Do I need a while loop to insert based on conditions?

I want to insert values in a column based on the following conditions. Name of the column to be inserted is project_renewal.

If decimal exists in project_code, insert the numbers after decimals.

SELECT substring_index(project_code, '.', -1)
FROM projects where
project_code like '%.%'

If no decimal, insert 0.

SELECT project_code
FROM projects where
project_code not like '%.%'

Order of insert must be the same as the order of reading values from project_code.