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mysql> select concat('a','b');
concat('a','b')
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| 'a'||'b' |  
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 1 row in set, 2 warnings (0.00 sec)  INSTR (string ,substring ) //·µ»ØsubstringÊ×´ÎÔÚstringÖгöÏÖµÄλÖÃ,²»´æÔÚ·µ»Ø0
 
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RPAD (string2 ,length ,pad) //ÔÚstrºóÓÃpad²¹³ä,Ö±µ½³¤¶Èlength
 
 
RTRIM (string2 ) //È¥³ýºó¶Ë¿Õ¸ñ
 
 
STRCMP (string1 ,string2 ) //Öð×Ö·û±È½ÏÁ½×Ö´®´óС,
 
 
SUBSTRING (str , position [,length ])
 
 
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mysql> select substring('abcd',0,2);
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