Junior
What are the restrictions on keys in a database?
sobes.tech AI
Answer from AI
In the context of relational databases, the main constraints on keys:
- Uniqueness: The value of the key (or a combination of values for a composite key) must be unique within the table. This guarantees the unambiguous identification of each row.
- Entity Integrity: For primary keys (PRIMARY KEY), NULL values are not allowed. Each row must have a defined primary key value.
- Referential Integrity: For foreign keys (FOREIGN KEY), the value must either exist in the parent table's referenced column (which is usually a primary or unique key in that table), or be NULL (if the column allows NULLs and does not violate other constraints).
- Stability: Key values, especially primary keys, should be stable and rarely change. Changing a primary key may require updating corresponding foreign keys in dependent tables.
- Size and data type: Although not a strict universal rule, it is preferable to use keys with a small size and efficient data types (e.g., integer IDs), as they are used for indexing and linking tables, affecting performance.
- Number of columns (for composite keys): Composite keys consisting of many columns can be less efficient due to larger index size and management complexity.