设有关系R，S和T如下：关系T是由R和S经过什么操作得到的？A．B．C．D．

A、cd

B、de

C、d

D、e

A、ALTER TABLE Student ADD Addr Char(20) ;

B、ALTER TABLE Student COLUMN ADD Addr Char(20) ;

C、ADD TABLE Student Addr Char(20) ;

D、ADD TABLE Student ALTER Addr Char(20);

A、SELECT子句

B、FROM子句

C、WHERE子句

D、UNION子句

A、SELECT DISTINCT Sno FROM SC WHERE Grade<60;<br> B、SELECT DISTINCT Sno FROM SC WHERE Grade IS NULL;

C、SELECT Sno FROM SC WHERE Grade<60;<br> D、SELECT ALL Sno FROM SC WHERE Grade<60;<br>

A、< >NULL

B、!=NULL

C、IS NOT NULL

D、NOT IS NULL

A、以“HUST_DB”为首的字符串

B、以“HUST_”为首，“DB_”为尾的字符串

C、以“HUST_”为首，“DB”为尾部倒数第3、第2个字符的字符串

D、以上选项都不正确

A、SELECT Sno, AVG(Grade) FROM SC GROUP BY Sno HAVING AVG(Grade)>=60;

B、SELECT Sno FROM SC WHERE Grade>=60 GROUP BY Sno;

C、SELECT Sno, Grade FROM SC GROUP BY Sno HAVING AVG(Grade)>=60;

D、SELECT Sno FROM SC GROUP BY Sno HAVING Grade >=60;

A、ORDER BY GRADE, SNO

B、ORDER BY GRADE DESC, SNO

C、ORDER BY GRADE DESC

D、ORDER BY SNO ASC, GRADE DESC

A、100

B、10

C、1000

D、不确定