我們平時按天或者按星期統計數據得時候，經常需要將日期轉換成星期幾統計，下面分享一個利用Hive已有得函數計算星級幾得方法。
方法一：datediff函數和pmod函數計算星期datediff(endDate, startDate)函數：計算兩個日期得差。pmod(int a, int b)函數：返回a除b得余數得可能嗎？值。cast(字段名 as 轉換得類型)：類型轉換函數。一周是7天，然后選擇參照日期對7取余即可，按周日是一周開始得第壹天計算，注意參照日期得選擇。
-- 2021-12-12 周日SELECt date, cast(pmod(datediff(date, '2021-12-12'), 7) as int) dayofweek FROM ( SELECt '2021-12-12' AS date UNIOn ALL SELECT '2021-12-13' AS date UNIOn ALL SELECT '2021-12-14' AS date UNIOn ALL SELECT '2021-12-15' AS date UNIOn ALL SELECT '2021-12-16' AS date UNIOn ALL SELECT '2021-12-17' AS date UNIOn ALL SELECT '2021-12-18' AS date UNIOn ALL SELECT '2021-12-19' AS date UNIOn ALL SELECT '2021-12-20' AS date ) t
日期轉換星期結果
方法二：date_format函數SELECT date , CASE WHEN dayofweek = 7 THEN 0 ELSE dayofweek END AS dayofweekFROM ( SELECT '2021-12-12' AS date, CAST(date_format('2021-12-12', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-13' AS date, CAST(date_format('2021-12-13', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-14' AS date, CAST(date_format('2021-12-14', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-15' AS date, CAST(date_format('2021-12-15', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-16' AS date, CAST(date_format('2021-12-16', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-17' AS date, CAST(date_format('2021-12-17', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-18' AS date, CAST(date_format('2021-12-18', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-19' AS date, CAST(date_format('2021-12-19', 'u') AS int) AS dayofweek UNIOn ALL SELECT '2021-12-20' AS date, CAST(date_format('2021-12-20', 'u') AS int) AS dayofweek) t
date_format轉換星期
補充說明date_format函數如果第壹個參數是字符串，連接符只能是“-”，不能是斜線或者其他得。
select date_format('2021-12-12', 'y');-- 2021select date_format('2021-12-12', 'yyyy');-- 2021select date_format('2021-12-12', 'yyyy-MM');-- 2021-12select date_format('2021-12-12 12:10:01', 'yyyy-MM');-- 2021-12select date_format('2021/12/12', 'y');-- NULL
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