Multiple assignment

We can perform multiple assignments in a single statement. This allows us to set inter-dependent values, since any constraints are only checked after all the assignments in a statement. We can assign multiple values to multiple variables using , to separate the assignments.

For example, here we'll remove a supplier from S and also from SP. The integrity constraints will be checked only after both of the assignments are complete and so will be satisfied.

S := S - S[SNO="S4"], SP := SP - SP[SNO="S4"]

SNO SNAME STATUS CITY

S1 Smith 20 London
S2 Jones 10 Paris
S3 Blake 30 Paris
S5 Adams 30 Athens

SNO	SNAME	STATUS	CITY
S1	Smith	20	London
S2	Jones	10	Paris
S3	Blake	30	Paris
S5	Adams	30	Athens

SP

SNO PNO QTY

S1 P1 300
S1 P2 200
S1 P3 400
S1 P4 200
S1 P5 100
S1 P6 100
S2 P1 300
S2 P2 400
S3 P2 200

SNO	PNO	QTY
S1	P1	300
S1	P2	200
S1	P3	400
S1	P4	200
S1	P5	100
S1	P6	100
S2	P1	300
S2	P2	400
S3	P2	200

And here's the view we defined earlier. Supplier `S4` has also been removed from here.

S20

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

SNO	PNO	QTY
S1	P1	300
S1	P2	200
S1	P3	400
S1	P4	200
S1	P5	100
S1	P6	100
S2	P1	300
S2	P2	400
S3	P2	200

SNO	PNO	QTY
S1	P1	300
S1	P2	200
S1	P3	400
S1	P4	200
S1	P5	100
S1	P6	100
S2	P1	300
S2	P2	400
S3	P2	200

SNO	PNO	QTY
S1	P1	300
S1	P2	200
S1	P3	400
S1	P4	200
S1	P5	100
S1	P6	100
S2	P1	300
S2	P2	400
S3	P2	200