A.#N/AB.#IHC.#valueD.??
A."D"B."H"C."Y"D."M"
A.字符個(gè)數(shù)必須大于或等于0B.如果字符個(gè)數(shù)大于文本長(zhǎng)度,則LEFT 返回字符個(gè)數(shù)C.如果省略字符個(gè)數(shù),則假定其為0D.如果省略字符個(gè)數(shù),則假定其為空