MariaDB 10.7

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select version();

version()

10.7.5-MariaDB-1:10.7.5+maria~deb11

CREATE TABLE t (j LONGTEXT);

INSERT INTO t (j) VALUES ('{"a":"1"}');

INSERT INTO t (j) VALUES ('{"b":"2"}');

INSERT INTO t (j) VALUES ('{"c":"0", "d":"0"}');

INSERT INTO t (j) VALUES ('{"c":"1", "d":"1"}');

SELECT JSON_KEYS(j) FROM t;

JSON_KEYS(j)

["a"]

["b"]

["c", "d"]

["c", "d"]

SELECT DISTINCT jt.*
  FROM t, 
       JSON_TABLE(JSON_KEYS(j), '$[*]' 
       COLUMNS (
                labelKey VARCHAR(40) PATH '$'
               )
       ) jt

labelKey

a

b

c

d