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                **窗口函數基本語法:** ```sql Function(arg1, ..., argn) over([partition by <...>] [order by <...>] [window_clause]) -- Function(arg1, ..., argn) 函數,如sum求和、first_value取第一個值 -- partition by 指明分區字段,如果省略則將所有數據作為一個分區 -- order by 指明每個分區排序的字段和方式,如果省略則按照表中原有順序排序 -- window_clause,指明相對于當前記錄的計算范圍,分別有向上preceding、向下following、上下邊界值between,省略則默認為當前分區 -- 只有在指定order by子句后才能指定window_clause,window_clause可取的值如下 -- rows between 也叫window子句 -- n+preceding 向前n條 -- n+following 向后n條件 -- current row 當前行 -- unbounded 無邊界,從最前面的點到最后面的點 -- unbounded preceding 向前無邊界 -- unbounded following 向后無邊界 ``` **窗口子句window_clause:** (1)用于進一步細分結果并應用分析函數。 (2)支持兩類窗口子句:行類型窗口(rows)、范圍類型窗口(range)。 (3)rank、ntile、dense_rank、cume_dist、percent_rank、lead、lag和row_number函數不支持與窗口子句一起使用。 <br/> **行窗口:** 根據當前行之前或之后的行號確定的窗口。 ```sql select name, dept_num as dept, salary as sal, max(salary) over (partition by dept_num order by name rows between 2 preceding and current row) win1, max(salary) over (partition by dept_num order by name rows between 2 preceding and unbounded following) win2, max(salary) over (partition by dept_num order by name rows between 1 preceding and 2 following) win3, max(salary) over (partition by dept_num order by name rows between 2 preceding and 1 preceding) win4, max(salary) over (partition by dept_num order by name rows between 1 following and 2 following) win5, max(salary) over (partition by dept_num order by name rows between current row and current row) win6, max(salary) over (partition by dept_num order by name rows between current row and 1 following) win7, max(salary) over (partition by dept_num order by name rows between current row and unbounded following) win8, max(salary) over (partition by dept_num order by name rows between unbounded preceding and current row) win9, max(salary) over (partition by dept_num order by name rows between unbounded preceding and 1 following) win10, max(salary) over (partition by dept_num order by name rows between unbounded preceding and unbounded following) win11, max(salary) over (partition by dept_num order by name rows 2 preceding) win12 from employee_contract order by dept, name; ``` <br/> **范圍窗口:** 是取分組內的值在指定范圍區間內的行。 (1)該范圍值/區間必須是數字或日期類型; (2)目前只支持一個`ORDER BY`列; ```sql -- 假設當前close值為3000,語句將包含分區內范圍從2500到4000的行 sum(close) range between 500 preceding and 1000 following -- 示例 select name, dept_num as dept, salary as sal, max(salary) over (partition by dept_num order by name rows between 2 preceding and current row) win1, salary - 1000 as sal_r_start,salary as sal_r_end, max(salary) over (partition by dept_num order by name range between 1000 preceding and current row) win13 from employee_contract order by dept, name; ```
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