 construct a
"minimum qualifying" relation (SSN, Part#)
:
The "minimum qualify" relation will contain the following:
Meaning of this relation:
 This relation contains the tuples
that is necessary for
each employee
to be selected by the query

The "qualify" relation
can be constructed as follows:

qualify
=
π _{ssn} (Emp_Proj)
× emp4_proj

 Compute the
difference "qualify  Emp_Proj":
If we project out the SSN
from the difference set, we will
obtain
all disqualified SSNs !!!
 The last step is to obtain the set of
qualified SSN,
that's simple use set difference again:

π _{ssn} (Emp_Proj)
− π _{ssn} (UnQualify)

