A.
SELECT 姓名 FROM 读者 WHERE NOT EXISTS; (SELECT 借书证号 FROM 借阅 WHERE 借阅.借书证号=读者.借书证号)
B.
SELECT 姓名,借书证号 FROM 读者 WHERE (SELECT *FROM 借阅 WHERE 借阅.借书证号=读者.借书证号)
C.
SELECT 姓名,借书证号 FROM 读者 WHERE NOT EXISTS; (SELECT *FROM 借阅 WHERE 借阅.借书证号=读者.借书证号)
D.
SELECT 姓名,借书证号 FROM 读者 WHERE 借阅=NULL (SELECT *FROM 借阅 WHERE 借阅.借书证号=读者.借书证号)