Question

can i use GROUP_CONCAT to update table? I have 2 tables
i

d    |  label
------------------------------
1     | ravi,rames,raja
------------------------------
2     |     ravi
------------------------------
3     | ravi,raja
------------------------------
4     | null
------------------------------
5     | null
------------------------------
6     | rames
------------------------------
and 
id    | values
------------------------------
12    | raja
------------------------------
13    | rames
------------------------------
14    | ravi
------------------------------

And i want the result like following table–

id    | label
------------------------------
1     | 12,13,14
------------------------------
2     |     14
------------------------------
3     | 14,12
------------------------------
4     | null
------------------------------
5     | null
------------------------------
6     | 13
------------------------------

but by using the following query –

SELECT `table1`.`id`, GROUP_CONCAT(`table2`.`id` ORDER BY `table2`.`id`) AS label
FROM `table1`
JOIN `table2` ON FIND_IN_SET(`table2`.`values`, `table1`.`nos`) 
GROUP BY `table1`.`id`;

Im getting-

id    | label
------------------------------
1     | 12,13,14
------------------------------
2     | 14
------------------------------
3     | 12,14
------------------------------
6     | 13
------------------------------

I want to keep the null value. otherwise the order of rows will be broken. please help.
sorry for the large font 🙁

Answer 1

You just need a LEFT JOIN to preserve the nulls:

SELECT `table1`.`id`, GROUP_CONCAT(`table2`.`id` ORDER BY `table2`.`id`) AS label
FROM `table1`
LEFT JOIN `table2` ON FIND_IN_SET(`table2`.`values`, `table1`.`nos`) 
GROUP BY `table1`.`id`;

However, I recommend against updating a table to include comma-separated values in a column. It forces you to use FIND_IN_SET() when querying it, and breaks the ability to index the column, affecting the performance of your queries. The more sustainable action would be to normalize table1 so that it doesn’t include a comma-separated column.

Update:

To use GROUP_CONCAT() in an UPDATE statement, you would use a syntax like the following. Substitute your correct table and column names, and in your case, you probably want to replace the entire JOIN subquery with your SELECT statement.

UPDATE 
  tbl_to_update
  JOIN (SELECT id, GROUP_CONCAT(concatcolumn) AS label FROM tbl GROUP BY id) tbl_concat
     ON tbl_to_update.id = tbl_concat.id
SET tbl_to_update.column_to_update = tbl_concat.label 
WHERE <where condition>

So in your case:

UPDATE
   table1 
   INNER JOIN (SELECT id, GROUP_CONCAT(id) AS label FROM table1 GROUP BY id) table2
      ON FIND_IN_SET(`table2`.`label`, `table1`.`nos`) 
SET table1.nos = table2.id