Please note: This page is imported from my wiki, which hasn't been updated in over 10 years. Some of the formatting was lost during the import. I'll try to get around to fixing it someday.
show index from <table_name>;
alter table <tablename> add index (<columnname>);
or if a text field. give a number of characters to index:
alter table <tablename> add index (<columnname>(<no_chars>);