Database Glossary Terms Beginning With The Letter P
A variable value that is designed to limit or provide information for what needs to be accomplished.
A table that represents the one side of a relationship between two tables.
See crosstab query.
Rules and methods an application uses to present data to the user.
A field or set of fields that create a value which uniquely identifies a record and is subsequently identified as the main key for a table or a relationship. It is the field(s) in a table that uniquely defines the row in the table; the values in the primary key are always unique.
Propagation of Nulls
The propagation of a Null value throughout an expression until an operator prevents it. For example, if any part of an expression evaluates to Null, then the expression as a whole evaluates to a Null, unless the operator prevents the propagation. Consider the difference of using + vs &: 1 + Null = Null, however, A & Null = A. The (+) operator propagates Nulls, the (&) operator does not.