MySQL 5.7

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create table test (j json)
insert into test values ('["A", "B", "C"]')
select json_extract(j, '$[1]') from test;
-- "B"

-- replace second element
select json_set(j, '$[1]', 'D') from test;
-- ["A", "D", "C"]

-- insert fourth element
select json_set(j, '$[3]', 'E') from test;
-- ["A", "B", "C", "E"]

-- attempt to insert second element fails as it already exists
select json_insert(j, '$[1]', 'F') from test;
-- ["A", "B", "C"]

-- use json_array_insert to insert a new second element and move the other elements right
select json_array_insert(j, '$[1]', 'F') from test;
-- ["A", "F", "B", "C"]

-- insert fourth element
select json_insert(j, '$[3]', 'F') from test;
-- ["A", "B", "C", "F"]

-- or use json_array_append to add an element at the end
select json_array_append(j, '$', 'F') from test;
-- ["A", "B", "C", "F"]

-- replace second element
select json_replace(j, '$[1]', 'G') from test;
-- ["A", "G", "C"]

-- attempt to replace non-existing element fails
select json_replace(j, '$[3]', 'G') from test;
-- ["A", "B", "C"]

json_extract(j, '$[1]')

"B"

json_set(j, '$[1]', 'D')

["A", "D", "C"]

json_set(j, '$[3]', 'E')

["A", "B", "C", "E"]

json_insert(j, '$[1]', 'F')

["A", "B", "C"]

json_array_insert(j, '$[1]', 'F')

["A", "F", "B", "C"]

json_insert(j, '$[3]', 'F')

["A", "B", "C", "F"]

json_array_append(j, '$', 'F')

["A", "B", "C", "F"]

json_replace(j, '$[1]', 'G')

["A", "G", "C"]

json_replace(j, '$[3]', 'G')

["A", "B", "C"]