreign Key. Foreign Key – a set of one or more columns used to reference a Primary Key in another table to ensure data integrity.
Why are foreign keys not implemented in DW?
Most data-warehouses (DW) do not have foreign keys implemented as constraints, because: In general, foreign key constraint would trigger on: an insert into a fact table, any key-updates, and a delete from a dimension table.
How are fact tables used in data warehousing?
The process of defining your data warehousing system (DWH) has started. You’ve outlined the relevant dimension tables, which tie to the business requirements. These tables define what we weigh, observe and scale. Now we need to define how we measure. Fact tables are where we store these measurements.
Which is the primary key for the fact table?
Now in the case of a Fact table, foreign keys flowing-in from the other dimension tables together already act as a compounded primary key. And these foreign-key combinations can uniquely identify each record in the fact table. So, this foreign key combination is the primary key for the fact table.
Is it good to use foreign keys in a query?
It is also possible that query performance will benefit because foreign keys permit certain types of query rewrite that are not normally possible without them. Data integrity is still the main reason to use foreign keys however. Yes, as a best practice, implement the FK constraints on your fact tables.