Big query update struct to null in repeated field - nested

Question

In Google bigquery, I'm trying to do an update on a repeated field.
For comparison, this works (or at least is flagged as valid), but of course isn't actually updating the field.
UPDATE my.table t
SET my_field = ARRAY(
SELECT AS STRUCT g.foo, g.bar, g.struct_to_set_null
FROM unnest(t.groups) as g
), ... FROM ... etc
Setting struct_to_set_null to null gives an error:
UPDATE my.table t
SET my_field = ARRAY(
SELECT AS STRUCT g.foo, g.bar, null as struct_to_set_null
FROM unnest(t.groups) as g
), ... FROM ... etc
Value of type ARRAY<STRUCT<... (really long and cut off) cannot be assigned to groups, which has type <ARRAY,STRUCT<... (same, really long, cut off)
I can see that the field in question is of type RECORD and NULLABLE, so I would think setting it to null is allowed. Is there a trick to getting this to work?

Answer 1

The problem is that BigQuery isn't inferring the type of the struct field just from the NULL literal; you need to be a bit more explicit. Here is an example:
CREATE TABLE tmp_elliottb.UpdateTable (
my_field ARRAY<STRUCT<foo INT64, bar INT64, struct_to_set_null STRUCT<x STRING, y BOOL, z INT64>>>
);
UPDATE tmp_elliottb.UpdateTable
SET my_field = ARRAY(
SELECT AS STRUCT foo, bar, NULL AS struct_to_set_null FROM UNNEST(my_field)
)
WHERE true;
This gives me:
Value of type ARRAY<STRUCT<foo INT64, bar INT64, struct_to_set_null INT64>> cannot be assigned to my_field, which has type ARRAY<STRUCT<foo INT64, bar INT64, struct_to_set_null STRUCT<x STRING, y BOOL, z INT64>>> at [4:16]
What I can do instead is to use an IF expression that produces NULL, but has struct_to_set_null on one of the branches in order to force the output type that I want:
UPDATE tmp_elliottb.UpdateTable
SET my_field = ARRAY(
SELECT AS STRUCT
foo, bar,
IF(false, struct_to_set_null, NULL) AS struct_to_set_null
FROM UNNEST(my_field)
)
WHERE true;
Or, alternatively, I can use SELECT * REPLACE:
UPDATE tmp_elliottb.UpdateTable
SET my_field = ARRAY(
SELECT AS STRUCT * REPLACE (IF(false, struct_to_set_null, NULL) AS struct_to_set_null )
FROM UNNEST(my_field)
)
WHERE true;

Answer 2

Repeated is an Array type, so it cannot be set to NULL.
Currently, BigQuery has two following limitations with respect to NULLs and ARRAYs:
BigQuery raises an error if query result has ARRAYs which contain NULL elements, although such ARRAYs can be used inside the query.
BigQuery translates NULL ARRAY into empty ARRAY in the query result, although inside the query NULL and empty ARRAYs are two distinct values.