user: Question: What was the minimum points against if the opponent is Port Adelaide?
Columns: ['Opponent', 'Played', 'Won', 'Lost', 'Drew', 'Premiership points', 'Points for', 'Points against', 'Percentage (%)']
Types: ['text', 'real', 'real', 'real', 'real', 'real', 'real', 'real', 'text']
Sample row: ['Richmond', 2.0, 2.0, 0.0, 0.0, 8.0, 242.0, 158.0, '153.16']
SQL:

assistant: SELECT MIN("Points against") FROM "Table" WHERE "Opponent" = 'Port Adelaide';