Find an SQL that can judge the data in the table and only fill in the SQL that is not overwritten

The phenomenon and background of the problem

The three fields in my main table need to be filled ( One of them is that the new fields are all null, The other two field sections contain data , The rest are null）.

demand

I need to fill the table with data , And the existing data in the table cannot be overwritten

Operation results and error reporting contents

My solution ideas and tried methods

I batch fill the records that are all empty in the three fields of the primary table into the corresponding data of the secondary table , Then enter the remaining records manually .

What I want to achieve

Find one that can judge the data in the table , Fill only those that are not overwritten sql