MySQL 寻常str函数

　　MySQL常见的字符串函数
 
　　整理自官档。
 
　　1.1     SUBSTR or SUBSTRING
　　SUBSTR(str,pos), SUBSTR(str FROM pos),SUBSTR(str,pos,len), SUBSTR(str FROM
 
　　pos FOR len)
 
　　SUBSTR() is a synonym for SUBSTRING().
 
　　SUBSTRING(str FROM pos FOR len)
 
　　The forms without a len argument return asubstring from string str starting at position pos.
 
　　The forms with a len argument return asubstring len characters long from string str, starting at
 
　　position pos. The forms that use FROM arestandard SQL syntax. It is also possible to use a negative
 
　　value for pos. In this case, the beginningof the substring is pos characters from the end of the
 
　　string, rather than the beginning. Anegative value may be used for pos in any of the forms of this
 
　　function.
 
　　For all forms of SUBSTRING(), the positionof the first character in the string from which the
 
　　substring is to be extracted is reckoned as1.
 
　　mysql> SELECTSUBSTRING('Quadratically',5);
 
　　-> 'ratically'
 
　　mysql> SELECT SUBSTRING('foobarbar' FROM4);
 
　　-> 'barbar'
 
　　mysql> SELECTSUBSTRING('Quadratically',5,6);
 
　　-> 'ratica'
 
　　mysql> SELECT SUBSTRING('Sakila', -3);
 
　　-> 'ila'
 
　　mysql> SELECT SUBSTRING('Sakila', -5,3);
 
　　-> 'aki'
 
　　mysql> SELECT SUBSTRING('Sakila' FROM -4FOR 2);
 
　　-> 'ki'
 
　　This function is multibyte safe.
 
　　If len is less than 1,the result is the empty string.
 
　　1.2    LENGTH
　　LENGTH(str) 返回字节数
 
　　Returns the length of the string str,measured in bytes. A multibyte character counts as multiple
 
　　bytes. This means that for a stringcontaining five 2-byte characters, LENGTH() returns 10, whereas
 
　　CHAR_LENGTH() returns 5.
 
　　mysql> SELECT LENGTH('text');