Use UNION instead of LEFT JOIN. (UNION is implemented in MySQL 4.0.0.)
select P.partno , CONCAT('Component: ', C.componentname) from parts P inner join components C on P.partno = C.partno union all select P.partno , CONCAT('Operation: ', O.operationname) from parts P inner join operations O on P.partno = O.partno
Note it's UNION ALL because there can be no duplicates across the two subqueries.
If you're not on MySQL 4.0.0, there is another way, a rather clever way, which I have not tested myself, although I expect it should work quite well. See the article Writing UNION statements in MySQL 3.x.
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This was first published in January 2003