1)which statement shows the maximum salary paid in each job category of each department?_______ a. 0select dept_id, job_cat,max(salary) from employees where salary > max(salary). b. 0select dept_id, job_cat,max(salary) from employees group by dept_id,job_cat. c. 0select dept_id, job_cat,max(salary) from employees. d. 0select dept_id, job_cat,max(salary) from employees group by dept_id. e. 0select dept_id, job_cat,max(salary) from employees group by dept_id,job_cat,salary. 2)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) 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 0select statement that includes a where clause,where is the group by clause placed in the 0select statement?______。 a. immediately after the 0select 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 0select statement that includes a where clause,where is the order by clause placed in the 0select statement?______. a.immediately after the 0select 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______. 0select last_name,salary from employees order by salary. 0select 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 0select statement should you use?______。 a. 0select to_date(sydate,’yearmmdd hh:mm:ss’)from dual. b. 0select to_char(sydate,’yearmonthday hh:mi:ss’)from dual. c. 0select to_date(sydate,’yyyymmdd hh24:mi:ss’)from dual. d. 0select to_char(sydate,’yyyymmdd hh24:mi:ss’)from dual. e. 0select to_char(sydate,’yy-mm-dd hh24:mi:ss’)from dual. 8)which 0select statement will the result ‘ello world’from the string‘hello world’?______. a. 0select substr(‘hello world’,1)from dual. b. 0select substr(trim(‘hello world’,1,1))from dual. c. 0select lower(substr(‘hello world’,1))from dual. d. 0select lower(trim(‘h’from‘hello world’))from dual. 9)which are dml statements(choose all that apply)______. a.commit b.merge c.0update d.0delete e.creat f.0drop 10)0select 语句中用来连接字符串的符号是______. 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？