What Causes The INSERT statement conflicted with the FOREIGN KEY constraint?

Question

This code has worked for me before but i am not sure anymore what has been causing this error. My only guess is that when i try to create a Player, the Team Data is being se

Answer 1

The reason behind this is when you are inserting a record on the child table which the value of the referencing column doesn't exist yet on the on the parent table.

Consider the following scenario:

// PARENT TABLE
CREATE TABLE TableA
(
   ID INT PRIMARY KEY
);

// CHILD TABLE
CREATE TABLE TableB
(
   ID INT PRIMARY KEY,
   TableA_ID INT,
   CONSTRAINT tb_FK FOREIGN KEY (TableA_ID) REFERENCES TableA(ID)
);


// RECORDS OF PARENT TABLE
INSERT INTO TableA (ID) VALUES (1);
INSERT INTO TableA (ID) VALUES (2);

// RECORDS OF CHILD TABLE
INSERT INTO TableB (ID, TableA_ID) VALUES (1,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (2,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (3,2);

If you execute the statements above, it will not fail because none of them violates the referential integrity rule.

Try executing the following statement:

INSERT INTO TableB (ID, TableA_ID) VALUES (3,4);

It fails because 4 which is the value of TableA_ID to be inserted doesn't exist on Table1.ID. Foreign Keys preserved referential integrity between the records.