How to append data to a concatenation, from another table, only if that data is available

I have 2 tables:

L0INS (ID_INS, Nume_INS)
1 ANB
2 ENEL
3 DIGI

L1AVZ (ID_AVZ, FK_INS, Nume_AVZ)
1 1 APA
2 1 CAN
3 1 HID
4 2 ELE

I want to make a query that displays the following:

Query1 (ID_INS, CONCAT(Nume_INS, Nume_AVZ) )
1 ANB, APA
1 ANB, CAN
1 ANB, HID
2 ENEL, ELE
3 DIGI, n/a

SO it only adds Name_T2 to the concatenation, if an entry in Table2 exists that has L0INS.ID_INS = L1AVZ.FK_INS

I made this version that does display a L0INS entry for each Nume_AVZ append but also one that includes 'n/a'. I'd Like to remove the 'n/a' entry, if there is another one.

SELECT DISTINCT     "L0INS"."ID_INS", 
                COALESCE ( "NUME_INS", 'n/a' ) || ', ' || CASE WHEN "L1AVZ"."FK_INS" = "L0INS"."ID_INS" THEN "L1AVZ"."NUME_AVZ" ELSE 'n/a' END "INS_SELECT", 
                LOWER ( COALESCE ( "NUME_INS", 'n/a' ) || ', ' || CASE WHEN "L1AVZ"."FK_INS" = "L0INS"."ID_INS" THEN "L1AVZ"."NUME_AVZ" ELSE 'n/a' END ) "INS_SEARCH" 

FROM        "L1AVZ", "L0INS" 
WHERE   "L0INS"."ID_INS" IS NOT NULL

I sense there's something in the WHERE part missing, that I can't wrap my head around.

Solution

Isn't a LEFT JOIN simpler?

SELECT l1."ID_INS", 
       ("NUME_INS" || ', ' || OALESCE(l0."NUME_AVZ", 'n/a') AS INS_SEARCH"
FROM "L1AVZ" l1 LEFT JOIN
     "L0INS" lO
     ON l1.id_ins = l0.fk_ins;