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 have been made. We can assign multiple values to multiple variables using , to separate the assignments.

If any constraints fail to be satisfied, all the assignments in the statement are undone. You can check the error relvar to see any constraint-failure messages. A common pattern might be to check, after every assignment that could fail, whether empty(error) is true (if error is not empty, then the last assignment statement failed and was undone).

For example, say we want to remove a supplier from S and also from SP, using two assignments. If we use a multiple-assignment then the integrity constraints will be checked only after both of the assignments are complete and so will be satisfied.

Here's a reminder of the constraints in effect:

Constraint

name
rule

Constraint_key
key(Constraint, {name})
P_SP_FK
SP{PNO} ⊆ P{PNO}
P_key
key(P, {PNO})
SP_key
key(SP, {SNO,PNO})
S_SP_FK
SP{SNO} ⊆ S{SNO}
S_key
key(S, {SNO})

So now the removal of "S4":

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

if not empty(error) then begin print("Removing S4 failed\n«error»") end

There were no errors. If we'd tried to do the assigments one at a time, with the S assigmment first, we would have caused constraint S_SP_FK to fail. e.g.

S := S - S[SNO="S3"]

if not empty(error) then begin print("Removing S3 failed\n«error»") end

Removing S3 failed
┌─────────────────────────────┐
│ message                     │
├─────────────────────────────┤
│ constraint 'S_SP_FK' failed │
└─────────────────────────────┘

S

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

And here's the view we defined earlier. Supplier `S4` is no longer in 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