Tuple Extraction

Since relations are comprised of tuples, we can extract a tuple from a relation, providing it has exactly one tuple in it. The print function displays the tuple horizontally on the console.

print($S[STATUS=10].tuple)

┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
  SNO   SNAME   STATUS   CITY   
  S2    Jones       10   Paris  
┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄

And we can go the other way: given a tuple, we can create a relation from it using to_rel().

to_rel(tuple{SNO:="S2", SNAME:="Jones", STATUS:=10, CITY:="Paris"})

We can compare tuples with other tuples, such as tuple literals, again using =.

$S[STATUS=10].tuple = tuple{SNO:="S2", SNAME:="Jones", STATUS:=10, CITY:="Paris"}

true

And we can check whether a tuple is a member of a relation using <=. Or using ∈ (if your keyboard can create it).

$S[STATUS=10].tuple <= $S

true

$S[STATUS=10].tuple ∈ $S

true

There is also a >= operator to check whether a relation contains a tuple (or ∋).

We can extract an attribute from a single-tuple relation using .tuple.name, e.g.

$S[STATUS=10]{CITY}.tuple.CITY

Paris

Given a single-tuple relation with a single attribute, we can extract that attribute using .tuple.* or the shorthand ..

$S[STATUS=10]{CITY}.tuple.*

Paris

$S[STATUS=10]{CITY}..

Paris

Limitations

When using .tuple, or the shorthand .., if the relation does not have exactly one tuple then an error will be given. Likewise if .tuple.* or .. is used, if the relation does not also have exactly one attribute then an error will be given, e.g.

print($S[STATUS=0].tuple)

print($S[STATUS>10].tuple)

$S[STATUS=10]..

Generally, tuple extraction should be avoided or left until the final presentation layer: working with relations affords more flexibility to combine data without having to check for the special case of exactly one tuple.