Introduction to Database Systems Queries in SQL The select ...

SQL and Relational Algebra (cntd)
It may be necessary to rename attributes
• in the target list (as in relational algebra)
• in the Cartesian product (in particular, when the query refers
twice to the same table)
select X.A1 as B1, ...
from R1 X, R2 Y, R1 Z
where X.A2 = Y.A3 and ...
which can also be written as
select X.A1 as B1, ...
from R1 as X, R2 as Y, R1 as Z
where X.A2 = Y.A3 and ...
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SQL and Relational Algebra (cntd)
select X.A1 as B1, Y.A4 as B2
from R1 X, R2 Y, R1 Z
where X.A2 = Y.A3 and Y.A4 = Z.A1
X ← R1, Y ← R2, Z ← R1,
ρ B1←X.A1, B2←Y.A4 (
π X.A1,Y.A4 (σ X.A2 = Y.A3 and Y.A4 = Z.A1 (X x Y x Z)))
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