mysql strange behaviour

I've no access to mysql NG, so I ask here.

I've a strange behaviour with mysql.

when I do a select from a simple table, then I get a result for a certain field
when I then put a "union" and create a query that return no result, I get an other result for that field. All other fields seems ok.

the field's type are different (one is a mediumint(8) and the other is a char(13)) it is a problem ? 


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