单项选择题 Examine the data in the EMPLOYEES and EMP_HIST tables: The EMP_HIST table is updated at the end of every year. The employee ID, name, job ID, and salary of each existing employee are modified with the latest data. New employee details are added to the table. Which statement accomplishes this task?（）

单项选择题 You define a multiple-row subquery in the WHERE clause of an SQL query with a comparison operator "=".What happens when the main query is executed?（）

多项选择题 Which three statements correctly describe the functions and use of constraints? （）