MySQL Trigger - Copy NEW. inserted data compare to existing column values and then insert them to other table if they match

Question

I am trying to create a trigger on a table (Cvr_flypn_hooks_501_1000) in my MySQL database that does the following:

• Checks the incoming inserted data's (new.user_id) value against the existing data's (post_author_id) value for each row in the table.
• If they match, Copy some of the NEW. INSERTED rows data column values and some of the matching rows existing column data to a single new row in a new table (Cvr_flypn_link_501_1000).
• If they do not match, just allow the NEW. INSERTED data to be inserted into the Cvr_flypn_hooks_501_1000 table.

Issue #1: I think my first issue is in the VALUES section, the last four value options I am trying to select the existing matching values to copy, but the trigger fails to fire when I add the additional existing data to the VALUES array so I think something is up here.
Issue #2: I realize that the query will run against the new data as well but I do not want it to compare its self to itself, I hope that makes sense.

I have included what I have so far below.

IF ( EXISTS(SELECT 1 FROM Cvr_flypn_hooks_501_1000 WHERE post_author_id =  new.user_id)) then
          
          INSERT INTO Cvr_flypn_1_link_501_1000 (a_hook_id, a_user_id, a_post_id, a_post_author_id, b_hook_id, b_user_id, b_post_id, b_post_author)

          VALUES (new.hook_id, 
                  new.user_id, 
                  new.post_id, 
                  new.post_author_id,
                  (SELECT hook_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id),
        (SELECT user_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id),
        (SELECT post_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id),
        (SELECT post_author_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id));
END IF

Any and all help is greatly appreciated. I am pretty new to programming so please go gently.

Answer 1

Please try to remember the next time to provide data in a fiddle like below, so we don't have to recreate it al the time

A Sub-SELECT is only allowed to return 1 value so it is better to add an ORDER BY and A LIMIT 1.

 CREATE tABLE Cvr_flypn_1_hooks_501_1000 (hook_id int, user_id int(11) ,post_id int ,post_author_id vaRCHAR(39))

 INSERT INTO Cvr_flypn_1_hooks_501_1000 VALUES (1,1,1,"1")

 CREATE TABLE account(hook_id int, user_id int(11) ,post_id int, post_author_id int)

 CREATE TABLE Cvr_flypn_1_link_501_1000 (a_hook_id int, a_user_id int(11) ,a_post_id int, a_post_author_id int , b_hook_id int, b_user_id int, b_post_id int, b_post_author int)

 CREATE TRIGGER upd_check AFTER INSERT ON account FOR EACH ROW BEGIN IF ( EXISTS(SELECT 1 FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id = new.user_id)) then INSERT INTO Cvr_flypn_1_link_501_1000 (a_hook_id, a_user_id, a_post_id, a_post_author_id, b_hook_id, b_user_id, b_post_id, b_post_author) VALUES (new.hook_id, new.user_id, new.post_id, new.post_author_id, (SELECT hook_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id ORDER BY hook_id DESC), (SELECT user_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id ORDER BY hook_id DESC), (SELECT post_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id ORDER BY hook_id DESC), (SELECT post_author_id FROM Cvr_flypn_1_hooks_501_1000 WHERE post_author_id=new.user_id ORDER BY hook_id DESC)); END IF; END

 INSERT INTO account VALUES(1,1,1,1)

 SELECT * FROM Cvr_flypn_1_link_501_1000

\na_hook_id |  a_user_id |  a_post_id |  a_post_author_id |  b_hook_id |  b_user_id |  b_post_id |  b_post_author \n--------: |  --------: |  --------: |  ---------------: |  --------: |  --------: |  --------: |  ------------: \n        1 |  1 |  1 |  1 |  1 |  1 |  1 |  1 \n

db<>fiddle here