这是三个等效的表达式：
select emp.*, (select count(*) from emp emp2 where emp2.dept_id = emp.dept_id and (emp2.salary > emp.salary or emp2.salary = emp.salary and emp2.emp_id <= emp.emp_id ) ) as "row_number", (select 1 + count(*) from emp emp2 where emp2.dept_id = emp.dept_id and emp2.salary > emp.salary ) ) as "rank", (select count(distinct salary) from emp emp2 where emp2.dept_id = emp.dept_id and emp2.salary >= emp.salary ) as "dense_rank",from emp;
假设存在，
emp_id以使行对于“ row_number”而言是唯一的。
