Convert JSON array in MySQL to rows
It's true that it's not a good idea to denormalize into JSON, but sometimes you need to deal with JSON data, and there's a way to extract a JSON array into rows in a query.
The trick is to perform a join on a temporary or inline table of indexes, which gives you a row for each non-null value in a JSON array. I.e., if you have a table with values 0, 1, and 2 that you join to a JSON array âfishâ with two entries, then fish[0] matches 0, resulting in one row, and fish1 matches 1, resulting in a second row, but fish[2] is null so it doesn't match the 2 and doesn't produce a row in the join. You need as many numbers in the index table as the max length of any array in your JSON data. It's a bit of a hack, and it's about as painful as the OP's example, but it's very handy.
Example (requires MySQL 5.7.8 or later):
CREATE TABLE t1 (rec_num INT, jdoc JSON);
INSERT INTO t1 VALUES
(1, '{"fish": ["red", "blue"]}'),
(2, '{"fish": ["one", "two", "three"]}');
SELECT
rec_num,
idx,
JSON_EXTRACT(jdoc, CONCAT('$.fish[', idx, ']')) AS fishes
FROM t1
-- Inline table of sequential values to index into JSON array
JOIN (
SELECT 0 AS idx UNION
SELECT 1 AS idx UNION
SELECT 2 AS idx UNION
-- ... continue as needed to max length of JSON array
SELECT 3
) AS indexes
WHERE JSON_EXTRACT(jdoc, CONCAT('$.fish[', idx, ']')) IS NOT NULL
ORDER BY rec_num, idx;
The result is:
+---------+-----+---------+
| rec_num | idx | fishes |
+---------+-----+---------+
| 1 | 0 | "red" |
| 1 | 1 | "blue" |
| 2 | 0 | "one" |
| 2 | 1 | "two" |
| 2 | 2 | "three" |
+---------+-----+---------+
It looks like the MySQL team may add a (The MySQL team has added a JSON_TABLE
function in MySQL 8 to make all this easier. (http://mysqlserverteam.com/mysql-8-0-labs-json-aggregation-functions/)JSON_TABLE
function.)
Here's how to do this with JSON_TABLE in MySQL 8+:
SELECT *
FROM
JSON_TABLE(
'[5, 6, 7]',
"$[*]"
COLUMNS(
Value INT PATH "$"
)
) data;
You can also use this as a general string split function which MySQL otherwise lacks (similar to PG's regexp_split_to_table or MSSQL's STRING_SPLIT) by taking a delimited string and turning it into a JSON string:
set @delimited = 'a,b,c';
SELECT *
FROM
JSON_TABLE(
CONCAT('["', REPLACE(@delimited, ',', '", "'), '"]'),
"$[*]"
COLUMNS(
Value varchar(50) PATH "$"
)
) data;