Mysql，如果表A中存在外键，则将新记录插入表B

Question

There are a few similar questions on here. None provide a solution. I would like to INSERT a NEW record into table B , but only if a foreign key exists in table A . To be clear, I do not wish to insert the result of a select. I just need to know that the foreign key exists.

INSERT INTO tableB (tableA_ID,code,notes,created) VALUES ('24','1','test',NOW())
        SELECT tableA_ID FROM tableA WHERE tableA_ID='24' AND owner_ID='9'

Clearly, the above does not work. But is this even possible? I want to insert the NEW data into tableB, only if the record for the row in tableA exists and belongs to owner_ID.

The queries I have seen so far relate to INSERTING the results from the SELECT query - I do not wish to do that.

Answer 1

Try this:

INSERT INTO tableB (tableA_ID,code,notes,created) 
SELECT id, code, notes, created
FROM ( SELECT '24' as id, '1' as code, 'test' as notes, NOW() as created) t
WHERE EXISTS 
(
   SELECT tableA_ID 
   FROM tableA 
   WHERE tableA_ID='24' AND owner_ID='9'
)

Answer 2

I know it's a pretty much old answered question but it's highly ranked now in google search results and I think some addition may help someone in the future.

In some DB configuration, you may want to insert a row in a table that have two or more foreign keys. Let's say we have four tables in a chat application : Users , Threads , Thread_Users and Messages

If we want a User to join a Thread we'll want to insert a row in Thread_Users in wich have two foreign keys : user_id , thread_id .

Then, we can use a query like this, to insert if both foreign keys exists, and silently fail otherwise :

INSERT INTO `thread_users` (thread_id,user_id,status,creation_date)
SELECT 2,3,'pending',1601465161690 FROM (SELECT 1 as nb_threads, 1 as nb_users) as tmp 
WHERE tmp.nb_threads = (SELECT count(*) FROM `threads` WHERE threads.id = 2)
      AND tmp.nb_users = (SELECT count(*) FROM `users` WHERE users.id = 3)

It's a little verbose but it does the job pretty well.

Application-side, we just have to raise an error if affectedRows = 0 and maybe trying to see which of the keys doesn'nt exists. IMHO, it's a better way to do the job than to execute two SELECT queries and THEN execute the INSERT especially when an inexistent foreign key probability is very low.