jsonb_array_elements_text() and json_array_elements_text()

Purpose: Transform the JSON values of JSON array into a SQL table of (i.e., SETOF) text values.

Signature: For the jsonb variant:

input value:       jsonb
return value:      SETOF text

Notes: The function jsonb_array_elements_text() bears the same relationship to jsonb_array_elements() that the other *text() functions bear to their plain counterparts: it's the same relationship that the ->> and #>>operators bear, respectively to -> and #> operators. (Compound values become the RFC 7159 text of the value; primitive values become the ::text representation of the SQL value that the JSON primitive value corresponds to.)

This example uses the same JSON array input that was used to illustrate jsonb_array_elements().

Notice that the JSON value null becomes a genuine SQL NULL. However, SQL array comparison uses IS NOT DISTINCT FROM semantics, and not the semantics that the comparison of scalars uses. So the simple ASSERT that elements = expected_elements is TRUE is sufficient. See the section Operators for comparing two arrays.

do $body$
  j_array constant jsonb := '["cat", "dog house", 42, true, {"x": 17}, null]';
  t text;

  elements text[];
  expected_elements constant text[] :=
      'dog house',
      '{"x": 17}',

  n int := 0;
  for t in (select jsonb_array_elements_text(j_array)) loop
    n := n + 1;
    elements[n] := t;
  end loop;

    elements = expected_elements,

This highlights the fact that the resulting values are the ::text typecasts of the equivalent SQL primitive values, rather than the RFC 7159 representations of the actual JSON values. In particular, 42 is the two characters 4 and 2 and true is the four characters t , r, u, and e.

This example emphasizes the impedance mismatch between a JSON array and a SQL array: the former allows values of heterogeneous data types; but the latter allows only values of the same data type—as was used to declare the array.

If you have prior knowledge of the convention to which the input JSON document adheres, you can cast the output of jsonb_array_elements_text() to, say, integer or boolean. For example, this:

create domain length as numeric
  not null
  check (value > 0);

select value::length
from jsonb_array_elements_text(
  '[17, 19, 42, 47, 53]'::jsonb

generates a table of genuine integer values that honor the constraints that the domain defines. If you make one of the input elements negative, then you get this error:

value for domain length violates check constraint "length_check"

And if you set one of the input elements to the JSON null, like this:

select value::length
from jsonb_array_elements_text(
  '[17, null, 42, 47, 53]'::jsonb

then you get this error:

domain length does not allow NULL values

Here's the same idea for boolean values:

create domain truth as boolean
  not null;

select (value::truth)::text
from jsonb_array_elements_text(
  '[true, false, true, false, false]'::jsonb

It produces this output in ysqlsh: