In a relational database, what enforces data integrity?

In a relational database, data integrity is primarily enforced through the use of data types and the establishment of relationships between tables. Data types ensure that the data entered into a field conforms to specific standards (such as integer, string, date, etc.), which helps prevent errors and inconsistencies in the data.

Relationships between tables are established through the use of primary keys and foreign keys. A primary key uniquely identifies each record in a table, ensuring that there are no duplicate entries. Foreign keys create links between tables, enforcing the rules that govern how data in one table relates to data in another. This helps maintain consistency across the database and ensures that relationships between entities are preserved, which is critical for data integrity.

In contrast, options that mention flexible schemas and unstructured data do not contribute to enforcing integrity within a relational database, as such databases are designed to work with structured data and predefined schemas. Additionally, the absence of primary and foreign keys would lead to a lack of relationships and potential inconsistencies, undermining data integrity. Lastly, manual data entry can introduce human error, which can also detract from data integrity. Thus, the establishment of data types and relationships is fundamental in maintaining accurate and reliable data within relational databases.