Is the keyword "ALIAS" actually used?
PostgreSQL maintains a list of reserved and non-reserved terms in the appendix. ALIAS is absent from that list. You can verify PostgreSQL does not use ALIAS by checking out the YACC grammar. Even as far back as Postgres95 ALIAS was not a reserved word (the first version in the migration from QUEL to SQL)


 MySQL does NOT reserve ALIAS
 MariaDB does NOT reserve ALIAS
 Oracle does NOT reserve ALIAS
 SQL Server does (in T-SQL) reserve ALIAS (as a potential future keyword on their "reserved keyword" list)


SQL Standard


In SQL-92, ALIAS was marked as a &lt;reserved word&gt;; but, there was no use assigned for that &lt;reserved word&gt;.
In SQL-99 ALIAS was marked as an "Additional Reserved Word", and added to the list of &lt;reserved word&gt;; but, there was no use assigned for that &lt;reserved word&gt;. Perhaps they reserved the term with the intent to define meaning later, and then withdrew it at a different point. Or, perhaps they reserved the term for vendor defined implementation. PostgreSQL reflected the spec's reservation in the docs, and then removed that reservation with the spec.
In SQL-2011, ALIAS is no where to be found and the word "alias" only appears in reference to 'Feature T053, “Explicit aliases for all-fields reference”'


ℹ There is no digitized copy of SQL-86, or SQL-89