单项选择题
A. ALTER TABLE students ADD PRIMARY KEY student_id;
B. ALTER TABLE students ADD CONSTRAINT PRIMARY KEY (student _ id);
C. ALTER TABLE students ADD CONSTRAINT stud _ id _pk PRIMARY KEY (student _ id);
D. ALTER TABLE students MODIFY CONSTRAINT stud _ id _pk PRIMARY KEY (student _ id);
单项选择题 The EMPLOYEE tables has these columns: You want to display the name and annual salary multiplied by the commission_pct for all employees. For records that have a NULL commission_pct, a zero must be displayed against the calculated column. Which SQL statement displays the desired results?（）
单项选择题 Examine the data from the ORDERS and CUSTOMERS table. ORDERS ORD_ID ORD_DATE CUST_ID ORD_TOTAL 100 12-JAN-2000 15 10000 09-MAR- 101 40 8000 09-MAR- 102 35 12500 15-MAR- 103 15 12000 104 25-JUN-2000 15 6000 105 18-JUL-2000 20 5000 106 18-JUL-2000 35 7000 107 21-JUL-2000 20 6500 04-AUG- 108 10 8000 CUSTOMERS CUST_ID CUST_NAME CITY 10 Smith Los Angeles 15 Bob San Francisco 20 Martin Chicago 25 Mary New York 30 Rina Chicago 35 Smith New York 40 Linda New York Which SQL statement retrieves the order ID, customer ID, and order total for the orders that are placed on the same day that Martin places his orders?（）
多项选择题 Examine the description of the STUDENTS table: Which two aggregate functions are valid on the START_DATE column?（）