user: Question: What is the total number of ratings among the 18 to 49 group that was aired on january 7, 2009?
Columns: ['Episode', 'Air Date', 'Timeslot (EST)', 'Season', 'Rating', 'Share', '18–49', 'Viewers (m)', 'Rank (#)']
Types: ['text', 'text', 'text', 'real', 'real', 'real', 'real', 'real', 'text']
Sample row: ['" I Knew You When "', 'October 1, 2008', 'Fri. 9:00PM', 2009.0, 2.8, 5.0, 1.6, 4.54, 'N/A']
SQL:

assistant: SELECT SUM("18–49") FROM "Table" WHERE "Air Date" = 'january 7, 2009';