1. The following SQL statement creates a primary key index named "Primary_Key" on the column "Column_X" of the table "Table_A":
CREATE INDEX Primary_Key ON Table_a (Column_X) WITH PRIMARY;
2. The first of the following two SQL statements deletes an existing index Named "IDX_Column_Y" based on the column "Column_Y" of the table "Table_A". The second SQL statement re-creates this "IDX_Column_Y" index with an ascending sort order and prohibits Null values in this column:
DROP INDEX Column_Y ON Table_a;
CREATE INDEX IDX_Column_Y ON Table_a (Column_Y ASC) WITH DISALLOW NULL;
I'm not sure whether the SQL statement ALTER INDEX works in Access or doesn't. For more explanations search for Microsoft Jet SQL DDL (Data Definition Language) in Access Help.