Loop

Experimental

1. 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:

1. def loop(step, initial):
2. result = []
3. current = initial
4. while current is not empty:
5. result = append(result, current)
6. current = step(current)

8. return result

Examples

PRQL

1. from [{n = 1}]
2. loop (
3. filter n<4
4. select n = n+1
5. )

7. # returns [1, 2, 3, 4]

SQL

1. WITH RECURSIVE table_1 AS (
2. SELECT
3. 1 AS n
4. ),
5. table_0 AS (
6. SELECT
7. n
8. FROM
9. table_1
10. UNION
11. ALL
12. SELECT
13. n + 1
14. FROM
15. table_0
16. WHERE
17. n < 4
18. )
19. SELECT
20. n
21. FROM
22. table_0

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.