[MariaDB discuss] Re: Joining two tables on a list of values

14 Sep 2024

      Hi,

You could use JSON_TABLE() <https://mariadb.com/kb/en/json_table/> if 
you store the values as JSON. The nice thing about JSON is that MariaDB 
already has a lot of functions to work with it.

The syntax for converting each list in a row into its own row is IMO a 
bit tricky to get right the first time but it seems to work. Here's an 
example that I tested it with:

CREATE TABLE main_table(id INT PRIMARY KEY NOT NULL, to_join INT NOT NULL);
CREATE TABLE joined_table(id INT PRIMARY KEY NOT NULL, int_join JSON NOT 
NULL);
INSERT INTO main_table VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5);
INSERT INTO joined_table VALUES (123, '[2,4]'), (456, '[3,5]'), (789, 
'[1, 5, 3]');

-- This one splits the join table's values into individual rows
SELECT jt.*, js_t.*
   FROM joined_table AS jt
   CROSS JOIN JSON_TABLE(
     jt.int_join, '$[*]' COLUMNS(
       id INT PATH '$'
     )
   ) js_t;

-- This one joins it to the main table
SELECT mt.id AS main_table_id, jt.id AS joined_table_id, js_t.id AS 
joined_on
   FROM joined_table AS jt
   CROSS JOIN JSON_TABLE(
     jt.int_join, '$[*]' COLUMNS(
       id INT PATH '$'
     )
   ) js_t
   JOIN main_table mt ON (js_t.id = mt.to_join)
   ORDER BY jt.id;

The SQLFiddle for it can be found here: 
https://sqlfiddle.com/mariadb/online-compiler?id=5f6d5772-815f-48a4-977c-990...

Markus Mäkelä

On 9/13/24 17:09, Gingko via discuss wrote:
...
Hello,
I would like to know if is there some (efficient!) way to join two 
tables on a list of values
contained in (likely a single column) of the joined table, whatever 
form could have
this list of values ?
For example, I have a main table containing:
main_table:
+------------+------------------+------+-----+------------+----------------+
| Field      | Type             | Null | Key | Default    | Extra |
+------------+------------------+------+-----+------------+----------------+
| index | int(11)          | NO   | PRI | 0          |                |
| to_join    | int(11) | NO   |     | 0          |                |
| <other fields, irrelevant>    |      |     |            |             |
+------------+------------------+------+-----+------------+----------------+
… and a secondary table containing:
joined_table:
+------------+------------------+------+-----+------------+----------------+
| Field      | Type             | Null | Key | Default    | 
Extra          |
+------------+------------------+------+-----+------------+----------------+
| index      | int(11)          | NO   | PRI |            | 
auto_increment |
| int_list   | <list_of_ints>   | YES  |     | (empty)    
|                |
| <other fields, irrelevant>    |      |     |            |            
    |
+------------+------------------+------+-----+------------+----------------+
We could suppose, here, as a simple test case, that main_table has 
five rows
where to_join contains respectively 1, 2, 3, 4, and 5,
and the joined_table has a single row with the single column int_list 
combining 2 and 4
in any form (but of course, this list could actually have any length).
I'd like to join these tables using a statement like:
SELECT
   <anything>
FROM main_table AS M
LEFT JOIN joined_table AS J
   ON M.to_join <some_operator to> J.int_list;
… in such a way that main_table would be joined to joined_table using 
any value
contained in int_list, and the statement is expected to give two rows 
as a result.
I tried to set int_list as a TINYTEXT containing "2,4" and use:
SELECT
   <anything>
FROM main_table AS M
LEFT JOIN joined_table AS J
   ON M.to_join IN (J.int_list);
… but this doesn't work, of course, my comma separated list is seen as 
a whole single value.
I also tried, still using TINYTEXT, the list as "[2][4]" and use:
SELECT
   <anything>
FROM main_table AS M
LEFT JOIN joined_table AS J
   ON J.int_listLIKE CONCAT('%[', M.to_join , ']%');
This works (more or less), but it is excruciatingly slow, especially 
knowing that main_table
can actually contain millions of rows, and joined_table several thousands.
Is there a better solution for doing that?
As the final goal implies, somewhere later, associating to_join in 
main_table, via int_list,
to the index auto increment primary key of the joined_table, using 
multiple rows there
is not an option.
Regards,
Gingko
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Markus Mäkelä, Senior Software Engineer
MariaDB Corporation