[mysql] How do I turn off autocommit for a MySQL client?

Do you mean the mysql text console? Then:

START TRANSACTION;
  ...
  your queries.
  ...
COMMIT;

Is what I recommend.

However if you want to avoid typing this each time you need to run this sort of query, add the following to the [mysqld] section of your my.cnf file.

init_connect='set autocommit=0'

This would set autocommit to be off for every client though.