What is the difference between a composite key, primary key and foreign key?
First, a primary key uniquely identifies each record in a database table. Any individual key that does this can be called a candidate key, but only one can be chosen by database engineers as a primary key.
How this works may have to do with the sophistication of the database.
In some cases, the homes may only be uniquely identified by a mortgage number — all other data (towns, streets, house numbers) is not unique to each record. The mortgage number would be the primary key. Suppose, however, that an MLS realtor’s listing technology assigns its own unique numbers to the records in the table.
Then, there will be two keys that developers might identify as “candidate keys”:
- The mortgage number.
One of them will qualify as the “primary key” in what some would consider an arbitrary way.
Meanwhile, if there is a key in a linked table, such as a buyer’s table that references the primary key, that will be a foreign key.
The relationship between a primary key and a foreign key is quite different. The key thing to understand here is that the primary key in one database table becomes a foreign key in another database table, and vice versa.
A foreign key in a database table is taken from some other table and applied in order to link database records back to that foreign table.
The foreign key in the database table where it resides is actually the primary key of the other table.
Here's an example to make this clearer: If we have a database table labeled, say, "orders," and we have a foreign key that's labeled "customers," we could perhaps link that foreign key to a primary key, identifying each customer in a separate table.
In sum, all of this interconnection of key categories shows the detailed nature of creating sophisticated patterns and relationships in relational database sy爱游戏网页入口stems.
Along with database normalization and promoting structured data, these key relationships are core parts of database sy爱游戏网页入口stem integrity and data storage design best practices.
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