mysql> #字符串类型
mysql> #char varchar text blob
mysql> #最容易用到char和varchar的区别
mysql>
mysql> #char 定长字符串，
mysql> #varchar 变长类型
mysql> #如姓名 char(6)
mysql> #1 张三
mysql> #2 李四
mysql> #3 王五
mysql> #4 李连杰
mysql> #5 阿拉木汗
mysql> #
mysql> #在查找行记录时，都是定长，可以通过行数与行的长度计算出文件指针的偏移量
mysql> #定长，不论够不够指定的长度，实际都占据
mysql> #如果不够指定长度，用空格在末尾补齐到指定长度
mysql> #
mysql> #对于varchar变长，不用空格补齐，在列内容前有一到两个字节，标识列的长度
mysql> #char 宽度M 可存M 实际I 实占空间M 利用率100%
mysql> #varchar宽度M 可存M 实际I 实占I+(1-2)字节 利用率小于100%
mysql> create table test(
-> ca char(6) not null default '',
-> vca varchar(6) not null default ''
-> )engine myisam charset utf8;
Query OK, 0 rows affected (0.01 sec)
mysql> desc
-> test;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| ca | char(6) | NO | | | |
| vca | varchar(6) | NO | | | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> select * from test;
Empty set (0.00 sec)
mysql> insert into test values('hello','hell0');
Query OK, 1 row affected (0.00 sec)
mysql> select * from test;
+-------+-------+
| ca | vca |
+-------+-------+
| hello | hell0 |
+-------+-------+
1 row in set (0.00 sec)
mysql> insert into test values('aa ','aa ');
Query OK, 1 row affected (0.00 sec)
mysql> select * from test;
+-------+-------+
| ca | vca |
+-------+-------+
| hello | hell0 |
| aa | aa |
+-------+-------+
2 rows in set (0.00 sec)
mysql> #concat 函数 连接字符串
mysql> select concat(ca,'!'),concat(vca,'!') from test;
+----------------+-----------------+
| concat(ca,'!') | concat(vca,'!') |
+----------------+-----------------+
| hello! | hell0! |
| aa! | aa ! |
+----------------+-----------------+
2 rows in set (0.03 sec)
mysql> #注意：char类型：存储时如果不够指定长度，内部用空格补齐
mysql> #在取出时，再把右侧的空格删掉；
mysql> #这就意味着，如果右侧有空格，将会丢失；
mysql> #区别：
mysql> #长度：
mysql> #取出长度：
mysql> #速度上：定长速度快；
mysql> #char长度0-255 varchar长度0-65535；
mysql> #
mysql> #char(M)和varchar(M)限制的是字符，不是字节
mysql> #char(2) charset utf8,能存2 个utf8字符，比如中国
mysql>
mysql> #cahr(M) varchar(M)都是限定的字符数，不是字节数
mysql> #一个utf8编码字符，由3个字节
mysql> insert into test values('中国人','华夏民族源头');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select * from test;
+--------+--------+
| ca | vca |
+--------+--------+
| hello | hell0 |
| aa | aa |
| 中国人 | 华夏民 |
+--------+--------+
3 rows in set (0.00 sec)
mysql> insert into test values('hello world','pretty woman');
Query OK, 1 row affected, 2 warnings (0.00 sec)
mysql> select * from test;
+--------+--------+
| ca | vca |
+--------+--------+
| hello | hell0 |
| aa | aa |
| 中国人 | 华夏民 |
| hello | pretty |
+--------+--------+
4 rows in set (0.00 sec)
mysql> set names utf8;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into test values('中国人','华夏民族源头');
Query OK, 1 row affected, 2 warnings (0.00 sec)
mysql> select * from test;
+--------------+--------------+
| ca | vca |
+--------------+--------------+
| hello | hell0 |
| aa | aa |
| 脰脨鹿煤脠脣 | 禄陋脧脛脙帽 |
| hello | pretty |
| | |
+--------------+--------------+
5 rows in set (0.00 sec)
mysql> set names gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+---------+--------+
| ca | vca |
+---------+--------+
| hello | hell0 |
| aa | aa |
| ???ú?? | ?????? |
| hello | pretty |
| | |
+---------+--------+
5 rows in set (0.02 sec)
mysql> set names utf8;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+--------------+--------------+
| ca | vca |
+--------------+--------------+
| hello | hell0 |
| aa | aa |
| 脰脨鹿煤脠脣 | 禄陋脧脛脙帽 |
| hello | pretty |
| | |
+--------------+--------------+
5 rows in set (0.00 sec)
mysql> #text 长度 65536存储文章
mysql> #注意：text没必要加默认值
mysql> #test 文本类型，一般用来存储文章内容，新闻内容
mysql> #声明text列，不必默认值
mysql> create table test2(
-> artitle text not null default ''
-> );
Query OK, 0 rows affected, 1 warning (0.08 sec)
mysql>
mysql> desc test2;
+---------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+------+------+-----+---------+-------+
| artitle | text | NO | | NULL | |
+---------+------+------+-----+---------+-------+
1 row in set (0.01 sec)
mysql> alter table test2 add img blob;
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc test2;
+---------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+------+------+-----+---------+-------+
| artitle | text | NO | | NULL | |
| img | blob | YES | | NULL | |
+---------+------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> #blob 是二进制类型，用来存储图像音频等二进制信息；
mysql> insert into test2 values('qingqing','zhangfei ');
Query OK, 1 row affected (0.00 sec)
mysql> select * from test2;
+----------+-----------+
| artitle | img |
+----------+-----------+
| qingqing | zhangfei |
+----------+-----------+
1 row in set (0.00 sec)
mysql> #意义在于：二进制从0-255每个字节都有可能出现，
mysql> #blob类型在于防止因为字符集问题，导致信息丢失
mysql> #如一张图片里有0xFF字节，这个在ascii字符集中认为非法，被过滤
mysql> #下次取的时候，图片因此被损坏；
mysql> #exit;
mysql> exit;