\!h Add an index to an existing table:
ALTER TABLE artist ADD INDEX by_name (artist_name); -- adds key called 'by_name' to artist_name column
\!h Add a primary key:
ALTER TABLE artist ADD PRIMARY KEY (artist_id) -- adds primary key to artist_id column
\!h Remove non-primary key index:
ALTER TABLE artist DROP INDEX by_name;
\!h Remove primary key:
ALTER TABLE artist DROP PRIMARY KEY;
\!h Modify an index:
-- can't really be done, you have to drop and re-create with new specifications
-- Example: have the by_name index include only the first 10 characters of the artist_name:
ALTER TABLE artist DROP INDEX by_name;
ALTER TABLE artist ADD INDEX by_name (artist_name(10));