pseudocolumn

A pseudocolumn is a special kind of column in a database that does not store data in the database itself but rather generates values on the fly. These values are typically derived from information related to the rows being queried, rather than being explicitly stored within those rows. Pseudocolumns are not part of the table's schema in the traditional sense, as they are not defined with a data type or constraints. Instead, their behavior and output are determined by the database system's query processor.

Common examples of pseudocolumns include functions that return the current date and time, or identifiers that are unique within a result set but not necessarily unique across the entire table or database. They are often used in SQL queries to retrieve contextual information or to perform calculations that are dependent on the query's execution environment or the data being processed. Because they don't require disk space for storage and are computed as needed, pseudocolumns can improve query efficiency for certain types of operations. However, their availability and specific syntax can vary significantly between different database management systems.