sql - How to write postgres query for below issue? -

      level1         |    level2      |     level3          |     level4 key       |  value   | key     |value |    key     | value  |  key     | value ---------------------|----------------|---------------------|-----------------   setting1  |  true    |                |  setting1  |  true  |setting1  | false                      |setting2 | false|                     |setting2  | false                          |                |                     |setting3  | true 

i have 4 tables. level1, level2, level3, level4. each has key value pair.

i want key, value pair such if key value pair present in level1 should not check level2,3,4.

if it's not present in level1 should go level2 if not level3.

value present in level4.

so final o/p of type map.

key       |  value --------------------- setting1  |  true setting2  |  false setting3  |  true 

is possible sql query or need write function or procedure it?

one approach use case - when structure.

see sqlfiddle

select case when t1."key" not null             t1."key"             else case when t2."key" not null                       t2."key"                       else case when t3."key" not null                                 t3."key"                                 else t4."key"                            end                  end        end        "key",        case when t1."value" not null             t1."value"             else case when t2."value" not null                       t2."value"                       else case when t3."value" not null                                 t3."value"                                 else t4."value"                            end                  end        end        "value" table4 t4 left join table1 t1 on t4."key" = t1."key" left join table2 t2 on t4."key" = t2."key" left join table3 t3 on t4."key" = t3."key"