补充下列语句: IF EXISTS(SELECT * FROM ? WHERE name='stuDB') DROP DATABASE stuDB 建表的语法? 补充下列语句: IF EXISTS(SELECT * FROM ? WHERE name='stuDB') DROP TABLE stuInfo 解释下列语句的含义? EXEC sp_addlogin 'zhangsan', '1234' EXEC sp_grantdbaccess 'zhangsan', 'zhangsanDBUser