Add CREATE SCHEMA ... [IF NOT EXISTS]

Also removed duplicate TABLE in CREATE TABLE, and add COLUMN noise to ALTER TABLE

Author: mrotteveel

doc/sql.extensions/README.ddl.txt

@@ -676,22 +676,22 @@ already exists.
 For ALTER TABLE ... ADD subclause, DDL triggers are not fired if there are only IF NOT EXISTS subclauses and all
 of them are related to existing columns or constraints.
 
-For others commands (except users currently) where IF NOT EXISTS is part of the main command,
+For other commands (except users currently) where IF NOT EXISTS is part of the main command,
 DDL triggers are not fired when the object already exists.
 
-The engine only verifies if the name (object, column or constraint) already exists, and if yes, do nothing.
-It never tries to match the existing object with the one being created.
+The engine only verifies if the name (object, column or constraint) already exists, and if so, it does nothing.
+It does not try to match the definition of the existing object with the one being created.
 
-Some objects share the same "namespace", for example, there cannot be a table and a procedure with the same name.
-In this case, if there is table XYZ and CREATE PROCEDURE IF NOT EXISTS XYZ is tried, the procedure will not be created
-and no error will be raised.
+Some objects share the same "namespace". For example, there cannot be a table and a procedure with the same name
+in the same schema. If there is a table XYZ and CREATE PROCEDURE IF NOT EXISTS XYZ is tried, the procedure will
+not be created, and no error will be raised.
 
 The following statements are supported:
 
 CREATE EXCEPTION [IF NOT EXISTS] ...
 CREATE INDEX [IF NOT EXISTS] ...
 CREATE PROCEDURE [IF NOT EXISTS] ...
-CREATE [{GLOBAL | LOCAL} TEMPORARY TABLE] TABLE [IF NOT EXISTS] ...
+CREATE [{GLOBAL | LOCAL} TEMPORARY] TABLE [IF NOT EXISTS] ...
 CREATE TRIGGER [IF NOT EXISTS] ...
 CREATE VIEW [IF NOT EXISTS] ...
 CREATE FILTER [IF NOT EXISTS] ...
@@ -707,7 +707,8 @@ CREATE USER [IF NOT EXISTS] ...
 CREATE PACKAGE [IF NOT EXISTS] ...
 CREATE PACKAGE BODY [IF NOT EXISTS] ...
 CREATE [GLOBAL] MAPPING [IF NOT EXISTS] ...
-ALTER TABLE <table> ADD [IF NOT EXISTS] <column name> ...
+CREATE SCHEMA [IF NOT EXISTS] ...
+ALTER TABLE <table> ADD [COLUMN] [IF NOT EXISTS] <column name> ...
 ALTER TABLE <table> ADD CONSTRAINT [IF NOT EXISTS] <constraint name> ...
 
 8) Creation of an inactive index