Literals, which are also called constants, specify a value.

    The following diagram shows the format of literals:

  literal =

  --+-> numeric-literal ----+--->
    +-> string-literal -----+
    +-> date-time-literal --+
    +-> interval-literal ---+
    +-> dbkey-literal ------+

    Literals are a type of value expression . Many SQL clauses that
    do not accept general value expressions require literal values.
    Literal values can be either numeric, character string, or date.
    In addition, SQL provides keywords that refer to literals, for
    example:

    o  NULL

    o  CURRENT_DATE

    o  SYSTEM_USER