postgres how to make conditional join

Question

I have json field that contains object or array of objects. I need to join one key from object if field contains only one object or use lateral join if there is array. I use jsonb_typeof() to determine whether it array or object and want to do something like this

SELECT DISTINCT ON
    (id) id,
    jsonb_typeof(field) AS type,
    CASE WHEN jsonb_typeof = 'object' THEN field->>'key' END
FROM
    test_table
    CASE WHEN jsonb_typeof = 'array' THEN lateral JOIN expression

is it possible to do something like this? At this point I get:

undefined column error(jsonb_typeof)

... in when condition

Answer 1

you can transform data in a subquery:

1. unnest the arrays with json_array_elements
2. union the unnested set with objects set
3. use this temporary set with the uniform object column in a normal join (by key or whatever)

Answer 2

You can split this into two subqueries:

SELECT DISTINCT ON (id) id, jsonb_typeof(field) AS type,
    CASE WHEN jsonb_typeof = 'object' THEN field->>'key' END
FROM ((SELECT id, jsonb_typeof(field) AS type, field->>'key' as col
       FROM test_table
       WHERE jsonb_typeof(field) = 'object'
      ) UNION ALL
      (SELECT id, jsonb_typeof(field) AS type, field->>'key' as col
       FROM test_table LEFT JOIN LATERAL
            . . .
       WHERE jsonb_typeof(field) = 'array'
      ) 
     ) t
ORDER BY id, ???;

Note that when you use DISTINCT ON , you should be using ORDER BY as well. Usually, there are additional columns beyond the DISTINCT ON keys to determine which row you want.