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Explain This!

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Hi,

Please Explain this query from inermost select.

Each step by stgep 

I don't understand this :(

select distinct s.sname from s
where not exists 
(select * from p 
where not exists 
(select * from spj
where SPJ.s# = s.s# and  spj.p# = p.p#))

And relation is :

S(s#,sname,city)
p(p#,pname,color)
j(j#,jname,city)
SPJ(S#,p#,j#,qty)


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