子查询变为联接查询
select mc.first_name,mc.last_name,jc.titlefrom job_current jcnatural join my_contacts mcwhere jc.title in(select title from job_listings);select mc.first_name,mc.last_name,jc.titlefrom job_current jc, my_contacts mcwhere jc.title in(select title from job_listings)and jc.contact_id=mc.contact_id;select mc.first_name,mc.last_name,jc.titlefrom job_current jcnatural join my_contacts mcinner join job_listings jlon jc.title = jl.title;select mc.first_name,mc.last_name,jc.titlefrom job_current jc, my_contacts mc, job_listings jlwhere jc.contact_id=mc.contact_idand jc.title = jl.title;?
