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

14 Sep 2024

      Hello, and thank you very much.

Actually I thought to that one already.

But I have reasons to think this doesn't have a lot of chances to be 
more efficient than the “LIKE” method against TINYTEXT.

How could I expect MariaDB to use any index with JSON_TABLE()?

Regards,

Gingko
...
*De :* Markus Mäkelä via discuss [mailto:discuss@lists.mariadb.org]
*Envoyé :* samedi 14 septembre 2024 à 9:19 AM
*Pour :* discuss@lists.mariadb.org
*Objet :* [MariaDB discuss] Re: Joining two tables on a list of values
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 |                |
|     |      | |            |                |
+------------+------------------+------+-----+------------+----------------+
… and a secondary table containing:
joined_table:
+------------+------------------+------+-----+------------+----------------+
| Field      | Type             | Null | Key | Default | 
Extra          |
+------------+------------------+------+-----+------------+----------------+
| index      | int(11)          | NO   | PRI | | auto_increment |
| int_list   |    | YES  |     | (empty)    
|                |
|     |      | |            |                |
+------------+------------------+------+-----+------------+----------------+
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  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_list LIKE 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
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