How to quote a variable for Mysql query

Question

I’m doing a trial, I need to put a variable varchar in the query, but it doesn’t go with the simple quotes, I tried to concatenate like this or something but it’s no use:

set @comando := concat(@comando, '"'); //ao contrário tbm
set @comando := concat(@comando, classe);
set @comando := concat(@comando, '"');

I’ve had to put triple quotes too:

set @comando := concat(@comando, '''classe''');

What better way to concatenate or make Mysql recognize as a varchar?

Code:

delimiter $$
create procedure eixos_caminhao (in classe varchar(3))
begin
    set @comando := 'select count(*) as qtdeCaminhoes';
    set @comando:= concat(@comando, ' from tb_vbv where classe = ');
    set @comando := concat(@comando, classe);

    PREPARE myquery FROM @comando;
    EXECUTE myquery;
end $$
delimiter;

Answer 1

Whenever we’re riding a string and within that string it is necessary to have another string, we have some exits:

1. As in the SQL so much ' as " begin and end a string, we can interlink these characters to form a string inside the other:

   set @comando = 'select count(*) as qtdeCaminhoes';
   set @comando = concat(@comando, ' from tb_vbv where classe = "');
   set @comando = concat(@comando, classe);
   set @comando = concat(@comando, '"');
   

   The result of this would be:

   select Count(*) as qtyCaminhoes from tb_vbv Where class = "content_de_class"

2. But if you really want to get around the string with ', you can use the previous reversed method("'"), or use the \ which is an escape character, the ' or " after it, will not be considered as closing a string:

   set @comando = 'select count(*) as qtdeCaminhoes';
   set @comando = concat(@comando, ' from tb_vbv where classe = \'');
   set @comando = concat(@comando, classe);
   set @comando = concat(@comando, '\'');
   

Further information about the function CONCAT is that it accepts several parameters to concatenate at once, its code could be summarized for this for example:

set @comando = 'select count(*) as qtdeCaminhoes';
set @comando = concat(@comando, ' from tb_vbv where classe = ');
set @comando = concat(@comando, '\'', classe, '\'');