How to replace null date in column a with the date in the same row of column b

[rtd123]

I have two columns that pull basically the exact same information on how a specific document is uploaded. Essentially, document A can be uploaded as “document a1” or “document a2”, and whichever tag is used means the document upload date filters into the specified column and the field for the other column remains null. I would like to combine these two columns in looker so that in a new column, the date from “document a1” will pull unless it is null. If it is null, then the date from document 2- example format in excel to demonstrate  

[IanT]

Google Coalesce() function for your database