Select column value if column exists in that table else create that column and set it's value to null in BigQuery

Question

I want to select total 450 fixed columns from the table which may or may not have all 450 columns always. When it doesn't have all columns then it should create the missing column and set it's value as null. In Sql there is a function if exists() But in bigquery I am unable to use it wisely.

Any suggestion will help a lot