mysql局部变量

从letecode上面看到的一个sql语句，使用局部变量，感觉比较好，记录下

SELECT e.Name, e.DepartmentId, e.Salary,

          @Rank := (CASE 
WHEN @PrevDept != e.DepartmentId THEN 1
                    WHEN @PrevSalary = e.Salary THEN @Rank
ELSE @Rank + 1 END) AS Rank, 
  @PrevDept := e.DepartmentId,
          @PrevSalary := e.Salary
FROM Employee e, (SELECT @Rank := 0, @PrevDept := 0, @PrevSalary := 0) r
ORDER BY DepartmentId ASC, Salary DESC