Even though you've been given advice how to concatenate the selected values into a single string, my advice is: Don't do this.
There are several things why this kind of approach is a bad design. For example:
- You need to write extra logic to concatenate the data and also to split it when you read the data
- If the selected values are text, a comma would then be an illegal character in the data
- If the values are numbers, you'll have to do unnecessary conversions
- Modifications are more troublesome since you have to rebuild the string
- You cannot use for example foreign keys to enforce data integrity and so on...
Instead, in my opinion you should create a small table where each row represents a selection and the table would be properly using foreign keys to the actual parent table and perhaps to a lookup table (possible values for the selection).