SELECT *
FROM players AS pl
LEFT JOIN positions AS pos ON (pl.position_id=pos.p_id)
Left JOIN content AS pla ON (CONCAT(pl.first_name,' ',pl.last_name) like pla.title) # pla.title нужно обернуть %pla.title%
SELECT *
FROM players AS pl
LEFT JOIN positions AS pos ON (pl.position_id=pos.p_id)
Left JOIN content AS pla ON (CONCAT(pl.first_name,' ',pl.last_name) like CONCAT('%',pla.title,'%'))