Dica rápida: Como remover entidades HTML no MySQL?

Trabalhando com uma migração me deparei com o problema de ter de substituir entidades HTML por seus respectivos caracteres. Melhor ainda se fossem removidos acentos e caracteres especiais, pois esse valor é usado para gerar links permanentes.

Após pesquisar um pouco, caí neste tópico do fórum do MySQL. Nele tem uma função pronta para substituir as entidades HTML por seus respectivos caracteres Unicode. Vou reproduzi-la aqui:

DELIMITER $$ 

DROP FUNCTION IF EXISTS `html_unencode`$$ 
CREATE DEFINER=`root`@`localhost` FUNCTION `html_unencode`(x VARCHAR(255)) RETURNS varchar(255) CHARSET latin1 
BEGIN 

DECLARE TextString VARCHAR(255) ; 
SET TextString = x ; 

#quotation mark 
IF INSTR( x , '"' ) 
THEN SET TextString = REPLACE(TextString, '"','"') ; 
END IF ; 
#apostrophe  
IF INSTR( x , ''' ) 
THEN SET TextString = REPLACE(TextString, ''','"') ; 
END IF ; 
#ampersand 
IF INSTR( x , '&' ) 
THEN SET TextString = REPLACE(TextString, '&','&') ; 
END IF ; 
#less-than 
IF INSTR( x , '<' ) 
THEN SET TextString = REPLACE(TextString, '&lt;','<') ; 
END IF ; 
#greater-than 
IF INSTR( x , '&gt;' ) 
THEN SET TextString = REPLACE(TextString, '&gt;','>') ; 
END IF ; 
#non-breaking space 
IF INSTR( x , '&nbsp;' ) 
THEN SET TextString = REPLACE(TextString, '&nbsp;',' ') ; 
END IF ; 
#inverted exclamation mark 
IF INSTR( x , '&iexcl;' ) 
THEN SET TextString = REPLACE(TextString, '&iexcl;','¡') ; 
END IF ; 
#cent 
IF INSTR( x , '&cent;' ) 
THEN SET TextString = REPLACE(TextString, '&cent;','¢') ; 
END IF ; 
#pound 
IF INSTR( x , '&pound;' ) 
THEN SET TextString = REPLACE(TextString, '&pound;','£') ; 
END IF ; 
#currency 
IF INSTR( x , '&curren;' ) 
THEN SET TextString = REPLACE(TextString, '&curren;','¤') ; 
END IF ; 
#yen 
IF INSTR( x , '&yen;' ) 
THEN SET TextString = REPLACE(TextString, '&yen;','¥') ; 
END IF ; 
#broken vertical bar 
IF INSTR( x , '&brvbar;' ) 
THEN SET TextString = REPLACE(TextString, '&brvbar;','¦') ; 
END IF ; 
#section 
IF INSTR( x , '&sect;' ) 
THEN SET TextString = REPLACE(TextString, '&sect;','§') ; 
END IF ; 
#spacing diaeresis 
IF INSTR( x , '&uml;' ) 
THEN SET TextString = REPLACE(TextString, '&uml;','¨') ; 
END IF ; 
#copyright 
IF INSTR( x , '&copy;' ) 
THEN SET TextString = REPLACE(TextString, '&copy;','©') ; 
END IF ; 
#feminine ordinal indicator 
IF INSTR( x , '&ordf;' ) 
THEN SET TextString = REPLACE(TextString, '&ordf;','ª') ; 
END IF ; 

#angle quotation mark (left) 
IF INSTR( x , '&laquo;' ) 
THEN SET TextString = REPLACE(TextString, '&laquo;','«') ; 
END IF ; 

#negation 
IF INSTR( x , '&not;' ) 
THEN SET TextString = REPLACE(TextString, '&not;','¬') ; 
END IF ; 
#soft hyphen 
IF INSTR( x , '&shy;' ) 
THEN SET TextString = REPLACE(TextString, '&shy;','­') ; 
END IF ; 
#registered trademark 
IF INSTR( x , '&reg;' ) 
THEN SET TextString = REPLACE(TextString, '&reg;','®') ; 
END IF ; 
#spacing macron 
IF INSTR( x , '&macr;' ) 
THEN SET TextString = REPLACE(TextString, '&macr;','¯') ; 
END IF ; 
#degree 
IF INSTR( x , '&deg;' ) 
THEN SET TextString = REPLACE(TextString, '&deg;','°') ; 
END IF ; 
#plus-or-minus  
IF INSTR( x , '&plusmn;' ) 
THEN SET TextString = REPLACE(TextString, '&plusmn;','±') ; 
END IF ; 
#superscript 2 
IF INSTR( x , '&sup2;' ) 
THEN SET TextString = REPLACE(TextString, '&sup2;','²') ; 
END IF ; 
#superscript 3 
IF INSTR( x , '&sup3;' ) 
THEN SET TextString = REPLACE(TextString, '&sup3;','³') ; 
END IF ; 
#spacing acute 
IF INSTR( x , '&acute;' ) 
THEN SET TextString = REPLACE(TextString, '&acute;','´') ; 
END IF ; 
#micro 
IF INSTR( x , '&micro;' ) 
THEN SET TextString = REPLACE(TextString, '&micro;','µ') ; 
END IF ; 
#paragraph 
IF INSTR( x , '&para;' ) 
THEN SET TextString = REPLACE(TextString, '&para;','¶') ; 
END IF ; 
#middle dot 
IF INSTR( x , '&middot;' ) 
THEN SET TextString = REPLACE(TextString, '&middot;','·') ; 
END IF ; 
#spacing cedilla 
IF INSTR( x , '&cedil;' ) 
THEN SET TextString = REPLACE(TextString, '&cedil;','¸') ; 
END IF ; 
#superscript 1 
IF INSTR( x , '&sup1;' ) 
THEN SET TextString = REPLACE(TextString, '&sup1;','¹') ; 
END IF ; 
#masculine ordinal indicator 
IF INSTR( x , '&ordm;' ) 
THEN SET TextString = REPLACE(TextString, '&ordm;','º') ; 
END IF ; 
#angle quotation mark (right) 
IF INSTR( x , '&raquo;' ) 
THEN SET TextString = REPLACE(TextString, '&raquo;','»') ; 
END IF ; 
#fraction 1/4 
IF INSTR( x , '&frac14;' ) 
THEN SET TextString = REPLACE(TextString, '&frac14;','¼') ; 
END IF ; 
#fraction 1/2 
IF INSTR( x , '&frac12;' ) 
THEN SET TextString = REPLACE(TextString, '&frac12;','½') ; 
END IF ; 
#fraction 3/4 
IF INSTR( x , '&frac34;' ) 
THEN SET TextString = REPLACE(TextString, '&frac34;','¾') ; 
END IF ; 
#inverted question mark 
IF INSTR( x , '&iquest;' ) 
THEN SET TextString = REPLACE(TextString, '&iquest;','¿') ; 
END IF ; 
#multiplication 
IF INSTR( x , '&times;' ) 
THEN SET TextString = REPLACE(TextString, '&times;','×') ; 
END IF ; 
#division 
IF INSTR( x , '&divide;' ) 
THEN SET TextString = REPLACE(TextString, '&divide;','÷') ; 
END IF ; 
#capital a, grave accent 
IF INSTR( x , '&Agrave;' ) 
THEN SET TextString = REPLACE(TextString, '&Agrave;','À') ; 
END IF ; 
#capital a, acute accent 
IF INSTR( x , '&Aacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Aacute;','Á') ; 
END IF ; 
#capital a, circumflex accent 
IF INSTR( x , '&Acirc;' ) 
THEN SET TextString = REPLACE(TextString, '&Acirc;','Â') ; 
END IF ; 
#capital a, tilde 
IF INSTR( x , '&Atilde;' ) 
THEN SET TextString = REPLACE(TextString, '&Atilde;','Ã') ; 
END IF ; 
#capital a, umlaut mark 
IF INSTR( x , '&Auml;' ) 
THEN SET TextString = REPLACE(TextString, '&Auml;','Ä') ; 
END IF ; 
#capital a, ring 
IF INSTR( x , '&Aring;' ) 
THEN SET TextString = REPLACE(TextString, '&Aring;','Å') ; 
END IF ; 
#capital ae 
IF INSTR( x , '&AElig;' ) 
THEN SET TextString = REPLACE(TextString, '&AElig;','Æ') ; 
END IF ; 
#capital c, cedilla 
IF INSTR( x , '&Ccedil;' ) 
THEN SET TextString = REPLACE(TextString, '&Ccedil;','Ç') ; 
END IF ; 
#capital e, grave accent 
IF INSTR( x , '&Egrave;' ) 
THEN SET TextString = REPLACE(TextString, '&Egrave;','È') ; 
END IF ; 
#capital e, acute accent 
IF INSTR( x , '&Eacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Eacute;','É') ; 
END IF ; 
#capital e, circumflex accent 
IF INSTR( x , '&Ecirc;' ) 
THEN SET TextString = REPLACE(TextString, '&Ecirc;','Ê') ; 
END IF ; 
#capital e, umlaut mark 
IF INSTR( x , '&Euml;' ) 
THEN SET TextString = REPLACE(TextString, '&Euml;','Ë') ; 
END IF ; 
#capital i, grave accent 
IF INSTR( x , '&Igrave;' ) 
THEN SET TextString = REPLACE(TextString, '&Igrave;','Ì') ; 
END IF ; 
#capital i, acute accent 
IF INSTR( x , '&Iacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Iacute;','Í') ; 
END IF ; 
#capital i, circumflex accent 
IF INSTR( x , '&Icirc;' ) 
THEN SET TextString = REPLACE(TextString, '&Icirc;','Î') ; 
END IF ; 
#capital i, umlaut mark 
IF INSTR( x , '&Iuml;' ) 
THEN SET TextString = REPLACE(TextString, '&Iuml;','Ï') ; 
END IF ; 
#capital eth, Icelandic 
IF INSTR( x , '&ETH;' ) 
THEN SET TextString = REPLACE(TextString, '&ETH;','Ð') ; 
END IF ; 
#capital n, tilde 
IF INSTR( x , '&Ntilde;' ) 
THEN SET TextString = REPLACE(TextString, '&Ntilde;','Ñ') ; 
END IF ; 
#capital o, grave accent 
IF INSTR( x , '&Ograve;' ) 
THEN SET TextString = REPLACE(TextString, '&Ograve;','Ò') ; 
END IF ; 
#capital o, acute accent 
IF INSTR( x , '&Oacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Oacute;','Ó') ; 
END IF ; 
#capital o, circumflex accent 
IF INSTR( x , '&Ocirc;' ) 
THEN SET TextString = REPLACE(TextString, '&Ocirc;','Ô') ; 
END IF ; 
#capital o, tilde 
IF INSTR( x , '&Otilde;' ) 
THEN SET TextString = REPLACE(TextString, '&Otilde;','Õ') ; 
END IF ; 
#capital o, umlaut mark 
IF INSTR( x , '&Ouml;' ) 
THEN SET TextString = REPLACE(TextString, '&Ouml;','Ö') ; 
END IF ; 
#capital o, slash 
IF INSTR( x , '&Oslash;' ) 
THEN SET TextString = REPLACE(TextString, '&Oslash;','Ø') ; 
END IF ; 
#capital u, grave accent 
IF INSTR( x , '&Ugrave;' ) 
THEN SET TextString = REPLACE(TextString, '&Ugrave;','Ù') ; 
END IF ; 
#capital u, acute accent 
IF INSTR( x , '&Uacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Uacute;','Ú') ; 
END IF ; 
#capital u, circumflex accent 
IF INSTR( x , '&Ucirc;' ) 
THEN SET TextString = REPLACE(TextString, '&Ucirc;','Û') ; 
END IF ; 
#capital u, umlaut mark 
IF INSTR( x , '&Uuml;' ) 
THEN SET TextString = REPLACE(TextString, '&Uuml;','Ü') ; 
END IF ; 
#capital y, acute accent 
IF INSTR( x , '&Yacute;' ) 
THEN SET TextString = REPLACE(TextString, '&Yacute;','Ý') ; 
END IF ; 
#capital THORN, Icelandic 
IF INSTR( x , '&THORN;' ) 
THEN SET TextString = REPLACE(TextString, '&THORN;','Þ') ; 
END IF ; 
#small sharp s, German 
IF INSTR( x , '&szlig;' ) 
THEN SET TextString = REPLACE(TextString, '&szlig;','ß') ; 
END IF ; 
#small a, grave accent 
IF INSTR( x , '&agrave;' ) 
THEN SET TextString = REPLACE(TextString, '&agrave;','à') ; 
END IF ; 
#small a, acute accent 
IF INSTR( x , '&aacute;' ) 
THEN SET TextString = REPLACE(TextString, '&aacute;','á') ; 
END IF ; 
#small a, circumflex accent 
IF INSTR( x , '&acirc;' ) 
THEN SET TextString = REPLACE(TextString, '&acirc;','â') ; 
END IF ; 
#small a, tilde 
IF INSTR( x , '&atilde;' ) 
THEN SET TextString = REPLACE(TextString, '&atilde;','ã') ; 
END IF ; 
#small a, umlaut mark 
IF INSTR( x , '&auml;' ) 
THEN SET TextString = REPLACE(TextString, '&auml;','ä') ; 
END IF ; 
#small a, ring 
IF INSTR( x , '&aring;' ) 
THEN SET TextString = REPLACE(TextString, '&aring;','å') ; 
END IF ; 
#small ae 
IF INSTR( x , '&aelig;' ) 
THEN SET TextString = REPLACE(TextString, '&aelig;','æ') ; 
END IF ; 
#small c, cedilla 
IF INSTR( x , '&ccedil;' ) 
THEN SET TextString = REPLACE(TextString, '&ccedil;','ç') ; 
END IF ; 
#small e, grave accent 
IF INSTR( x , '&egrave;' ) 
THEN SET TextString = REPLACE(TextString, '&egrave;','è') ; 
END IF ; 
#small e, acute accent 
IF INSTR( x , '&eacute;' ) 
THEN SET TextString = REPLACE(TextString, '&eacute;','é') ; 
END IF ; 
#small e, circumflex accent 
IF INSTR( x , '&ecirc;' ) 
THEN SET TextString = REPLACE(TextString, '&ecirc;','ê') ; 
END IF ; 
#small e, umlaut mark 
IF INSTR( x , '&euml;' ) 
THEN SET TextString = REPLACE(TextString, '&euml;','ë') ; 
END IF ; 
#small i, grave accent 
IF INSTR( x , '&igrave;' ) 
THEN SET TextString = REPLACE(TextString, '&igrave;','ì') ; 
END IF ; 
#small i, acute accent 
IF INSTR( x , '&iacute;' ) 
THEN SET TextString = REPLACE(TextString, '&iacute;','í') ; 
END IF ; 
#small i, circumflex accent 
IF INSTR( x , '&icirc;' ) 
THEN SET TextString = REPLACE(TextString, '&icirc;','î') ; 
END IF ; 
#small i, umlaut mark 
IF INSTR( x , '&iuml;' ) 
THEN SET TextString = REPLACE(TextString, '&iuml;','ï') ; 
END IF ; 
#small eth, Icelandic 
IF INSTR( x , '&eth;' ) 
THEN SET TextString = REPLACE(TextString, '&eth;','ð') ; 
END IF ; 
#small n, tilde 
IF INSTR( x , '&ntilde;' ) 
THEN SET TextString = REPLACE(TextString, '&ntilde;','ñ') ; 
END IF ; 
#small o, grave accent 
IF INSTR( x , '&ograve;' ) 
THEN SET TextString = REPLACE(TextString, '&ograve;','ò') ; 
END IF ; 
#small o, acute accent 
IF INSTR( x , '&oacute;' ) 
THEN SET TextString = REPLACE(TextString, '&oacute;','ó') ; 
END IF ; 
#small o, circumflex accent 
IF INSTR( x , '&ocirc;' ) 
THEN SET TextString = REPLACE(TextString, '&ocirc;','ô') ; 
END IF ; 
#small o, tilde 
IF INSTR( x , '&otilde;' ) 
THEN SET TextString = REPLACE(TextString, '&otilde;','õ') ; 
END IF ; 

#small o, umlaut mark 
IF INSTR( x , '&ouml;' ) 
THEN SET TextString = REPLACE(TextString, '&ouml;','ö') ; 
END IF ; 
#small o, slash 
IF INSTR( x , '&oslash;' ) 
THEN SET TextString = REPLACE(TextString, '&oslash;','ø') ; 
END IF ; 
#small u, grave accent 
IF INSTR( x , '&ugrave;' ) 
THEN SET TextString = REPLACE(TextString, '&ugrave;','ù') ; 
END IF ; 
#small u, acute accent 
IF INSTR( x , '&uacute;' ) 
THEN SET TextString = REPLACE(TextString, '&uacute;','ú') ; 
END IF ; 
#small u, circumflex accent 
IF INSTR( x , '&ucirc;' ) 
THEN SET TextString = REPLACE(TextString, '&ucirc;','û') ; 
END IF ; 
#small u, umlaut mark 
IF INSTR( x , '&uuml;' ) 
THEN SET TextString = REPLACE(TextString, '&uuml;','ü') ; 
END IF ; 
#small y, acute accent 
IF INSTR( x , '&yacute;' ) 
THEN SET TextString = REPLACE(TextString, '&yacute;','ý') ; 
END IF ; 
#small thorn, Icelandic 
IF INSTR( x , '&thorn;' ) 
THEN SET TextString = REPLACE(TextString, '&thorn;','þ') ; 
END IF ; 
#small y, umlaut mark 
IF INSTR( x , '&yuml;' ) 
THEN SET TextString = REPLACE(TextString, '&yuml;','ÿ') ; 
END IF ; 
RETURN TextString ; 

END$$ 
DELIMITER ;

Não é uma função otimizada, nem tem como, pois o objetivo de bancos de dados não é processar textos e, por isso, fornecem poucas funções e métodos nativos para tal. Mas já ajudou muito, porém meu problema era me livrar das entidades, dos caracteres especiais e remover os acentos, então fiz algumas alterações nessa função e ficou assim:

DELIMITER $$ 

DROP FUNCTION IF EXISTS `html_unencode`$$ 
CREATE DEFINER=`root`@`localhost`
FUNCTION `html_unencode`(x VARCHAR(255)) RETURNS varchar(255) CHARSET latin1 
BEGIN 

DECLARE TextString VARCHAR(255) ; 
SET TextString = x ; 
#quotation mark 
IF INSTR( x , '"' ) 
THEN SET TextString = REPLACE(TextString, '"','') ; 
END IF ; 
#apostrophe  
IF INSTR( x , ''' ) 
THEN SET TextString = REPLACE(TextString, ''','') ; 
END IF ; 
#ampersand 
IF INSTR( x , '&' ) 
THEN SET TextString = REPLACE(TextString, '&','') ; 
END IF ; 
#less-than 
IF INSTR( x , '<' ) 
THEN SET TextString = REPLACE(TextString, '<','') ; 
END IF ; 
#greater-than 
IF INSTR( x , '>' ) 
THEN SET TextString = REPLACE(TextString, '>','') ; 
END IF ; 
#non-breaking space 
IF INSTR( x , ' ' ) 
THEN SET TextString = REPLACE(TextString, ' ','') ; 
END IF ; 
#inverted exclamation mark 
IF INSTR( x , '¡' ) 
THEN SET TextString = REPLACE(TextString, '¡','') ; 
END IF ; 
#cent 
IF INSTR( x , '¢' ) 
THEN SET TextString = REPLACE(TextString, '¢','') ; 
END IF ; 
#pound 
IF INSTR( x , '£' ) 
THEN SET TextString = REPLACE(TextString, '£','') ; 
END IF ; 
#currency 
IF INSTR( x , '¤' ) 
THEN SET TextString = REPLACE(TextString, '¤','') ; 
END IF ; 
#yen 
IF INSTR( x , '¥' ) 
THEN SET TextString = REPLACE(TextString, '¥','') ; 
END IF ; 
#broken vertical bar 
IF INSTR( x , '¦' ) 
THEN SET TextString = REPLACE(TextString, '¦','') ; 
END IF ; 
#section 
IF INSTR( x , '§' ) 
THEN SET TextString = REPLACE(TextString, '§','') ; 
END IF ; 
#spacing diaeresis 
IF INSTR( x , '¨' ) 
THEN SET TextString = REPLACE(TextString, '¨','') ; 
END IF ; 
#copyright 
IF INSTR( x , '©' ) 
THEN SET TextString = REPLACE(TextString, '©','') ; 
END IF ; 
#feminine ordinal indicator 
IF INSTR( x , 'ª' ) 
THEN SET TextString = REPLACE(TextString, 'ª','') ; 
END IF ; 
#angle quotation mark (left) 
IF INSTR( x , '«' ) 
THEN SET TextString = REPLACE(TextString, '«','') ; 
END IF ; 
#negation 
IF INSTR( x , '¬' ) 
THEN SET TextString = REPLACE(TextString, '¬','') ; 
END IF ; 
#soft hyphen 
IF INSTR( x , '­' ) 
THEN SET TextString = REPLACE(TextString, '­','') ; 
END IF ; 
#registered trademark 
IF INSTR( x , '®' ) 
THEN SET TextString = REPLACE(TextString, '®','') ; 
END IF ; 
#spacing macron 
IF INSTR( x , '¯' ) 
THEN SET TextString = REPLACE(TextString, '¯','') ; 
END IF ; 
#degree 
IF INSTR( x , '°' ) 
THEN SET TextString = REPLACE(TextString, '°','') ; 
END IF ; 
#plus-or-minus  
IF INSTR( x , '±' ) 
THEN SET TextString = REPLACE(TextString, '±','') ; 
END IF ; 
#superscript 2 
IF INSTR( x , '²' ) 
THEN SET TextString = REPLACE(TextString, '²','') ; 
END IF ; 
#superscript 3 
IF INSTR( x , '³' ) 
THEN SET TextString = REPLACE(TextString, '³','') ; 
END IF ; 
#spacing acute 
IF INSTR( x , '´' ) 
THEN SET TextString = REPLACE(TextString, '´','') ; 
END IF ; 
#micro 
IF INSTR( x , 'µ' ) 
THEN SET TextString = REPLACE(TextString, 'µ','') ; 
END IF ; 
#paragraph 
IF INSTR( x , '¶' ) 
THEN SET TextString = REPLACE(TextString, '¶','') ; 
END IF ; 
#middle dot 
IF INSTR( x , '·' ) 
THEN SET TextString = REPLACE(TextString, '·','') ; 
END IF ; 
#spacing cedilla 
IF INSTR( x , '¸' ) 
THEN SET TextString = REPLACE(TextString, '¸','') ; 
END IF ; 
#superscript 1 
IF INSTR( x , '¹' ) 
THEN SET TextString = REPLACE(TextString, '¹','') ; 
END IF ; 
#masculine ordinal indicator 
IF INSTR( x , 'º' ) 
THEN SET TextString = REPLACE(TextString, 'º','') ; 
END IF ; 
#angle quotation mark (right) 
IF INSTR( x , '»' ) 
THEN SET TextString = REPLACE(TextString, '»','') ; 
END IF ; 
#fraction 1/4 
IF INSTR( x , '¼' ) 
THEN SET TextString = REPLACE(TextString, '¼','') ; 
END IF ; 
#fraction 1/2 
IF INSTR( x , '½' ) 
THEN SET TextString = REPLACE(TextString, '½','') ; 
END IF ; 
#fraction 3/4 
IF INSTR( x , '¾' ) 
THEN SET TextString = REPLACE(TextString, '¾','') ; 
END IF ; 
#inverted question mark 
IF INSTR( x , '¿' ) 
THEN SET TextString = REPLACE(TextString, '¿','') ; 
END IF ; 
#multiplication 
IF INSTR( x , '×' ) 
THEN SET TextString = REPLACE(TextString, '×','') ; 
END IF ; 
#division 
IF INSTR( x , '÷' ) 
THEN SET TextString = REPLACE(TextString, '÷','') ; 
END IF ; 
#capital a, grave accent 
IF INSTR( x , 'À' ) 
THEN SET TextString = REPLACE(TextString, 'À','a') ; 
END IF ; 
#capital a, acute accent 
IF INSTR( x , 'Á' ) 
THEN SET TextString = REPLACE(TextString, 'Á','a') ; 
END IF ; 
#capital a, circumflex accent 
IF INSTR( x , 'Â' ) 
THEN SET TextString = REPLACE(TextString, 'Â','a') ; 
END IF ; 
#capital a, tilde 
IF INSTR( x , 'Ã' ) 
THEN SET TextString = REPLACE(TextString, 'Ã','a') ; 
END IF ; 
#capital a, umlaut mark 
IF INSTR( x , 'Ä' ) 
THEN SET TextString = REPLACE(TextString, 'Ä','a') ; 
END IF ; 
#capital a, ring 
IF INSTR( x , 'Å' ) 
THEN SET TextString = REPLACE(TextString, 'Å','a') ; 
END IF ; 
#capital ae 
IF INSTR( x , 'Æ' ) 
THEN SET TextString = REPLACE(TextString, 'Æ','ae') ; 
END IF ; 
#capital c, cedilla 
IF INSTR( x , 'Ç' ) 
THEN SET TextString = REPLACE(TextString, 'Ç','c') ; 
END IF ; 
#capital e, grave accent 
IF INSTR( x , 'È' ) 
THEN SET TextString = REPLACE(TextString, 'È','e') ; 
END IF ; 
#capital e, acute accent 
IF INSTR( x , 'É' ) 
THEN SET TextString = REPLACE(TextString, 'É','e') ; 
END IF ; 
#capital e, circumflex accent 
IF INSTR( x , 'Ê' ) 
THEN SET TextString = REPLACE(TextString, 'Ê','e') ; 
END IF ; 
#capital e, umlaut mark 
IF INSTR( x , 'Ë' ) 
THEN SET TextString = REPLACE(TextString, 'Ë','e') ; 
END IF ; 
#capital i, grave accent 
IF INSTR( x , 'Ì' ) 
THEN SET TextString = REPLACE(TextString, 'Ì','i') ; 
END IF ; 
#capital i, acute accent 
IF INSTR( x , 'Í' ) 
THEN SET TextString = REPLACE(TextString, 'Í','i') ; 
END IF ; 
#capital i, circumflex accent 
IF INSTR( x , 'Î' ) 
THEN SET TextString = REPLACE(TextString, 'Î','i') ; 
END IF ; 
#capital i, umlaut mark 
IF INSTR( x , 'Ï' ) 
THEN SET TextString = REPLACE(TextString, 'Ï','i') ; 
END IF ; 
#capital eth, Icelandic 
IF INSTR( x , 'Ð' ) 
THEN SET TextString = REPLACE(TextString, 'Ð','d') ; 
END IF ; 
#capital n, tilde 
IF INSTR( x , 'Ñ' ) 
THEN SET TextString = REPLACE(TextString, 'Ñ','n') ; 
END IF ; 
#capital o, grave accent 
IF INSTR( x , 'Ò' ) 
THEN SET TextString = REPLACE(TextString, 'Ò','o') ; 
END IF ; 
#capital o, acute accent 
IF INSTR( x , 'Ó' ) 
THEN SET TextString = REPLACE(TextString, 'Ó','o') ; 
END IF ; 
#capital o, circumflex accent 
IF INSTR( x , 'Ô' ) 
THEN SET TextString = REPLACE(TextString, 'Ô','o') ; 
END IF ; 
#capital o, tilde 
IF INSTR( x , 'Õ' ) 
THEN SET TextString = REPLACE(TextString, 'Õ','o') ; 
END IF ; 
#capital o, umlaut mark 
IF INSTR( x , 'Ö' ) 
THEN SET TextString = REPLACE(TextString, 'Ö','o') ; 
END IF ; 
#capital o, slash 
IF INSTR( x , 'Ø' ) 
THEN SET TextString = REPLACE(TextString, 'Ø','o') ; 
END IF ; 
#capital u, grave accent 
IF INSTR( x , 'Ù' ) 
THEN SET TextString = REPLACE(TextString, 'Ù','u') ; 
END IF ; 
#capital u, acute accent 
IF INSTR( x , 'Ú' ) 
THEN SET TextString = REPLACE(TextString, 'Ú','u') ; 
END IF ; 
#capital u, circumflex accent 
IF INSTR( x , 'Û' ) 
THEN SET TextString = REPLACE(TextString, 'Û','u') ; 
END IF ; 
#capital u, umlaut mark 
IF INSTR( x , 'Ü' ) 
THEN SET TextString = REPLACE(TextString, 'Ü','u') ; 
END IF ; 
#capital y, acute accent 
IF INSTR( x , 'Ý' ) 
THEN SET TextString = REPLACE(TextString, 'Ý','y') ; 
END IF ; 
#capital THORN, Icelandic 
IF INSTR( x , 'Þ' ) 
THEN SET TextString = REPLACE(TextString, 'Þ','') ; 
END IF ; 
#small sharp s, German 
IF INSTR( x , 'ß' ) 
THEN SET TextString = REPLACE(TextString, 'ß','') ; 
END IF ; 
#small a, grave accent 
IF INSTR( x , 'à' ) 
THEN SET TextString = REPLACE(TextString, 'à','a') ; 
END IF ; 
#small a, acute accent 
IF INSTR( x , 'á' ) 
THEN SET TextString = REPLACE(TextString, 'á','a') ; 
END IF ; 
#small a, circumflex accent 
IF INSTR( x , 'â' ) 
THEN SET TextString = REPLACE(TextString, 'â','a') ; 
END IF ; 
#small a, tilde 
IF INSTR( x , 'ã' ) 
THEN SET TextString = REPLACE(TextString, 'ã','a') ; 
END IF ; 
#small a, umlaut mark 
IF INSTR( x , 'ä' ) 
THEN SET TextString = REPLACE(TextString, 'ä','a') ; 
END IF ; 
#small a, ring 
IF INSTR( x , 'å' ) 
THEN SET TextString = REPLACE(TextString, 'å','a') ; 
END IF ; 
#small ae 
IF INSTR( x , 'æ' ) 
THEN SET TextString = REPLACE(TextString, 'æ','ae') ; 
END IF ; 
#small c, cedilla 
IF INSTR( x , 'ç' ) 
THEN SET TextString = REPLACE(TextString, 'ç','c') ; 
END IF ; 
#small e, grave accent 
IF INSTR( x , 'è' ) 
THEN SET TextString = REPLACE(TextString, 'è','e') ; 
END IF ; 
#small e, acute accent 
IF INSTR( x , 'é' ) 
THEN SET TextString = REPLACE(TextString, 'é','e') ; 
END IF ; 
#small e, circumflex accent 
IF INSTR( x , 'ê' ) 
THEN SET TextString = REPLACE(TextString, 'ê','e') ; 
END IF ; 
#small e, umlaut mark 
IF INSTR( x , 'ë' ) 
THEN SET TextString = REPLACE(TextString, 'ë','e') ; 
END IF ; 
#small i, grave accent 
IF INSTR( x , 'ì' ) 
THEN SET TextString = REPLACE(TextString, 'ì','i') ; 
END IF ; 
#small i, acute accent 
IF INSTR( x , 'í' ) 
THEN SET TextString = REPLACE(TextString, 'í','i') ; 
END IF ; 
#small i, circumflex accent 
IF INSTR( x , 'î' ) 
THEN SET TextString = REPLACE(TextString, 'î','i') ; 
END IF ; 
#small i, umlaut mark 
IF INSTR( x , 'ï' ) 
THEN SET TextString = REPLACE(TextString, 'ï','i') ; 
END IF ; 
#small eth, Icelandic 
IF INSTR( x , 'ð' ) 
THEN SET TextString = REPLACE(TextString, 'ð','') ; 
END IF ; 
#small n, tilde 
IF INSTR( x , 'ñ' ) 
THEN SET TextString = REPLACE(TextString, 'ñ','n') ; 
END IF ; 
#small o, grave accent 
IF INSTR( x , 'ò' ) 
THEN SET TextString = REPLACE(TextString, 'ò','o') ; 
END IF ; 
#small o, acute accent 
IF INSTR( x , 'ó' ) 
THEN SET TextString = REPLACE(TextString, 'ó','o') ; 
END IF ; 
#small o, circumflex accent 
IF INSTR( x , 'ô' ) 
THEN SET TextString = REPLACE(TextString, 'ô','o') ; 
END IF ; 
#small o, tilde 
IF INSTR( x , 'õ' ) 
THEN SET TextString = REPLACE(TextString, 'õ','o') ; 
END IF ; 
#small o, umlaut mark 
IF INSTR( x , 'ö' ) 
THEN SET TextString = REPLACE(TextString, 'ö','o') ; 
END IF ; 
#small o, slash 
IF INSTR( x , 'ø' ) 
THEN SET TextString = REPLACE(TextString, 'ø','o') ; 
END IF ; 
#small u, grave accent 
IF INSTR( x , 'ù' ) 
THEN SET TextString = REPLACE(TextString, 'ù','u') ; 
END IF ; 
#small u, acute accent 
IF INSTR( x , 'ú' ) 
THEN SET TextString = REPLACE(TextString, 'ú','u') ; 
END IF ; 
#small u, circumflex accent 
IF INSTR( x , 'û' ) 
THEN SET TextString = REPLACE(TextString, 'û','u') ; 
END IF ; 
#small u, umlaut mark 
IF INSTR( x , 'ü' ) 
THEN SET TextString = REPLACE(TextString, 'ü','u') ; 
END IF ; 
#small y, acute accent 
IF INSTR( x , 'ý' ) 
THEN SET TextString = REPLACE(TextString, 'ý','y') ; 
END IF ; 
#small thorn, Icelandic 
IF INSTR( x , 'þ' ) 
THEN SET TextString = REPLACE(TextString, 'þ','') ; 
END IF ; 
#small y, umlaut mark 
IF INSTR( x , 'ÿ' ) 
THEN SET TextString = REPLACE(TextString, 'ÿ','y') ; 
END IF ; 
RETURN TextString ; 

END$$ 
DELIMITER ;

Depois é só utilizar o html_unencode em suas consultas como se fosse alguma função nativa, como lower, substr, etc. Um exemplo:

select html_unencode(titulo) from tabela_exemplo;

Hoje é só isso! ;)