MySQL用2个外键插入表

Question

I'm working on a C# application that uses a MySQL Database. I'm trying to insert into one table with 2 foreign keys, but am getting an error:

Cannot add or update a child row: a foreign key constraint fails ( thedatabase . tblC , CONSTRAINT tblC_ibfk_1 FOREIGN KEY ( ID ) REFERENCES tblA ( ID ) ON DELETE NO ACTION ON UPDATE CASCADE)

The table structure is

tblA - ID, col2, col3, col4...

tblB - ID, col2, col3, col4...

tblC - tblA.ID, tblB.ID, col3, col4, col5...

I've been looking over the forums, and for the life of me I can't find the syntax to make it work.

Here is what I'm using to connect. I've confirmed that the ID's to exist in the parent tables.

//Connect to the database
MySqlConnection connection = new MySqlConnection(connectionString);
MySqlCommand cmd;
connection.Open(); 

//Insert new record into database
cmd = connection.CreateCommand();
cmd.CommandText = "INSERT INTO tblC(tblA_ID, tblB_ID, col3, col4, col5) VALUES(@tblA_ID, @tblB_ID, @col3, @col4, @col5);";
cmd.Parameters.AddWithValue("@tblA_ID", tblA_ID);
cmd.Parameters.AddWithValue("@tblB_ID", tblB_ID);
cmd.Parameters.AddWithValue("@col3", col3);
cmd.Parameters.AddWithValue("@col4", col4);
cmd.Parameters.AddWithValue("@col5", col5);

cmd.ExecuteNonQuery();
connection.Close();

Answer 1

Your syntax is fine but a foreign key error indicates a data issue.

The error is happening because the ID being referenced does not exist in tblA.

So if you are doing something like:

INSERT INTO tblC SET tblA_id = 100, tblB_id = 101

tblA MUST have a record with an ID = 100 and tblB MUST have a record with ID = 101 or a foreign key error will occur.

Answer 2

The foreign records must be added first ; use a transaction over all the inserts to ensure consistency.

This is because MySQL does not support deferred FK constraints , but then neither does SQL Server..

Like MySQL in general, in an SQL statement that inserts, deletes, or updates many rows, InnoDB checks UNIQUE and FOREIGN KEY constraints row-by-row. When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records it has to look at. InnoDB checks foreign key constraints immediately ; the check is not deferred to transaction commit. ..

(Besides, for surrogate/auto-increment PKs, how can one access IDs that do not exist yet?)

Inserting such a relation can be illustrated with LAST_INSERT_ID and the following SQL:

 BEGIN TRANSACTION;

 DECLARE a_id INT DEFAULT 0,
         b_id INT DEFAULT 0;

 INSERT INTO tblA (..) VALUES (..)
 SET a_id = LAST_INSERT_ID();

 INSERT INTO tblA (..) VALUES (..)
 SET b_id = LAST_INSERT_ID();

 INSERT INTO tblC (..) VALUES (@a_id, @b_id, ..)

 COMMIT;

The exact same approach must be followed when doing the inserts from the client . Create the foreign records; get the IDs, use the IDs in the dependent record.

For a "generic" ADO.NET solution one can use the result of ExecuteScalar for the insert along with a trailing select (vary for database). If using MySqlCommand , from MySqlConnector, this is natively supported via LastInsertedId .

When using another Framework and/or ORM, follow the rules used therein for building object graphs and/or determining inserted IDs.