SQL Interview Question and Answers | What is Candidate key, Primary Key and Super Key
In the context of relational databases, "candidate key," "primary key," and "super key" are terms related to the identification of unique records in a table. Let me explain each of these concepts:
1. Super Key:
- A super key is a set of one or more attributes (columns) in a table that can be used to uniquely identify records within that table. It can contain more attributes than necessary to uniquely identify records. In other words, a super key is a superset of a candidate key.
2. Candidate Key:
- A candidate key is a minimal super key, meaning it is a set of attributes that can uniquely identify each record in a table, and removing any attribute from this set would cause it to lose its uniqueness property. Tables may have multiple candidate keys, but one of them will be chosen as the primary key.
3. Primary Key:
- The primary key is a specific candidate key chosen by the database designer to be the main method of uniquely identifying records in a table. It enforces two important constraints:
a. Uniqueness: No two records in the table can have the same value in the primary key column(s).
b. Not Null: The primary key columns must have a value for every record; they cannot contain NULL values.
In summary, a super key is a broader concept that can include more attributes than needed to uniquely identify records, while a candidate key is a minimal set of attributes that can uniquely identify records. The primary key is a specific candidate key chosen to enforce uniqueness and not-null constraints in a table. In practice, primary keys are essential for maintaining data integrity and enabling efficient data retrieval and indexing in relational databases.
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