Postgres 12

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with cte(id,description,letters) as (
  values
    (1, 'good', '["a","b", "c"]'::json),
    (2, 'bad', '[]'::json)
)
select id,description, COALESCE(l.value::text, 'x') as letter
from cte
left join json_array_elements_text(letters) l on true


id
description
letter


1
good
a

1
good
b

1
good
c

2
bad
x



SELECT 4


with cte(id,description,letters) as (
  values
    (1, 'good', '["a","b", "c"]'::json),
    (2, 'bad', '[]'::json)
)
select id,description, COALESCE(l.value::text, 'x') as letter
from cte
left join json_array_elements(letters) l on true


id
description
letter


1
good
"a"

1
good
"b"

1
good
"c"

2
bad
x



SELECT 4


with cte(id,description,letters) as (
  values
    (1, 'good', '["a","b", "c"]'::json),
    (2, 'bad', '[]'::json)
)
select id,description, COALESCE(l.value::text, 'x') as letter
from cte,
json_array_elements_text(case when letters::text = '[]' then '[null]'::json else letters end) l;


id
description
letter


1
good
a

1
good
b

1
good
c

2
bad
x



SELECT 4