user: Question: What is the "season joined league" for the club that had 15 seasons at this level?
Columns: ['Clubs', 'Seasons at this level', 'Settlements', 'Season joined league', 'Position in 1959–1960']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Benfica', '27 seasons', 'Lisbon', '1934–1935', '1']
SQL:

assistant: SELECT "Season joined league" FROM "Table" WHERE "Seasons at this level" = '15 seasons';