Oracle 21c

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create table classInfo (classId integer, category CLOB);
BEGIN
insert into classInfo VALUES (1, '{"category":"AAA"}');
insert into classInfo VALUES(2, '{"category":"BBB"}');
END;
/
1 rows affected

select json_arrayagg(category FORMAT JSON RETURNING CLOB) from classInfo;


JSON_ARRAYAGG(CATEGORYFORMATJSONRETURNINGCLOB)


[{"category":"AAA"},{"category":"BBB"}]