Composite key definition
A composite key is a combination of two or more columns that uniquely identify a row within a database table. While each column may not be unique on its own, their combined values are guaranteed to be unique across the table.
How composite keys work
In relational databases, primary keys are used to uniquely identify rows. Sometimes, no single column can serve as a primary key because its values are not unique. In these cases, a combination of columns can be used. This combination is called a composite key. When establishing relationships between tables using foreign keys, the same combination of columns must be referenced.
Advantages of using composite keys
- They often represent data relationships more naturally than single artificial keys.
- Composite keys ensure that the combination of values is unique, maintaining data integrity.
- Allows for a wider range of unique combinations than a single column.
Disadvantages of using composite keys
- Complicates queries and joins, leading to performance issues.
- The combined length of columns could exceed the system’s maximum index size.
- It requires constant maintenance because of changes in table structure or business requirements.