MySQL是一個常用的開源關系型數據庫管理系統,其擁有多種查詢語句,其中并列查詢是很常用的一種。本文就講述如何實現不同天數的值并列查詢。
SELECT COUNT(*) AS day_1, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 2, 1, 0)) AS day_2, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 3, 1, 0)) AS day_3, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 4, 1, 0)) AS day_4, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 5, 1, 0)) AS day_5, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 6, 1, 0)) AS day_6 FROM my_table;
上述代碼中,我們使用了IF函數來判斷日期差值是否符合條件,然后根據條件返回不同的結果,最終利用SUM函數來求和。其中,COUNT(*)是求總數,得到day_1的結果。其余的day_2、day_3、day_4、day_5、day_6分別表示當前日期與記錄日期的差值為2~6天的記錄數量。
此外,我們還可以增加WHERE條件來限制查詢的記錄數,例如:
SELECT COUNT(*) AS day_1, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 2, 1, 0)) AS day_2, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 3, 1, 0)) AS day_3, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 4, 1, 0)) AS day_4, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 5, 1, 0)) AS day_5, SUM(IF(DATEDIFF(CURRENT_DATE, date) = 6, 1, 0)) AS day_6 FROM my_table WHERE date BETWEEN '2022-01-01' AND '2022-01-31';
此時,我們只會查詢指定日期范圍內的記錄。
總之,MySQL的并列查詢使得我們可以很方便地獲取一些特定的數據信息,同時也可以結合各種函數進行更加靈活的查詢操作。