Compose

It's often useful to remove the common attributes used by a join.

Compose

We can join two relations and then remove their common attributes by composing them with parentheses ( ).

S & SP

SNO SNAME STATUS CITY PNO QTY

S1 Smith 20 London P1 300
S1 Smith 20 London P2 200
S1 Smith 20 London P3 400
S1 Smith 20 London P4 200
S1 Smith 20 London P5 100
S1 Smith 20 London P6 100
S2 Jones 10 Paris P1 300
S2 Jones 10 Paris P2 400
S3 Blake 30 Paris P2 200
S4 Clark 20 London P2 200
S4 Clark 20 London P4 300
S4 Clark 20 London P5 400

SNO	SNAME	STATUS	CITY	PNO	QTY
S1	Smith	20	London	P1	300
S1	Smith	20	London	P2	200
S1	Smith	20	London	P3	400
S1	Smith	20	London	P4	200
S1	Smith	20	London	P5	100
S1	Smith	20	London	P6	100
S2	Jones	10	Paris	P1	300
S2	Jones	10	Paris	P2	400
S3	Blake	30	Paris	P2	200
S4	Clark	20	London	P2	200
S4	Clark	20	London	P4	300
S4	Clark	20	London	P5	400

S(SP)

SNAME STATUS CITY PNO QTY

Blake 30 Paris P2 200
Clark 20 London P2 200
Clark 20 London P4 300
Clark 20 London P5 400
Jones 10 Paris P1 300
Jones 10 Paris P2 400
Smith 20 London P1 300
Smith 20 London P2 200
Smith 20 London P3 400
Smith 20 London P4 200
Smith 20 London P5 100
Smith 20 London P6 100

SNAME	STATUS	CITY	PNO	QTY
Blake	30	Paris	P2	200
Clark	20	London	P2	200
Clark	20	London	P4	300
Clark	20	London	P5	400
Jones	10	Paris	P1	300
Jones	10	Paris	P2	400
Smith	20	London	P1	300
Smith	20	London	P2	200
Smith	20	London	P3	400
Smith	20	London	P4	200
Smith	20	London	P5	100
Smith	20	London	P6	100

Notice how the SNO attribute, the one used to join the relations, has been removed in the compose version.

Although compose can make some results tidier, it can be simpler to debug without it. Compose can also restrict the join ordering options, and not removing attributes too early can make complex expressions simpler to construct.

We can also use the compose syntax as a shorthand to compose with a relation created from scalar(s), e.g.

S(CITY:="Paris", STATUS:=30)

SNO SNAME

S3 Blake

SNO	SNAME
S3	Blake

Which is short for:

(S & to_rel(tuple{CITY:="Paris", STATUS:=30}))~{STATUS, CITY}

SNO SNAME

S3 Blake

SNO	SNAME
S3	Blake

And composition with dee, the relation with 1 tuple and 0 attributes, can be used to create a relation from scalar(s). The attribute types are inferred from the values, e.g.

dee(name:="This is a relation", value:=6.28)

name value

This is a relation 6.28