15.00 - Redesigning the Database to Eliminate the Need for Nulls - Teradata Database

Teradata Database Design

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Redesigning the Database to Eliminate the Need for Nulls

In the first order predicate calculus, propositions can have one of two values: they can either be true or they can be false. Recall that the rows of a relational table correspond to logical propositions that evaluate to TRUE (see “How Relational Databases Are Built From Logical Propositions” on page 631), because all false propositions are excluded from the database by means of various integrity constraints.

One can build a set of axioms deriving from such a set of true propositions in addition to a set of well‑defined inference rules (referred to as a calculus in formal logic). Additional true propositions, formally known as theorems, can be derived from this calculus. These derived propositions correspond to valid relational queries if and only if the following statements about the original propositions are true:

  • The initial set of rows represents only true propositions.
  • All operations on a relation that contains tuples corresponding to these true propositions obey the formal inference rules of bivalent logic.
  • If either of these statements is false, then the correctness (truth of the derived propositions) of any query made against the data cannot be guaranteed. Because SQL nulls represent data values that cannot be asserted, it is not possible to know whether rows that contain nulls represent true propositions or not, hence, the tables that contain them are not true relations in the mathematical sense.

    Noting that one of the most frequently provided justifications null supporters give for their use is that maintaining rows with missing values is often a necessary and useful thing to do in the real world, Pascal asks, why it would be considered useful to record what he calls “partial nothings” in the database if it is obviously nonsensical to maintain “full nothings”? Pascal then uses the following extreme example to support his case against nulls.