一、文本处理函数
- LENGTH()：以字节为单位返回字符串的长度。
mysql> SELECT LENGTH('1234');
+----------------+
| LENGTH('1234') |
+----------------+
| 4 |
+----------------+
1 row in set (0.00 sec)
- 大小写转换，UPPER()转换为大写，LOWER()转换为小写。
mysql> SELECT LOWER('Hello'), UPPER('Hello');
+----------------+----------------+
| LOWER('Hello') | UPPER('Hello') |
+----------------+----------------+
| hello | HELLO |
+----------------+----------------+
1 row in set (0.00 sec)
- 获取子字符串。
LEFT()返回从左边取指定长度的子串，RIGHT()返回从右边取指定长度的子串。
mysql> SELECT LEFT('1234567890', 3) AS left_3, RIGHT('1234567890', 3) AS right_3;
+--------+---------+
| left_3 | right_3 |
+--------+---------+
| 123 | 890 |
+--------+---------+
1 row in set (0.00 sec)
SUBSTRING()返回指定的子字符串，参数可以为起始位置或起始位置和长度，起始位置为负数。
mysql> SELECT SUBSTRING('function',2),SUBSTRING('function',2,4),SUBSTRING('function',-4,2);
+-------------------------+---------------------------+----------------------------+
| SUBSTRING('function',2) | SUBSTRING('function',2,4) | SUBSTRING('function',-4,2) |
+-------------------------+---------------------------+----------------------------+
| unction | unct | ti |
+-------------------------+---------------------------+----------------------------+
1 row in set (0.00 sec)
- 去除空格。
LTRIM()去除左边空格，RTRIM()去除右边空格，TRIM()去除两边空格。
mysql> SELECT CONCAT('(',LTRIM(' hello '),')') AS ltrim ,
CONCAT('(',RTRIM(' hello '),')') AS rtrim ,
CONCAT('(',TRIM(' hello '),')') AS trim ;
+-----------+-----------+---------+
| ltrim | rtrim | trim |
+-----------+-----------+---------+
| (hello ) | ( hello) | (hello) |
+-----------+-----------+---------+
1 row in set (0.00 sec)
- 查找子字符串第一次出现的位置。
LOCATE(substr,str)， LOCATE(substr,str,pos)，INSTR(str,substr)，两种函数的参数出现的顺序不同。
mysql> SELECT LOCATE('bar', 'foobarbar'),LOCATE('xbar', 'foobar'),LOCATE('bar', 'foobarbar', 5),INSTR('foobarbar', 'bar');
+----------------------------+--------------------------+-------------------------------+---------------------------+
| LOCATE('bar', 'foobarbar') | LOCATE('xbar', 'foobar') | LOCATE('bar', 'foobarbar', 5) | INSTR('foobarbar', 'bar') |
+----------------------------+--------------------------+-------------------------------+---------------------------+
| 4 | 0 | 7 | 4 |
+----------------------------+--------------------------+-------------------------------+---------------------------+
1 row in set (0.00 sec)
二、日期和时间处理函数
- Now()返回当前日期和时间。
mysql> select Now();
+---------------------+
| Now() |
+---------------------+
| 2019-03-31 23:49:02 |
+---------------------+
1 row in set (0.01 sec)
- CurDate()返回当前日期。
mysql> select CurDate();
+------------+
| CurDate() |
+------------+
| 2019-03-31 |
+------------+
1 row in set (0.00 sec)
- CurTime()返回当前时间。
mysql> select CurTime();
+-----------+
| CurTime() |
+-----------+
| 23:52:03 |
+-----------+
1 row in set (0.00 sec)
- DATE()提取日期或日期/时间表达式的日期部分。DAY()返回一个日期的天数部分。YEAR()返回一个日期的年份部分。
mysql> SELECT DATE('2003-12-31 01:02:03'), DAY('2007-02-03'), YEAR('2007-02-03');
+-----------------------------+-------------------+--------------------+
| DATE('2003-12-31 01:02:03') | DAY('2007-02-03') | YEAR('2007-02-03') |
+-----------------------------+-------------------+--------------------+
| 2003-12-31 | 3 | 2007 |
+-----------------------------+-------------------+--------------------+
1 row in set (0.00 sec)
- DATE_ADD()给日期添加指定的时间间隔，DATE_SUB()从日期减去指定的时间间隔。两者的用法相同。
mysql> SELECT DATE_ADD('2018-05-01',INTERVAL 30 DAY), DATE_ADD('2025-01-01 00:00:00',INTERVAL '1 1:1:1' DAY_SECOND);
+----------------------------------------+---------------------------------------------------------------+
| DATE_ADD('2018-05-01',INTERVAL 30 DAY) | DATE_ADD('2025-01-01 00:00:00',INTERVAL '1 1:1:1' DAY_SECOND) |
+----------------------------------------+---------------------------------------------------------------+
| 2018-05-31 | 2025-01-02 01:01:01 |
+----------------------------------------+---------------------------------------------------------------+
1 row in set (0.00 sec)
另外，ADDDATE()为增加一个日期(天，周等)，ADDTIME()增加一个时间(时，分等)，功能与DATE_ADD()类似，当为以下第一种形式时则完全相同。同样地，SUBDATE()与DATE_SUB()类似。
mysql> SELECT ADDDATE('2008-01-02', INTERVAL 31 DAY), ADDDATE('2008-01-02', 31), ADDTIME('01:00:00', '02:00:05');
+----------------------------------------+---------------------------+---------------------------------+
| ADDDATE('2008-01-02', INTERVAL 31 DAY) | ADDDATE('2008-01-02', 31) | ADDTIME('01:00:00', '02:00:05') |
+----------------------------------------+---------------------------+---------------------------------+
| 2008-02-02 | 2008-02-02 | 03:00:05 |
+----------------------------------------+---------------------------+---------------------------------+
1 row in set (0.00 sec)
- DATEDIFF()返回两个日期之间的天数。
mysql> SELECT DATEDIFF('2007-12-31 23:59:59','2007-12-30'), DATEDIFF('2007-11-30 23:59:59','2007-12-31');
+----------------------------------------------+----------------------------------------------+
| DATEDIFF('2007-12-31 23:59:59','2007-12-30') | DATEDIFF('2007-11-30 23:59:59','2007-12-31') |
+----------------------------------------------+----------------------------------------------+
| 1 | -31 |
+----------------------------------------------+----------------------------------------------+
1 row in set (0.00 sec)
- DATE_FORMAT()用不同的格式显示日期/时间。其中%W等为说明符，可以通过查表了解不同说明符的作用。
mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y %H:%i:%s');
+---------------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y %H:%i:%s') |
+---------------------------------------------------------+
| Sunday October 2009 22:23:00 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
