least_not_null - BigQuery

WITH sample AS (
SELECT 5 as col_1, 3 as col_2, NULL as col_3 UNION ALL SELECT NULL, 10, 7 UNION ALL SELECT 4, NULL, 6 UNION ALL SELECT NULL, NULL, 1
)
SELECT `justfunctions.eu.least_not_null`([col_1, col_2, col_3])
FROM sample
                                            
/*--Output--
[3,7,4,1]

| col_1 | col_2 | col_3 | greatest_value |
|-------|-------|-------|----------------|
|   5   |   3   |  NULL |        3       |
|  NULL |   10  |   7   |        7       |
|   4   |  NULL |   6   |        4       |
|  NULL |  NULL |   1   |        1       |

*/

CREATE OR REPLACE FUNCTION `your_project_id.your_dataset_id.least_not_null`(`arr` any type)  AS ((SELECT min(a) FROM UNNEST(arr) a WHERE a is not NULL)
)
  OPTIONS ( description = '''This SQL function returns the least (minimum) value among multiple columns in the array <arr>, while properly handling NULL values.''')

SQL User Defined Function (SQL UDF)