CSVs in database columns - not a good idea?<>
This question already has an answer here:
Putting comma-separated values into a single field in a database is not just a bad idea, it is the incarnation of Satan expressed in a database model.
It cannot represent a great many situations accurately (cases in which the value contains a comma or something else that your CSV-consuming code has trouble with), often has problems with values nested in other values, cannot be properly indexed, cannot be used in database JOINs, is difficult to dedupe, cannot have additional information added to it (number of times the skill was earned, in your case, or a skill level), cannot participate in relational integrity, cannot enforce type constraints, and so on. The list is almost endless.
This is especially true of MySQL which has the very convenient group_concat function that makes it easy to present this data as a comma-separated string when needed while still maintaining the full functionality and speed of a normalized database.
You gain nothing from using the comma-separate approach but lose searchability and performance. Get Satan behind thee, and normalize your data.