日期計算 · mysql 8.0 中文文檔

#### 3.3.4.5 日期計算 MySQL 提供了幾個函數可以用來完成日期計算, 例如, 計算年齡或者提取部分日期. 要確定每個寵物的年齡, 可以使用 [`TIMESTAMPDIFF()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_timestampdiff) 函數. 它的參數是你想要表達結果的單位, 以及取差的兩個日期. 下面的查詢顯示了每只寵物的出生日期, 當前日期和年齡. ***`alias`*** (`age`) 用于讓最終的輸出列名更有意義. ```sql mysql> SELECT name, birth, CURDATE(), TIMESTAMPDIFF(YEAR,birth,CURDATE()) AS age FROM pet; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | +----------+------------+------------+------+ ``` 查詢可以正常工作, 但是如果按照某種順序顯示行, 則可以更容易的掃描結果. 可以通過添加 `ORDER BY name` 子句按照名稱排序來輸出: ```sql mysql> SELECT name, birth, CURDATE(), TIMESTAMPDIFF(YEAR,birth,CURDATE()) AS age FROM pet ORDER BY name; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | +----------+------------+------------+------+ ``` 按照 `age` 而不是 `name` 進行排序輸出, 只需要使用不同的 `ORDER BY` 子句: ```sql mysql> SELECT name, birth, CURDATE(), TIMESTAMPDIFF(YEAR,birth,CURDATE()) AS age FROM pet ORDER BY age; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | +----------+------------+------------+------+ ``` 相似的查詢可以用來確定已死亡動物的死亡年齡. 你可以通過那些 `death` 的值為 `NULL` 來確定動物是那些. 然后, 對那些非 `NULL` 值, 計算出 `death` 和 `birth` 的差值: ```sql mysql> SELECT name, birth, death, TIMESTAMPDIFF(YEAR,birth,death) AS age FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5 | +--------+------------+------------+------+ ``` 查詢使用 `death IS NOT NULL` 而不是 `death <> NULL` 是因為 `NULL` 是一個特殊值, 不能使用常規的比較操作符來操作. 稍后將進行討論. 參閱 [Section 3.3.4.6, “處理 NULL 值”](https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html). 如果你想知道下個月那些動物過生日呢? 對于這類運算, 年月日無關緊要; 你只需取出 `birth` 列的月份部分. MySQL 提供了幾個提取日期部分的函數, 比如 [`YEAR()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_year), [`MONTH()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month), 和 [`DAYOFMONTH()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_dayofmonth). [`MONTH()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month) 函數適合這里. 要查看它是如何工作的, 運行一個顯示出 `birth` 和 [`MONTH(birth)`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month) 的簡單查詢: ```sql mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+ ``` 找出下個月有生日的動物非常簡單. 假設當前月份是四月. 然后這個月份的值就是 `4`, 你可以這樣查找 `5` 月份出生的動物: ```sql mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+ ``` 如果當前月份是 12 月, 就會出現一個小問題. 你不能僅僅在 (`12`) 上面加 1, 然后尋找第 `13` 個月出生的動物, 因為沒有這個月份. 相反, 你要尋找 `1` 月的動物. 你可以寫個查詢無論在哪個月份都可以正常工作, 這樣你就不必為特定的月份使用數字. [`DATE_ADD()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-add) 允許向指定日期添加間隔. 如果添加一個月到 [`CURDATE()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_curdate) 的值中, 然后使用 [`MONTH()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month) 提取月份部分, 然后產生了尋找生日的月份: ```sql mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(),INTERVAL 1 MONTH)); ``` 另一種完成任務的方法是使用 `MOD` 方法對其取模, 如果當前月份是 `12`, 那么取值為 `0`, 加 `1` 來獲得當前月份的下一個月的值: ```sql mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1; ``` [`MONTH()`](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month) 返回 `1` 到 `12` 之間的數字. [`MOD(something,12)`](https://dev.mysql.com/doc/refman/8.0/en/mathematical-functions.html#function_mod) 返回 `0` 和 `11` 之間的數字. 因此, 必須在 [`MOD()`](https://dev.mysql.com/doc/refman/8.0/en/mathematical-functions.html#function_mod) 之后添加, 否則將從 `11` 月份直接到 `1` 月份. 如果使用無效日期, 則會計算失敗并產生警告: ```sql mysql> SELECT '2018-10-31' + INTERVAL 1 DAY; +-------------------------------+ | '2018-10-31' + INTERVAL 1 DAY | +-------------------------------+ | 2018-11-01 | +-------------------------------+ mysql> SELECT '2018-10-32' + INTERVAL 1 DAY; +-------------------------------+ | '2018-10-32' + INTERVAL 1 DAY | +-------------------------------+ | NULL | +-------------------------------+ mysql> SHOW WARNINGS; +---------+------+----------------------------------------+ | Level | Code | Message | +---------+------+----------------------------------------+ | Warning | 1292 | Incorrect datetime value: '2018-10-32' | +---------+------+----------------------------------------+ ```