insert where is not in

One way to do this would be this:

CREATE Table materiales
(
 tag nvarchar(100) , /* unique?*/
 unidad nvarchar(100),
 descripcion nvarchar(255),
 proveedor nvarchar(255),
 tipmater nvarchar(255),
)

/*
* Add anything
*/
INSERT INTO materiales(tag,unidad,descripcion,proveedor,tipmater)
values ( 'HV9900', 'PZA', 'VALVULAS DE CONTROL ON-OFF', 'ADOBE SYSTEMS INCORPORATED', 'INSTRUMENTACION')


/*
* You can use Left Join and a sub query
*/

-- THIS query does not add a new row
insert into materiales(tag,unidad,descripcion,proveedor,tipmater)

SELECT * FROM
(
      SELECT 'HV9900' AS tag, 'PZA' AS unidad, 'VALVULAS DE CONTROL ON-OFF' AS descripcion, 'ADOBE SYSTEMS INCORPORATED' AS proveedor, 'INSTRUMENTACION' AS tipmater      
) to_insert
LEFT JOIN materiales m
ON to_insert.tag = m.tag
WHERE m.tag IS NULL /* if returns NULL then you know that the line you'd like to add is not already present in the table materiales */



-- THIS query adds a new row
insert into materiales(tag,unidad,descripcion,proveedor,tipmater)

SELECT * FROM
(
      SELECT 'HV9901' AS tag, 'PZA' AS unidad, 'VALVULAS DE CONTROL ON-OFF' AS descripcion, 'ADOBE SYSTEMS INCORPORATED' AS proveedor, 'INSTRUMENTACION' AS tipmater      
) to_insert
LEFT JOIN materiales m
ON to_insert.tag = m.tag
WHERE m.tag IS NULL /* if returns NULL then you know that the line you'd like to add is not already present in the table materiales */

You can also use MERGE. This makes code easy to understand