BigQuery - cannot UNION String and NULL

Question

I have two tables that I am trying to UNION. Column id in table1 is of type STRING. I do not have that column in table2 so I am writing SELECT NULL AS id FROM table2 but BigQuery thinks that column id in table2 is of type INT64.

I get this error - Column 2 in UNION ALL has incompatible types: STRING, INT64.

Query works if I write SELECT "" AS id FROM table2 instead.

Answer 1

SELECT id FROM table1 UNION ALL SELECT CAST(NULL AS STRING) FROM table2