Loop

Experimental

prql

loop {step_function} {initial_relation}

Iteratively applies step function to initial relation until the step returns an empty table. Returns a relation that contains rows of initial relation and all intermediate relations.

This behavior could be expressed with following pseudo-code:

python

def loop(step, initial):
    result = []
    current = initial
    while current is not empty:
        result = append(result, current)
        current = step(current)

    return result

Examples

prql

from [{n = 1}]
loop (
    filter n<4
    select n = n+1
)

# returns [1, 2, 3, 4]

[!NOTE] The behavior of WITH RECURSIVE may depend on the database configuration in MySQL. The compiler assumes the behavior described by the Postgres documentation and will not produce correct results for alternative configurations of MySQL.

[!NOTE] Currently, loop may produce references to the recursive CTE in sub-queries, which is not supported by some database engines, e.g. SQLite. For now, we suggest step functions are kept simple enough to fit into a single SELECT statement.