SQL内置函数

    掌握以下函数的用法，并在命令行进行实践操作：concat()、group_concat()、concat_ws()、load_file()、hex()、char()、count()、substring()、into+outfile、LOAD DATA INFIL、limit。

• concat(str1,str2…)

  • 将多个字符串连接起来，中间并无分隔符

  • such:

    mysql>  select concat('hello','jerrybird');
    +-----------------------------+
    | concat('hello','jerrybird') |
    +-----------------------------+
    | hellojerrybird              |
    +-----------------------------+
    1 row in set (0.00 sec)
    

• group_concat()

  • 将按组分开的数据，以组的方式显示出来，以逗号分隔
  • such：
    

    mysql> select *from teacher;
    +----+------+--------+----------+
    | id | name | sex    | addr     |
    +----+------+--------+----------+
    |  1 | Leo  | Male   | hangzhou |
    |  2 | Able | Male   | hangzhou |
    |  3 | John | Male   | beijing  |
    |  4 | Mary | Female | hangzhou |
    |  5 | Bird | Male   | huainan  |
    +----+------+--------+----------+
    5 rows in set (0.00 sec)
    
    mysql> select sex,group_concat(name) from teacher group by sex;
    +--------+--------------------+
    | sex    | group_concat(name) |
    +--------+--------------------+
    | Female | Mary               |
    | Male   | Leo,Able,John,Bird |
    +--------+--------------------+
    2 rows in set (0.00 sec)

• concat_ws(separator,str1,str2…)

  • 将多个字符串以指定的分隔符连接起来
  • such：
    

    mysql> select concat_ws('_','author','jerrybird','age','20');
    +------------------------------------------------+
    | concat_ws('_','author','jerrybird','age','20') |
    +------------------------------------------------+
    | author_jerrybird_age_20                        |
    +------------------------------------------------+
    1 row in set (0.00 sec)    

• load_file(path)

  • 将某个路径下的文件读取并显示出来
  • 若文件不存在会显示 null
  • 若使用phpstudy 5.3…而未进行设置则会提示错误。因而需将mysql的配置文件修改下.打开my.ini 添加如下一行：secure_file_priv=”e:/code/MySQL/”
  • 之后load data infile or into outfile 等只能对此文件夹下的文件有权限操作
    

    mysql> select load_file('e:/code/MySQL/test.txt');
    +-------------------------------------+
    | load_file('e:/code/MySQL/test.txt') |
    +-------------------------------------+
    | hello jerrybird,how are you?        |
    +-------------------------------------+
    1 row in set (0.00 sec)

• hex(number)
  -将number 转换为16进制整数

  mysql> select hex(100);
  +----------+
  | hex(100) |
  +----------+
  | 64       |
  +----------+
  1 row in set (0.00 sec)

• char(number)

  • 将number根据ascii码转为相应的字符
  • 0

    mysql> select char(65);
    +----------+
    | char(65) |
    +----------+
    | A        |
    +----------+
    1 row in set (0.00 sec)

• count(column1…)

  • 这是一个聚合函数，用来计算符合指定条件的记录的行数
  • such：
    

    mysql> select count(*) from teacher;
    +----------+
    | count(*) |
    +----------+
    |        5 |
    +----------+
    1 row in set (0.00 sec)
    
    mysql> select count(*) from teacher where sex='male';
    +----------+
    | count(*) |
    +----------+
    |        4 |
    +----------+
    1 row in set (0.00 sec)

• substring(str,pos[,len]) or substring(str from pos [for len])
  
  • len 为可选的参数
  • 从str中选取 从pos位置开始长度为len的字串
  • pos可以为负数，代表从右往左第几位
  • str的位置从1开始
    
mysql> SELECT SUBSTRING('Quadratically',5)
-> 'ratically'
mysql> SELECT SUBSTRING('foobarbar' FROM 4)
-> 'barbar'
mysql> SELECT SUBSTRING('Quadratically',5,6)
-> 'ratica'
mysql> SELECT SUBSTRING('Sakila', -3)
-> 'ila'
mysql> SELECT SUBSTRING('Sakila', -5, 3)
-> 'aki'
mysql> SELECT SUBSTRING('Sakila' FROM -4 FOR 2)
-> 'ki'

• select colomn into outfile ‘path/file’ from table-name where (…);
  
  • 将数据表中符合条件的数据备份到path路径下的file中
  • file为新建的
  • path为之前设置的secure_file_priv
    
mysql> select * into outfile 'e:/code/MySQL/back_up.sql' from teacher where sex=
'male';
Query OK, 4 rows affected (0.00 sec)

• load data infile
  
  • load data infile ‘path/file’ into table tb_name;
  • 将file中的数据导入到指定的table中
  • path为secure_file_priv
  • 
    
mysql> load data infile 'e:/code/MySQL/back_up.sql' into table teacher;
Query OK, 4 rows affected (0.00 sec)
Records: 4 Deleted: 0 Skipped: 0 Warnings: 0


• limit

  • select (colomn…) from table table_name limit number;
  • 从指定table中将前number个记录的colomn…显示出来
    

    mysql> select *from teacher limit 3;
    +----+------+------+----------+
    | id | name | sex  | addr     |
    +----+------+------+----------+
    |  1 | Leo  | Male | hangzhou |
    |  2 | Able | Male | hangzhou |
    |  3 | John | Male | beijing  |
    +----+------+------+----------+
    3 rows in set (0.00 sec)

• 清空表中的记录

  • delete from table_name;

    mysql> delete from teacher;
    Query OK, 4 rows affected (0.00 sec)

  • truncate table table_name;

    mysql> truncate table teacher;
    Query OK, 0 rows affected (0.00 sec)