Convert JSON Array In MySQL To Rows

Answer :

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 “fish” 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 JSON_TABLE function in MySQL 8 to make all this easier. (http://mysqlserverteam.com/mysql-8-0-labs-json-aggregation-functions/) (The MySQL team has added a 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;

In 2018. What I do for this case.

1. Prepare a table with just continually number in rows.

   
   
   

   CREATE TABLE `t_list_row` (
   `_row` int(10) unsigned NOT NULL,
   PRIMARY KEY (`_row`)
   ) ENGINE=MyISAM DEFAULT CHARSET=latin1;
   
   INSERT t_list_row VALUES (0), (1), (2) .... (65535) big enough;
   




2. Enjoy easy JSON array to rows in the future.

   
   
   

   SET @j = '[1, 2, 3]';
   SELECT 
   JSON_EXTRACT(@j, CONCAT('$[', B._row, ']'))
   FROM (SELECT @j AS B) AS A
   INNER JOIN t_list_row AS B ON B._row < JSON_LENGTH(@j);
   

For this way. is some kind like 'Chris Hynes' way. but you don't need to know array size.

Good: Clear, short, easy code, no need to know array size, no loop, no invoke other function will be fast.

Bad: You need one more table with enough rows.