Data

Compare relations and extract tuples

We can compare relations. Since the restrict operation returns a relation, we can compare two filtered results. 

S[STATUS=20]
SNOSNAMESTATUSCITY


S1Smith20London


S4Clark20London


S[CITY="London"]
SNOSNAMESTATUSCITY


S1Smith20London


S4Clark20London

The equality comparison, using = will return true for this case. 

S[STATUS=20] = S[CITY="London"]
true

We can also test whether one relation is a subset of another, using <=. This symbol is meant to resemble the subset symbol, ⊆ (which you can use instead, if your keyboard can create it). 

S[STATUS=20] <= S[CITY="London"]
true
S[STATUS=20] <= S
true
S[STATUS=20] <= S[STATUS=30]
false

There is also a superset operator: >= (or ⊇). And there are pure subset and pure superset operators (i.e. excluding the cases when both are equal): < and > (or ⊂ and ⊃)

We can add a boolean attribute to a relation (or use an existing one). Then we can use the all or exists (or ∀ or ∃ if your keyboard can create them) aggregate operators to check if all or any tuples have the attribute set true. 

exists(S{*, test:=(STATUS=20)}, test)
true
all(S{test:=(STATUS=20)})
false
all(S{test:=(STATUS<100)})
true

exists and all should be more efficient than counting the matches since they can short-circuit and finish early without necessarily checking all the tuples.

Since relations are comprised of tuples, we can extract a tuple from a relation, providing it has only 1 tuple in it. The print function displays the tuple horizontally. 

print(S[STATUS=10].tuple)
┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
  SNO   SNAME   STATUS   CITY   
  S2    Jones       10   Paris  
┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄

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

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


S2Jones10Paris

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 tuple using .tuple.name, e.g. 

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

Given a tuple 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