Certain expressions that return a fixed value are called constant
expressions. These include variant function operators that
read the system clock, but that are valid in contexts where literal
constants are also valid.
Among these expressions are the following operators (or system
constants) whose returned values are determined at runtime:
CURRENT returns the current time and date from the system
clock.
CURRENT_ROLE returns the name of the role, if any, whose
privileges are enabled for the current user.
CURRENT_USER is a synonym for USER.
DEFAULT_ROLE returns the name of the role, if any, that
is the default role for the current user.
DBSERVERNAME returns the name of the current database server.
SITENAME is a synonym for DBSERVERNAME.
SYSDATE reads the DATETIME value from the system clock
like the CURRENT operator, but has a different default precision.
TODAY returns the current calendar date from the system
clock.
USER returns the login name (also called the authorization
identifier) of the current user.
Besides these operators, the term constant expression can
also refer to a quoted string, to a literal value, or to the UNITS operator
with its operands.
The Constant Expression segment
has the following syntax.