declare @search varchar(100)
set @search = 'mrtBHvqovElgBsnn'


declare @cat table
( id int)

declare @com table
( id int)

declare @ut table
( id int)

insert into @cat
select id from tab_categorie where descrizione like '%' + @search + '%'

insert into @com
select id from tab_comuni where descrizione like '%' + @search + '%'  

insert into @ut
select id from tab_utenti where nominativo like '%' + @search + '%' or azienda like '%' + @search + '%'

insert into @ut
select u.id from tab_utenti u inner join @com c on u.idcomune=c.id

insert into @ut
select u.id from tab_utenti u inner join @cat c on u.idcategoria=c.id

select  u.nominativo, u.azienda, c.descrizione as comune, ca.descrizione as categoria
from  tab_utenti u
    inner join tab_comuni c
      on c.id=u.idcomune inner join
    tab_categorie ca
      on u.idcategoria=ca.id inner join
    @ut x on x.id=u.id