3.3.4.5日期计算

MySQL提供了一些函数，可用于对日期执行计算，例如，计算年龄或提取部分日期。

要确定您的每只宠物几岁，请使用该 TIMESTAMPDIFF()功能。它的参数是要表示结果的单位，以及两个日期之间的差值。以下查询为每只宠物显示出生日期，当前日期和年龄（以岁为单位）。一个别名（age）是用来制造最终输出列标签更有意义。

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 |
+----------+------------+------------+------+

该查询有效，但是如果以某种顺序显示行，则可以更轻松地扫描结果。这可以通过添加一个ORDER BY name子句来按名称对输出进行排序来完成：

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 |
+----------+------------+------------+------+

要使用age而不是排序输出name，只需使用其他ORDER BY子句：

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 |
+----------+------------+------------+------+

可以使用类似的查询来确定已死亡动物的死亡年龄。您可以通过检查death值是否为来确定这些动物是哪些NULL。然后，对于那些没有NULL值的人，计算death和 birth值之间的差：

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 |
+--------+------------+------------+------+

该查询使用death IS NOT NULL而不是，death <> NULL因为它 NULL是一个特殊值，无法使用常规比较运算符进行比较。稍后讨论。请参见第3.3.4.6节“使用空值”。

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the birth column. MySQL provides several functions for extracting parts of dates, such as YEAR(), MONTH(), and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth):

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 |
+----------+------------+--------------+

在接下来的一个月中找到有生日的动物也很简单。假设当前月份为4月。然后，月份值为4，您可以5像这样查找五月（月份）出生的动物：

mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name  | birth      |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+

如果当前月份为十二月，则情况会很复杂。您不能仅在月份号（12）上加上一个并查找月份中出生的动物 13，因为没有这样的月份。相反，您寻找一月（月1）出生的动物。

您可以编写查询以便无论当前月份是什么都可以使用，从而不必在特定月份使用该数字。 DATE_ADD()使您可以将时间间隔添加到给定的日期。如果您将的值加上一个月CURDATE()，则使用提取月份部分MONTH()，结果将生成要在其中寻找生日的月份：

mysql> SELECT name, birth FROM pet
       WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(),INTERVAL 1 MONTH));

完成相同任务的另一种方法是，1在使用模函数（MOD）将月份值包装0为当前值之后，将其添加到当前月份之后的下个月12：

mysql> SELECT name, birth FROM pet
       WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1;

MONTH()返回介于1和之间的数字12。并 MOD(something,12)返回介于0和之间的数字11。因此加法必须在之后 MOD()，否则我们将从11月（11）到1月（1）。

如果计算使用无效日期，则计算将失败并产生警告：

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' |
+---------+------+----------------------------------------+