在book表中，将工具书类型（tool）的书的书架序号都减少2，下列语句正确的是（）
UPDATE books SET shelf = shelf - 2 WHERE type IS 'tool';
INSERT books SET shelf = shelf - 2 WHERE type IS 'tool';
UPDATE books SET shelf = shelf - 2 WHERE type = 'tool';
UPDATE books INTO shelf = shelf - 2 WHERE type = 'tool';
