Personnel table stores personnel information, such as personnel ID, name, gender, etc.
The skill table stores skill information, and 22 skills have 22 records. Only technical related information is recorded, and the certification time, such as skill ID and skill name, is not recorded.
The skill demonstration table stores information of personnel and skill certification, such as personnel ID, skill ID, certification time and validity period.
Just connect these three tables in series when querying.
If you want to query people with skills of 1:
Select a from personnel table a.* Add skill certification table B. Personnel id = B. Personnel ID, where B. Skill ID= 1.