[Oracle]箱型圖(BoxPlot)統計值運算

繪製箱型圖顯示一組數據的分散情況，而若從資料庫中的Raw Data，來繪製箱型圖，則需結算出六個必要數值：最大值、最小值、中位數、平均值、Q1下四分位數、Q3下四分位數

以Oracle範例資料庫中的employees資料表為例，計算每個部門的箱型圖數值：

select department_id,
       max(salary) Upper_whisker,
       min(salary) Lower_whisker,
       round(avg(salary),3) Average,
       PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY salary ASC) Q1,
       PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY salary ASC) Median,
       PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY salary ASC) Q3
  from employees t
 group by department_id