Cardinality
Cardinality Definition
Cardinality describes how records in one database table relate to records in another table. It defines how many items on one side of a relationship can connect to items on the other side.
The most common cardinality patterns include one-to-one, one-to-many, and many-to-many relationships. Defining these relationships clearly helps keep data organized and consistent. It prevents duplicate entries, broken links between records, and conflicting information inside a database.
How Cardinality Works
Databases store information in tables, and those tables often need to connect to each other. Cardinality sets the rules for those connections. It determines whether a record can link to just one record, several records, or many records in another table.
For example, a single customer might place multiple orders, while each order belongs to only one customer. In another case, multiple students might enroll in several courses. Cardinality sets these boundaries so the database knows what relationships are valid and how to structure the data.
By clearly defining these rules, databases can store information efficiently and avoid broken or inconsistent links between tables.
Types of Cardinality Relationships
- One-to-one (1:1): Each record in one table connects to only one record in another table, and vice versa. This type is used when data must remain strictly paired, often to separate related information into distinct tables while maintaining a single connection.
- One-to-many (1:N): One record in a table can connect to multiple records in another table, but each of those records connects back to only one original record. This is the most common relationship type, often used to organize grouped data, like departments and employees.
- Many-to-many (N:M): Multiple records in one table can connect to multiple records in another table. Because relational databases cannot store this structure directly, an additional linking table is required to manage the associations between both sides.
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FAQ
In a database, cardinality describes how many records in one table can relate to records in another table. It defines whether a record links to one, many, or multiple records on the other side. This helps structure data logically and prevents inconsistent relationships.
Cardinality describes the relationship rule between tables. A foreign key is the actual column used to enforce that relationship. In simple terms, cardinality defines the rule, and the foreign key is the tool that applies it.
Relational databases can’t directly store many-to-many connections in two tables alone. They need a junction table to act as a bridge by storing pairs of related record IDs. This keeps the relationship structured and prevents data duplication.
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