add column to table if not exists

Trying to add column column1 to table1 if it doesn't exist yet:

create table if not exists table1( column2 varchar(20));

CREATE PROCEDURE column_present()
        MODIFIES SQL DATA
    BEGIN ATOMIC
        DECLARE column_count integer;
        set column_count = select COUNT(*) from
information_schema.system_columns Where table_name = 'table1' and
column_name = 'column1';
        if column_count = 0 then alter table table1 ADD column1 integer;
end if;
END;

call column_present();

results in:

[2019-05-03 22:28:13] [42581][-5581] unexpected token: ALTER : line: 6
[2019-05-03 22:28:13] java.lang.RuntimeException: org.hsqldb.HsqlException:
unexpected token: ALTER : line: 6
[2019-05-03 22:28:13]   at org.hsqldb.error.Error.parseError(Unknown Source)

what is the proper way to create the column in existing table (if it
doesn't exist yet) in the HSQLDB?

Please note: ignoring the error on creation if it already exists, is not an
option for me.

_______________________________________________
Hsqldb-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/hsqldb-user