How to include empty values using in a query to create relationship between column and row

I have a query who returns the next results +-----------+--------+-------+ | idPeriodo | codigo | valor | +-----------+--------+-------+ | 1 | 7 | 1000 | | 1 | 8 | 1000 | | 1 | 9 | 1000 | | 2 | 7 | 1000 | | 2 | 8 | 1000 | | 3 | 7 | 1000 | | 3 | 9 | 1000 | +-----------+--------+-------+ I need to organize the query because I'm creating a text plain file with the results, so in the first row I need to put each "codigo" split by tab, something like this. --This is the first row, is like a head 7 8 9 Afther this, I need to put the column "valor" associated with its respective codigo and idPeriodo. The final result must be something like this: 7 8 9 1 1000 1000 1000 2 1000 1000 3 1000 1000 Now I have two queries, the first one is to obtain the first row(head with codigo values) and second one I'm doing a query to obtain all values from column "valor" grouped by idPeriodo. These are my queries looks like. -------Query to return head(codigo values)--------------- SELECT DISTINCT CONCAT(conjunto.codigo, central.confterm_id) as codigo_central FROM mantenimiento_central_termica mantenimiento JOIN configuracion_central_termica central ON central.confterm_id = mantenimiento.confterm_id JOIN conjunto_centrales conjunto ON central.id_conjuntocentral = conjunto.id -------Query to return valor grouped by idPeriodo--------------- SELECT id_periodo , valor_mantenimiento = string_agg(valor_mantenimiento, CHAR(9)) WITHIN GROUP (ORDER BY codigo_central) FROM( SELECT CONCAT(conjunto.codigo, central.confterm_id) as codigo_central, per_id AS id_periodo , mantenimcentraltermica_potencia AS valor_mantenimiento FROM mantenimiento_central_termica mantenimiento JOIN configuracion_central_termica central ON central.confterm_id = mantenimiento.confterm_id JOIN conjunto_centrales conjunto ON central.id_conjuntocentral = conjunto.id WHERE cas_id = 2) main_thermal GROUP BY id_periodo The problem I have is the second query returns only the values from column valor who is not null and when I'm going to split the first query with the second one I obtain something like this: 7 8 9 1 1000 1000 1000 2 1000 1000 3 1000 2000 --The error is here, because the 2000 value must be in the third column, with the 9(codigo) value. I need the query put a blank space when find a null valor in a relation between codigo and idPeriodo. How Can i do that ?