1)description of the students table:

　　sid_id number

　　start_date date

　　end_date date

　　which two function are valid on the start_date column?_________。

　　a. sum(start_date)

　　b. avg(start_date)

　　c. count(start_date)

　　d. avg(start_date,end_date)

　　e. min(start_date)

　　f. maximum(start_date)

　　2)which statement shows the maximum salary paid in each job category of each department?_______

　　a. select dept_id, job_cat,max(salary) from employees where salary > max(salary);

　　b. select dept_id, job_cat,max(salary) from employees group by dept_id,job_cat;

　　c. select dept_id, job_cat,max(salary) from employees;

　　d. select dept_id, job_cat,max(salary) from employees group by dept_id;

　　e. select dept_id, job_cat,max(salary) from employees group by dept_id,job_cat,salary;

　　3)for which two constraints does the oracle server implicitly create a unique index?______。

　　a. not null

　　b. primary

　　c. foreign key

　　d. check

　　e. unique

　　4)in a select statement that includes a where clause,where is the group by clause placed in the select statement?______。

　　a. immediately after the select clause

　　b. before the where clause

　　c. before the from clause

　　d. after the order by clause

　　e. after the where clause

　　5)in a select statement that includes a where clause,where is the order by clause placed in the select statement?______.

　　a.immediately after the select clause

　　b.before the where clause

　　c.after all clause

　　d.after the where clause

　　e.before the from clause

　　6)evaluate there two sql statements______.

　　select last_name,salary from employees order by salary;

　　select last_name,salary from employees order by 2 asc;

　　a.the same result b.different result c.the second statement returns a syntax error

　　7) you would like to display the system date in the format“20051110 14：44：17”。which select statement should you use?______。

　　a. select to_date(sydate,’yearmmdd hh:mm:ss’)from dual;

　　b. select to_char(sydate,’yearmonthday hh:mi:ss’)from dual;

　　c. select to_date(sydate,’yyyymmdd hh24:mi:ss’)from dual;

　　d. select to_char(sydate,’yyyymmdd hh24:mi:ss’)from dual;

　　e. select to_char(sydate,’yy-mm-dd hh24:mi:ss’)from dual;

　　8)which select statement will the result ‘ello world’from the string‘hello world’?______.

　　a. select substr(‘hello world’,1)from dual;

　　b. select substr(trim(‘hello world’,1,1))from dual;

　　c. select lower(substr(‘hello world’,1))from dual;

　　d. select lower(trim(‘h’from‘hello world’))from dual;

　　9)which are dml statements(choose all that apply)______.

　　a.commit b.merge c.update d.delete e.creat f.drop

　　10)select 语句中用来连接字符串的符号是___c___.

　　a. “+” b. “&” c.“||” d.“|”

　　问答题：什么是聚集索引，什么是非聚集索引，什么又是主键?