首先我們需要了解字符串截取函數：
left(), right(), substring(), substring_index()。還有 mid(), substr()。其中，mid(), substr() 等價于 substring() 函數，substring() 的功能非常強大和靈活。??
具體實例如下：
1.?字符串截取：left(str,?length)?? mysql>?select?left('sqlstudy.com',?3);?? +-------------------------+?? |?left('sqlstudy.com',?3)?|?? +-------------------------+?? |?sql?????????????????????|?? +-------------------------+?? 2.?字符串截取：right(str,?length)?? mysql>?select?right('sqlstudy.com',?3);?? +--------------------------+?? |?right('sqlstudy.com',?3)?|?? +--------------------------+?? |?com??????????????????????|?? +--------------------------+?? 3.?字符串截取：substring(str,?pos);?substring(str,?pos,?len)?? 3.1?從字符串的第?4?個字符位置開始取，直到結束。?? mysql>?select?substring('sqlstudy.com',?4);?? +------------------------------+?? |?substring('sqlstudy.com',?4)?|?? +------------------------------+?? |?study.com????????????????????|?? +------------------------------+?? 3.2?從字符串的第?4?個字符位置開始取，只取?2?個字符。?? mysql>?select?substring('sqlstudy.com',?4,?2);?? +---------------------------------+?? |?substring('sqlstudy.com',?4,?2)?|?? +---------------------------------+?? |?st??????????????????????????????|?? +---------------------------------+?? 3.3?從字符串的第?4?個字符位置（倒數）開始取，直到結束。?? mysql>?select?substring('sqlstudy.com',?-4);?? +-------------------------------+?? |?substring('sqlstudy.com',?-4)?|?? +-------------------------------+?? |?.com??????????????????????????|?? +-------------------------------+?? 3.4?從字符串的第?4?個字符位置（倒數）開始取，只取?2?個字符。?? mysql>?select?substring('sqlstudy.com',?-4,?2);?? +----------------------------------+?? |?substring('sqlstudy.com',?-4,?2)?|?? +----------------------------------+?? |?.c???????????????????????????????|?? +----------------------------------+?? 我們注意到在函數?substring(str,pos,?len)中，?pos?可以是負值，但?len?不能取負值。?? 4.?字符串截取：substring_index(str,delim,count)?? 4.1?截取第二個?'.'?之前的所有字符。?? mysql>?select?substring_index('www.sqlstudy.com.cn',?'.',?2);?? +------------------------------------------------+?? |?substring_index('www.sqlstudy.com.cn',?'.',?2)?|?? +------------------------------------------------+?? |?www.sqlstudy???????????????????????????????????|?? +------------------------------------------------+?? 4.2?截取第二個?'.'?（倒數）之后的所有字符。?? mysql>?select?substring_index('www.sqlstudy.com.cn',?'.',?-2);?? +-------------------------------------------------+?? |?substring_index('www.sqlstudy.com.cn',?'.',?-2)?|?? +-------------------------------------------------+?? |?com.cn??????????????????????????????????????????|?? +-------------------------------------------------+?? 4.3?如果在字符串中找不到?delim?參數指定的值，就返回整個字符串?? mysql>?select?substring_index('www.sqlstudy.com.cn',?'.coc',?1);?? +---------------------------------------------------+?? |?substring_index('www.sqlstudy.com.cn',?'.coc',?1)?|?? +---------------------------------------------------+?? |?www.sqlstudy.com.cn???????????????????????????????|?? +---------------------------------------------------+?? ?4.4?截取一個表某個字段數據的中間值?如該字段數據為??1,2,3?? mysql>?select?substring_index(substring_index(該字段,?',',?2)?,?',',?-1)?from?表名;???? +--------------------------------------------------------------+???? |?substring_index(substring_index(該字段,?',',?2);??,?',',?-1)|???? +--------------------------------------------------------------+???? |?2????????????????????????????????????????|???? +--------------------------------------------------------------+
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