Recursive Queries

Andl now supports recursive queries. This can be seen as a direct extension of the Relational Algebra for the situation where a query cannot be satisfied in a single operation.

Imagine a company organisation chart. One query will find every employee who reports to a given boss, but it takes a succession of queries to find out who reports to that employee, and who reports to them, until at last we reach the workers and no-one reports to them.

Here is the data for a simple orgchart (data thanks to SQLite).

name  | boss
-------------
Alice |
Bob   | Alice
Cindy | Alice
Dave  | Bob
Emma  | Bob
Fred  | Cindy
Gail  | Cindy

Here is the Andl code to query it.

def orgchart:db(csv)
orgchart
ua := {{ name:= 'Alice', level := 0 }} recurse( {{ boss := name, level := level+1 }} compose orgchart)
ua
ua [{ t:=fill('.', level*3) & name }]

And here is the result. The recurse function repeatedly evaluates the join with successive tuples starting from the seed, until no more can be found.

name  | level
--------------
Alice |      0
Bob   |      1
Cindy |      1
Dave  |      2
Emma  |      2
Fred  |      2
Gail  |      2
t
----------
Alice
...Bob
...Cindy
......Dave
......Emma
......Fred
......Gail

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