According to the Postgres documentation for every version from 7.1 through 9.1, 'ALIAS' is listed as a reserved word, at least for SQL99. Later versions do not show it - suggesting that it has been dropped as a reserved word. When aliasing a table or column one may use 'AS', but I have never seen the word 'ALIAS' used anywhere - not for Postgres, SQL Server or any other RDBMS I have encountered. The Postgres docs do say "the presence of a key word does not indicate the existence of a feature." Is it as simple as that - that someone thought "oh we'd better make this reserved because it looks like we might want to use it" but no one ever did use it? Or is there a case where the word itself is or was actually used (and possibly required) and I simply have never stumbled over it?

According to the Postgres (and other) documentation, 'ALIAS' is listed as a reserved word, at least for SQL99.

That's not true. 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 You can see that in Postgres95 (the first version in the migration) it did not have ALIAS as a reserved word

• 👍 MySQL does NOT reserve ALIAS
  


• 👍 Oracle does NOT reserve ALIAS
  


• 👎 SQL Server does (in T-SQL) reserve ALIAS
  

SQL Standard

• In SQL-92, ALIAS was marked as a <reserved word>; but, there was no use assigned for that <reserved word>.




• In SQL-99 ALIAS was marked as an "Additional Reserved Word", and added to the list of <reserved word>; but, there was no use assigned for that <reserved word>. 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

It is used at least in various flavours of Db2: ALIAS is an object that allows you to specify a different name for another object, like table. It's frequently used to allow references in one schema to objects in another schema without explicitly specifying that schema.

create table SOMEOTHERSCHEMA.FOOBAR (...);

set schema MYSCHEMA;

create alias FOOBAR for SOMEOTHERSCHEMA.FOOBAR;

select * from FOOBAR;

ALIAS is an alias for SYNONYM; the latter concept also exists in Oracle and SQL Server.