Add SQL92 "constants" CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP,

 CURRENT_USER.
Add syntax for primary and foreign keys.
Change optional syntax in CREATE INDEX to avoid parsing conflict with
 TIMESTAMP WITH TIME ZONE data type (use USING <class> rather than WITH...).
Decouple various categories of data type syntax to allow the most possible
 non-ambiguous extensions to SQL92 for column names and labels. This should
 make the parser a bit more understandable, or at least easier to find
 where and how the data types are handled.
Support syntax for IN and EXISTS clauses with subselects.
Support SQL92 syntax for IS TRUE/IS FALSE/IS NOT TRUE/IS NOT FALSE.

Author: Thomas G. Lockhart