CREATE TABLE table_40542 ("Air Date" TEXT, "Timeslot" TEXT, "Rating" FLOAT, "Share" FLOAT, "18-49" TEXT, "Viewers" FLOAT, "Weekly Rank" TEXT)

What is the lowest share with an air date of March 21, 2008 and had viewers larger than 5.15?
SELECT MIN("Share") FROM table_40542 WHERE "Air Date" = 'march 21, 2008' AND "Viewers" > '5.15'
SELECT MIN("Share") FROM "table_40542" WHERE "Air Date" = 'march 21, 2008' AND "Viewers" > '5.15'