Syntax `WHERE NOT EXISTS` using MySQL

This is my table tb_group on DB MySQL

The values ​​of columns _tdate and _tnumber of the table tb_group are obtained from a table where accesses are recorded for individual groups, therefore in column _tdate you find the access date, in column _tnumber you find the number of accesses that occurred with that group…

To find missing groups in the table tb_group I use the syntax WHERE NOT EXISTS by crossing the values ​​of the _tgroup columns of the two tables ta_group and tb_group.

SELECT
    `_tgroup` 
FROM
    `ta_group` q 
WHERE
    NOT EXISTS ( SELECT 1 FROM `tb_group` t WHERE q.`_tgroup` = t.`_tgroup` );

This is the result

Now I need insert into my table tb_group the values ​​of the _tgroup missing in the table tb_group for this output

I have tried this solution fiddle demo

I have this problem:

1. the date is not just the day 2025-05-12

In my simplified example I only reported the data for the day 2025-05-12 but in the tb_group table there are all the days of the year up to yesterday...

I have to insert into the tb_group table all the groups that are missing in the tb_group table, taking them from the ta_group table, for each day recorded in the tb_group table

Solution fiddle demo

To achieve your goal of inserting missing _tgroup values from ta_group into tb_group for every distinct _tdate present in tb_group, you'll need to generate a cross join between all dates and all groups, then filter out those already existing in tb_group.

INSERT INTO tb_group (_tdate, _tgroup, _tnumber)
SELECT 
    d._tdate,
    g._tgroup,
    0 AS _tnumber
FROM 
    (SELECT DISTINCT _tdate FROM tb_group) d
CROSS JOIN 
    ta_group g
WHERE 
    NOT EXISTS (
        SELECT 1 
        FROM tb_group t 
        WHERE t._tdate = d._tdate AND t._tgroup = g._tgroup
    );

Explanation:

• (SELECT DISTINCT _tdate FROM tb_group): Gets all unique dates from your existing table.
• CROSS JOIN ta_group: Pairs each date with every group in ta_group, forming all possible combinations.
• NOT EXISTS: Ensures you only insert combinations that don't already exist in tb_group.
• _tnumber = 0: You insert 0 for missing records as required.

📌 Example:

• Given:
  • Dates in tb_group: 2025-05-12, 2025-05-13
  • Groups in ta_group: 71AB1, 71AB2, 71AB3
• The query will check for every combination like:
  2025-05-12, 71AB1
2025-05-12, 71AB2
2025-05-13, 71AB1
  …and so on.
• It will only insert combinations that are not already present in tb_group.