Which of the following characteristics pertaining to databases is not true?
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A. B. C. D.B.
Justifications should be provided when data is renormalized, not when it is normalized, because it introduces risk of data inconsistency.
Renormalization is usually introduced for performance purposes.
Option C, "No NULLs should be allowed for primary keys" is the characteristic pertaining to databases that is not true.
In a relational database, a primary key is a unique identifier for each record in a table. It is used to enforce data integrity and ensure that each record is unique. While it is preferable to have a primary key for each record, it is not necessary for the primary key to have a value for every record in a table. The value of a primary key can be null in certain situations.
Therefore, it is not true that no NULLs should be allowed for primary keys. However, it is important to note that while allowing nulls in a primary key is not prohibited, it is generally not a good practice because it can lead to data inconsistency and difficulty in querying the data.
Option A states that a data model should exist, and all entities should have significant names. This is true, as a data model is a visual representation of data and the relationships between data entities, and having significant names for entities makes it easier to understand the data model.
Option B states that justifications must exist for normalized data. This is true, as normalization is a process of organizing data in a database to reduce redundancy and dependency, and it is important to have justifications for why a particular level of normalization was chosen.
Option D states that all relations must have a specific cardinality. This is also true, as cardinality defines the relationship between two entities in terms of the number of instances of one entity that can be associated with one instance of the other entity.
In summary, option C is the characteristic that is not true pertaining to databases because while it is generally not a good practice to allow NULLs in a primary key, it is not prohibited.