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Submitted by gouki on 2008, August 14, 11:55 PM
mysql数据库也像ORACLE数据库一样,可以动态的修改参数,可以修改会话级变量只对当前会话产生影响;也可以修改全局变量,对所有新连接的会话都产生影响。
修改会话级变量
用show variables 命令查看当前参数的值,like 'pattern'用于模式匹配,查找指定的参数
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 6291448 |
+---------------------------+------------+
1 rows in set (0.00 sec)
用set SESSION命令设置会话级变量的新值
mysql> set SESSION sort_buffer_size=7000000;
Query OK, 0 rows affected (0.00 sec)
--修改会话级变量对当前会话来说立刻生效
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 7000000 |
+---------------------------+------------+
1 rows in set (0.00 sec)
mysql> exit
Bye
退出重新连接后,此参数恢复原值
[root@devdbc_stb root]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 40 to server version: 5.0.37-log
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 6291448 |
+---------------------------+------------+
1 rows in set (0.00 sec)
修改全局变量
[root@devdbc_stb root]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 40 to server version: 5.0.37-log
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 6291448 |
+---------------------------+------------+
1 rows in set (0.00 sec)
用set GLOBAL 命令设置全局变量
mysql> set GLOBAL sort_buffer_size = 7000000;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 6291448 |
+---------------------------+------------+
1 rows in set (0.00 sec)
当前此参数的值并不发生变化,先退出,然后重新连进去
mysql> exit
Bye
[root@devdbc_stb root]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 41 to server version: 5.0.37-log
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql> show variables like '%sort_buffer_size%';
+---------------------------+------------+
| Variable_name | Value |
+---------------------------+------------+
| sort_buffer_size | 7000000 |
+---------------------------+------------+
1 rows in set (0.00 sec)
新的参数值生效
Tags: mysql, 优化, database, 修改
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Submitted by gouki on 2008, August 14, 11:54 PM
mysql的大小写敏感性主要分为两部份:一个是表名区别大小写;二是字段值不区分大小写.这个跟我们希望的恰好相反,而这却是mysql 的默认设置,不得不令人费解,而这些却是oracle数据库比较基本的东西。
首先说说MYSQL建表的特点?如果你创建一张test表,然后你到data目录下会发现以下三个文件:
test.frm
test.MYD
test.MYI
即创建一个表,它会自动生成三个文件。这点跟oracle也有很大的不同,如果这样,那如何使用裸设备呢?看来要用mysql数据库,是不得不要用文件系统。而通常操作系统都对用户同时打开的文件数有限制,一般为1024个,使用mysql要注意.
那如何设置让mysql的表名不区分大小写呢?修改/etc/my.cnf文件,在[mysqld] 下增加参数lower_case_table_names = 1
这个参数的含义是使所有的表名都转化成小写来处理,如果你在原系统中已有大写的表名,要先把它们重命名为小写,以免加了此参数后,以前大写的表无法识别.
另外一个问题,就是默认的字段值不区分大小写?这点是比较令人头痛的事。如果你在一有唯一约束的列上插入两行值'A'和'a',Mysql会认为它是相同的,而在oracle中就不会。请看下面的测试:
mysql> create table test4(nick varchar(20) primary key);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into test4 values('A');
Query OK, 1 row affected (0.00 sec)
mysql> insert into test4 values('a');
ERROR 1062: Duplicate entry 'a' for key 1
而如何设置让其列值区分大小写呢?
mysql> create table test4(nick varchar(20) binary primary key);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into test4 values('A');
Query OK, 1 row affected (0.00 sec)
mysql> insert into test4 values('a');
Query OK, 1 row affected (0.00 sec)
在声明字符类型后,在后面加一个binary,mysql就可以区分大小写了。
Tags: mysql, database, 大小写
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Submitted by gouki on 2008, August 14, 11:52 PM
1、concat()函数
1.1 MySQL的concat函数可以连接一个或者多个字符串,如
mysql> select concat('10');
+--------------+
| concat('10') |
+--------------+
| 10 |
+--------------+
1 row in set (0.00 sec)
mysql> select concat('11','22','33');
+------------------------+
| concat('11','22','33') |
+------------------------+
| 112233 |
+------------------------+
1 row in set (0.00 sec)
而Oracle的concat函数只能连接两个字符串
SQL> select concat('11','22') from dual;
1.2 MySQL的concat函数在连接字符串的时候,只要其中一个是NULL,那么将返回NULL
mysql> select concat('11','22',null);
+------------------------+
| concat('11','22',null) |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)
而Oracle的concat函数连接的时候,只要有一个字符串不是NULL,就不会返回NULL
SQL> select concat('11',NULL) from dual;
CONCAT
--
11
2、concat_ws()函数, 表示concat with separator,即有分隔符的字符串连接
如连接后以逗号分隔
mysql> select concat_ws(',','11','22','33');
+-------------------------------+
| concat_ws(',','11','22','33') |
+-------------------------------+
| 11,22,33 |
+-------------------------------+
1 row in set (0.00 sec)
和concat不同的是, concat_ws函数在执行的时候,不会因为NULL值而返回NULL
mysql> select concat_ws(',','11','22',NULL);
+-------------------------------+
| concat_ws(',','11','22',NULL) |
+-------------------------------+
| 11,22 |
+-------------------------------+
1 row in set (0.00 sec)
3、group_concat()可用来行转列, Oracle没有这样的函数
完整的语法如下
group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])
如下例子
mysql> select * from aa;
+------+------+
| id | name |
+------+------+
| 1 | 10 |
| 1 | 20 |
| 1 | 20 |
| 2 | 20 |
| 3 | 200 |
| 3 | 500 |
+------+------+
6 rows in set (0.00 sec)
3.1 以id分组,把name字段的值打印在一行,逗号分隔(默认)
mysql> select id,group_concat(name) from aa group by id;
+------+--------------------+
| id | group_concat(name) |
+------+--------------------+
| 1 | 10,20,20 |
| 2 | 20 |
| 3 | 200,500 |
+------+--------------------+
3 rows in set (0.00 sec)
3.2 以id分组,把name字段的值打印在一行,分号分隔
mysql> select id,group_concat(name separator ';') from aa group by id;
+------+----------------------------------+
| id | group_concat(name separator ';') |
+------+----------------------------------+
| 1 | 10;20;20 |
| 2 | 20 |
| 3 | 200;500 |
+------+----------------------------------+
3 rows in set (0.00 sec)
3.3 以id分组,把去冗余的name字段的值打印在一行,逗号分隔
mysql> select id,group_concat(distinct name) from aa group by id;
+------+-----------------------------+
| id | group_concat(distinct name) |
+------+-----------------------------+
| 1 | 10,20 |
| 2 | 20 |
| 3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)
3.4 以id分组,把name字段的值打印在一行,逗号分隔,以name排倒序
mysql> select id,group_concat(name order by name desc) from aa group by id;
+------+---------------------------------------+
| id | group_concat(name order by name desc) |
+------+---------------------------------------+
| 1 | 20,20,10 |
| 2 | 20 |
| 3 | 500,200 |
+------+---------------------------------------+
3 rows in set (0.00 sec)
4、repeat()函数,用来复制字符串,如下'ab'表示要复制的字符串,2表示复制的份数
mysql> select repeat('ab',2);
+----------------+
| repeat('ab',2) |
+----------------+
| abab |
+----------------+
1 row in set (0.00 sec)
又如
mysql> select repeat('a',2);
+---------------+
| repeat('a',2) |
+---------------+
| aa |
+---------------+
1 row in set (0.00 sec)
Tags: mysql, database, 字符串
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