如何在 PostgreSQL V9.4 的新 json-array 元素中添加和删除

Question

I`m making a loop to create a new json matrix with only certain elements, for this I need to go through the previously created json-array and PUSH (ADD ELEMENT) and POP (REMOVE ELEMENT) the elements according to the condition.

I receive something like that

[
{"id":1 , "data": "test_a"},
{"id":2 , "data": "test_t"},
{"id":3 , "data": "test_h"},
.....
]

and I`m doing something like this to the loop

DECLARE
_test       json;
i           json;
_new_jsn    json;
BEGIN
_test := SELECT json_agg(json_build_object('id', 1, 'data', 'test'))

        FOR i IN SELECT * FROM json_array_elements(_test) LOOP


            RAISE NOTICE 'element %',i;
        END LOOP;

From here, I don't know how to continue to make to ADD and REMOVE element in a new param.

My filter is based in an IF-ELSE so if I have

       FOR i IN SELECT * FROM json_array_elements(_test) LOOP
         IF i->>id > 2 THEN
         ..... Add the element in _new_jsn
         ELSE
         .... Remove/Ignore the element
         END IF; 
        END LOOP;

I want to receive after the loop something like this

[
{"id":3 , "data": "test_h"},
{"id":4 , "data": "test_c"},
.....
]

Answer 1

I believe you are looking for

DECLARE
  test CONSTANT json := json_build_array(
   json_build_object('id', 1, 'data', 'test_a'),
   json_build_object('id', 2, 'data', 'test_t'),
   json_build_object('id', 3, 'data', 'test_h'),
   json_build_object('id', 4, 'data', 'test_c')
  );
  result json;
BEGIN
  result := (SELECT json_agg(el)
    FROM json_array_elements(test) AS t(el)
    WHERE (el->>'id')::int > 2);
  -- …
END;

You don't need any loop or IF statements here. Just a SELECT query with a WHERE clause.

Online demo (I haven't found a better way to print the result than to raise an exception:-/)