能够在时间上体现税收强制性，固定性特征的税制要素是（）。A.税率B.纳税环节C.

A．SELECT* FROM R WHERE ＜'2'

B．SELECTB, E FROM R WHERE B＜'2'

C．SELECTB, E FROM R HAVING E＜'2'

D．SELECT* FROM R WHERE '5'＜B

A．SELECT*FROM R WHERE E＜'2'

B．SELECT B,E FROM R WHERE E＜'2'

C．SELECT B,E FROM R HAVING E＜'2'

D．SELECT*FROM R WHERE'5'＜B

A．SELECT A,B FROM R,S WHERE C＜B

B．SELECT R.A,S.B FROM R,S WHERE R.C＜S.B

C．SELECT 1,5 FROM R WHERE C＜(SELECT B FROM S)

D．SELECT A,B FROM RWHERE C＜(SELECT B FROM S)

A．SELECT*FROM R,SWHERE R.C=S.C

B．SELECT A，B，D FROM R，S WHERE R.C=S.C

C．SELECT A，B，D FROM R，S WHERE R=S

D． SELECT A，B FROM R WHERE (SELECT D FROM S WHERE R．C= S．C)

设有关系R(A，B，C)和S(C，D)。与关系代数表达式πA,B,D(σR.C=S.C(R×S)等价的SQL语句是

A．SELECT*FROM R,S WHERE R.C=S.C

B．SELECT A,B,D FROM R,S WHERE R.C=S.C

C．SELE4T 1,2,8 FROM R,S WHERE R=S

D．SELECT A,B FROM R WHERE(SELECT D FROM S WHERE R.C=S.

设有关系R(A,B,C)和S(C，D)，与关系代数表达式ΠA，B，D(σR,C=S.C(R×S))等价的SQL语句是______。

A．SELECT*FROMR，S WHERE R.C=S.C

B．SELECT A,B,D FROM R,S WHERE R.C=S.C

C．SELECT A,B,D FROM R,S WHERE R=S

D．SELECT A,B FROM R WHERE(SELECT D FROM S WHERE R.C=S.

A．SELECTA，B FROM R，SWHERE C＜B

B．SELECT R．A，S．B From R，S WHERE R.C＜S.B

C．SELECT A，B FROM RWHERE C＜(SELECTB FROM S)

D．SELECT 1，5 FROM RWHERE C＜(SELECTB FROM S)

A．SELECT S_NAME FROM R WHERE S_NO=′2005％′

B．SELECT S_NAME FROM R WHERE S_NO LIKE ′2005％′

C．SELECT S_NAME FROM R WHERE S_NO=′2005%′

D．SELECT S_NAME FROM R WHERE S_NO ′2005%′

设有关系R(A，B，C)和S(A，B，C)，下面的SOL语句：

SELECT*FROM R WHERE A＜=20

UNION

SELECT*FROM S WHERE A＞=80

所对应的关系代数操作，除选择外还有()操作。

A．交

B．差

C．并

D．笛卡尔积