109. Examine the structure of the MARKS table:
name Null Type
STUDENT_ID NOT NULL VARCHAR2(4)
STUDENT_NAME VARCHAR2(25)
SUBJECT1 NUMBER(3)
SUBJECT2 NUMBER(3)
SUBJECT3 NUMBER(3)
Which two statements would execute successfully? (Choose two.)
A. SELECT student_name,subject1
FROM marks
WHERE subject1 > AVG(subject1); where不能跟表达式
B. SELECT student_name,SUM(subject1)
FROM marks
WHERE student_name LIKE 'R%'; 少了group by
C. SELECT SUM(subject1+subject2+subject3)
FROM marks
WHERE student_name IS NULL;
D. SELECT SUM(DISTINCT NVL(subject1,0)), MAX(subject1)
FROM marks
WHERE subject1 > subject2;
Answer: CD
答案解析：
实验测试：
首先创建marks表
scott@TEST0924> create table marks
2 (student_id varchar2(4) not null,
3 student_name varchar(25),
4 subject1 number(3),
5 subject2 number(3),
6 subject3 number(3)
7 );
插入几条数据后查询如下：
scott@TEST0924> select * from marks;
STUD STUDENT_NAME SUBJECT1 SUBJECT2 SUBJECT3
---- ------------------------- ---------- ---------- ----------
1 aa 88 89 90
2 bb 85 85 80
3 cc 87 81 95
A答案：AGV分组函数不能用在where子句里
scott@TEST0924> SELECT student_name,subject1
2 FROM marks
3 WHERE subject1 > AVG(subject1);
WHERE subject1 > AVG(subject1)
*
ERROR at line 3:
ORA-00934: group function is not allowed here
scott@TEST0924> SELECT student_name,subject1
2 FROM marks
3 WHERE subject1>(select AVG(subject1) from marks);
STUDENT_NAME SUBJECT1
------------------------- ----------
aa 88
cc 87
B答案：缺少group by
scott@TEST0924> SELECT student_name,SUM(subject1)
2 FROM marks
3 WHERE student_name LIKE 'a%';
SELECT student_name,SUM(subject1)
*
ERROR at line 1:
ORA-00937: not a single-group group function
scott@TEST0924> SELECT student_name,SUM(subject1)
2 FROM marks
3 WHERE student_name LIKE 'a%'
4 group by student_name;
STUDENT_NAME SUM(SUBJECT1)
------------------------- -------------
aa 88
C答案：因为测试数据里没有student_name没有为空的，所有无返回值。
cott@TEST0924> SELECT SUM(subject1+subject2+subject3)
2 FROM marks
3 WHERE student_name IS NULL;
SUM(SUBJECT1+SUBJECT2+SUBJECT3)
-------------------------------
故用is not null 一样的测试效果。
scott@TEST0924>
scott@TEST0924> SELECT SUM(subject1+subject2+subject3)
2 FROM marks
3 WHERE student_name IS not NULL;
SUM(SUBJECT1+SUBJECT2+SUBJECT3)
-------------------------------
780
D答案：
cott@TEST0924> SELECT SUM(DISTINCT NVL(subject1,0)), MAX(subject1)
2 FROM marks
3 WHERE subject1 > subject2;
SUM(DISTINCTNVL(SUBJECT1,0)) MAX(SUBJECT1)
---------------------------- -------------
87 87