user: Question: What is the lowest share with an air date of March 21, 2008 and had viewers larger than 5.15?
Columns: ['Air Date', 'Timeslot', 'Rating', 'Share', '18-49', 'Viewers', 'Weekly Rank']
Types: ['text', 'text', 'real', 'real', 'text', 'real', 'text']
Sample row: ['February 22, 2008', '9:00 P.M.', 4.6, 8.0, '2.4/7', 7.44, 'TBA']
SQL:

assistant: SELECT MIN("Share") FROM "Table" WHERE "Air Date" = 'March 21, 2008' AND "Viewers" > 5.15;