1、MySQL
-----运算符和函数-----
字符函数，数值运算符，比较运算等
----字符函数
---
CONCAT()--字符连接
CONCAT_WS()--使用指定的分隔符进行字符连接
mysql> SELECT CONCAT('imooc','-','MySQL');
+-----------------------------+
| CONCAT('imooc','-','MySQL') |
+-----------------------------+
| imooc-MySQL |
+-----------------------------+
mysql> USE TEST;
Database changed
mysql> SELECT * FROM TEST;
+----+----------+
| id | username |
+----+----------+
| 1 | Tom |
| 2 | Ben |
+----+----------+
2 rows in set (0.02 sec)
mysql> SELECT CONCAT(id,username) AS fullname FROM test;
+----------+
| fullname |
+----------+
| 1Tom |
| 2Ben |
+----------+
以上是CONCAT()的实例，而CONCAT_WS()如下
mysql> SELECT CONCAT_WS('-',id,username) FROM test;
+----------------------------+
| CONCAT_WS('-',id,username) |
+----------------------------+
| 1-Tom |
| 2-Ben |
+----------------------------+
CONCAT_WS()需要至少三个参数，第一个是分隔符，后面才是需要连接的东西
---
FORMAT()，数字格式化
mysql> SELECT FORMAT(234234.23423,2);
+------------------------+
| FORMAT(234234.23423,2) |
+------------------------+
| 234,234.23 |
+------------------------+
就是标准化，当然是外国人那一套
---
LOWER()&UPPER()，大小写变换
---
LEFT()&RIGHT()，获取左右侧字符
需要两位参数,从哪个字段取，从第几位开始的左右侧
mysql> SELECT UPPER(LEFT('andy',1));
+-----------------------+
| UPPER(LEFT('andy',1)) |
+-----------------------+
| A |
+-----------------------+
---
LENGTH()，获取字符串长度。注意，空格也要算在内
---
LTRIM(),RTRIM(),TRIM()，删除前/后/前后的空格
TRIM还能删除其他字符
mysql> SELECT TRIM(LEADING '!' FROM '!!!ANDY!!');
+------------------------------------+
| TRIM(LEADING '!' FROM '!!!ANDY!!') |
+------------------------------------+
| ANDY!! |
+------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT TRIM(BOTH '!' FROM '!!!ANDY!!');
+---------------------------------+
| TRIM(BOTH '!' FROM '!!!ANDY!!') |
+---------------------------------+
| ANDY |
+---------------------------------+
1 row in set (0.00 sec)
mysql> SELECT TRIM(TRAILING '!' FROM '!!!ANDY!!');
+-------------------------------------+
| TRIM(TRAILING '!' FROM '!!!ANDY!!') |
+-------------------------------------+
| !!!ANDY |
+-------------------------------------+
1 row in set (0.00 sec)
---
REPLACE()
mysql> SELECT REPLACE('!!ANDY!SDL!!','!','');
+--------------------------------+
| REPLACE('!!ANDY!SDL!!','!','') |
+--------------------------------+
| ANDYSDL |
+--------------------------------+
1 row in set (0.00 sec)
mysql> SELECT REPLACE('!!ANDY!SDL!!','!','LALAL');
+-------------------------------------+
| REPLACE('!!ANDY!SDL!!','!','LALAL') |
+-------------------------------------+
| LALALLALALANDYLALALSDLLALALLALAL |
+-------------------------------------+
1 row in set (0.00 sec)
可以看到，替换是比较灵活的
---
SUBSTRING()
mysql> SELECT SUBSTRING('MYSQL','1','2');
+----------------------------+
| SUBSTRING('MYSQL','1','2') |
+----------------------------+
| MY |
+----------------------------+
1 row in set (0.00 sec)
注意mysql是从1开始，不是从0开始数数
---
做匹配
mysql> SELECT 'mysql' LIKE 'M%';
+-------------------+
| 'mysql' LIKE 'M%' |
+-------------------+
| 1 |
+-------------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM test WHERE username LIKE '%m%';
+----+----------+
| id | username |
+----+----------+
| 1 | Tom |
+----+----------+
这里，%代表任意，类似window中的*
_代表任意一个字符
----数值运算符以及函数
几个很通用的函数简单的介绍下
CEIL()-向上取整---------FLOOR()-向下取整
DIV()-整数除法，也就是结果只有整数
MOD()-取余数
POWER()-幂运算
ROUND()-四舍五入
TRUNCATE()-数字截取（不再四舍五入）
不是两位参数，就是一位参数，大家自己试试哈
----比较运算符与函数
---
[NOT] BETWEEN...AND...
mysql> SELECT 123 BETWEEN 2 AND 123123;
+--------------------------+
| 123 BETWEEN 2 AND 123123 |
+--------------------------+
| 1 |
+--------------------------+
---
[NOT] IN()，给定几个区间来做判断
mysql> SELECT 123 IN(123,23,12);
+-------------------+
| 123 IN(123,23,12) |
+-------------------+
| 1 |
+-------------------+
1 row in set (0.00 sec)
mysql> SELECT 123 IN(120,23,12);
+-------------------+
| 123 IN(120,23,12) |
+-------------------+
| 0 |
+-------------------+
---
IS [NOT] NULL,判断是空么
mysql> SELECT 0 IS NULL;
+-----------+
| 0 IS NULL |
+-----------+
| 0 |
+-----------+
常用在检查是否为空
mysql> SELECT * FROM test WHERE username IS NULL;
Empty set (0.00 sec)
----日期时间函数
---
NOW()，返回当时的日期和时间
CURDATE(),CURTIME()
---
DATE_ADD()，当前日期的变化
mysql> SELECT DATE_ADD('2012-2-23', INTERVAL 234 DAY);
+-----------------------------------------+
| DATE_ADD('2012-2-23', INTERVAL 234 DAY) |
+-----------------------------------------+
| 2012-10-14 |
+-----------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT DATE_ADD('2012-2-23', INTERVAL -234 DAY);
+------------------------------------------+
| DATE_ADD('2012-2-23', INTERVAL -234 DAY) |
+------------------------------------------+
| 2011-07-04 |
+------------------------------------------+
1 row in set (0.00 sec)
---
DATEDIFF()，得到两个日期间的差值
mysql> SELECT DATEDIFF('2304-2-2','1234-3-22');
+----------------------------------+
| DATEDIFF('2304-2-2','1234-3-22') |
+----------------------------------+
| 390760 |
+----------------------------------+
1 row in set (0.02 sec)
---
DATE_FORMAT()，日期的格式化，日期的格式是可以选的，也就是说日期的格式转换
mysql> SELECT DATE_FORMAT('2013-2-22','%m/%d/%y');
+-------------------------------------+
| DATE_FORMAT('2013-2-22','%m/%d/%y') |
+-------------------------------------+
| 02/22/13 |
+-------------------------------------+
1 row in set (0.00 sec)
----信息函数
USER(),VERSION(),DATEBASE(),CONNECTION_ID(),LAST_INSERT_ID()
----聚合函数
只有一个返回值是他们的特点
AVG(),COUNT(),MAX(),MIN(),SUM()
直接调用可能会有错误，一般是针对数据表的字段进行的操作
mysql> SELECT AVG(id) FROM test;
+---------+
| AVG(id) |
+---------+
| 1.5000 |
+---------+
----加密函数
MD5(),PASSWORD()
mysql> SELECT MD5('HELLOWORLD');
+----------------------------------+
| MD5('HELLOWORLD') |
+----------------------------------+
| e81e26d88d62aba9ab55b632f25f117d |
+----------------------------------+
1 row in set (0.00 sec)
mysql> SELECT PASSWORD('HELLOWORLD');
+-------------------------------------------+
| PASSWORD('HELLOWORLD') |
+-------------------------------------------+
| *3456E7782A7F539BC823C715DB60231B0C7DE847 |
+-------------------------------------------+
1 row in set (0.00 sec)
一般而言，网页的编程用的都是MD5，password仅仅用于修改当前用户的密码
----
注重自带函数的熟悉、了解，灵活的调用和嵌套运用
