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[数据库]贤者时间太久了么?


hi

给自己放了大概三天的假,没有一点点防备,没有一点点准备,无意的

是不是贤者时间过不去了我不知道啊。。。继续看东西吧

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

 

 

今天烦躁,不想看。。。回去看电影了