The EMPLOYEES table has these columns:

Management wants to add a default value to the SALARY column. You plan to alter the table by using this SQL statement:

What is true about your ALTER statement?（）

多项选择题Exhibit The COMMISSION column shows the monthly commission earned by the employee. Which two tasks would require subqueries or joins in order to be performed in a single step? （）

A. listing the employees who earn the same amount of commission as employee 3
B. finding the total commission earned by the employees in department 10
C. finding the number of employees who earn a commission that is higher than the average commission of the company
D. listing the departments whose average commission is more that 600
E. listing the employees who do not earn commission and who are working for department 20 in descending order of the employee ID 
F. listing the employees whose annual commission is more than 6000

单项选择题User Mary has a view called EMP_DEPT_LOC_VU that was created based on the EMPLOYEES, DEPARTMENTS, and LOCATIONS tables. She has the privilege to create a public synonym, and would like to create a synonym for this view that can be used by all users of the database.Which SQL statement can Mary use to accomplish that task?（）

A. CREATE PUBLIC SYNONYM EDL_VU ON emp_dept_loc_vu
B. CREATE PUBLIC SYNONYM EDL:VU FOR mary (emp_dept_loc_vu);
C. CREATE PUBLIC SYNONYM EDL_VU FOR emp _dept_loc_vu;
D. CREATE SYNONYM EDL_VU ON emp_dept_loc_vu FOR EACH USER;
E. CREATE SYNONYM EDL_VU FOR EACH USER ON emp_dept_loc_vu
F. CREATE PUBLIC SYNONYM EDL_VU ON emp_dept_loc_vu FOR ALL USERS;

多项选择题Which two statements about subqueries are true? （）

A. A single row subquery can retrieve data from only one table.
B. A SQL query statement cannot display data from table B that is referred to in its subquery, unless table B is included in the main query's FROM clause.
C. A SQL query statement can display data from table B that is referred to in its subquery, without including B in its own FROM clause.
D. A single row subquery can retrieve data from more than one table.
E. A single row subquery cannot be used in a condition where the LIKE operator is used for comparison.
F. A multiple-row subquery cannot be used in an INSERT statement to insert multiple rows at a time.

单项选择题Which substitution variable would you use if you want to reuse the variable without prompting the user each time?（）

A. &
B. ACCEPT
C. PROMPT
D. &&

单项选择题Which constraint can be defines only at the column level?（）

A. UNIQUE
B. NOT NULL
C. CHECK
D. PRIMARY KEY
E. FOREIGN KEY