MYSQL错误＃1452

Question

I have two tables

One is Customer table With ---

CustomerID INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
Name CHAR(50) NOT NULL
Address CHAR (100) Not NULL
CITY CHAR (30) Not NULL

Then I have another Table called Orders with ----

OrdersID  INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
CustomerID INT UNSIGNED NOT NULL
Amount FLOAT(6,2)
Date DATE NOT NULL

---

Now I am trying to add a foreign Key for CustomerID and REFERENCE Customers(CustomerID)

I am using this command but am getting a #1452 - Cannot add or update a child row: a foreign key constraint fails

ALTER TABLE LA_Orders
ADD CONSTRAINT fk_CustomerID FOREIGN KEY (CustomerID) REFERENCES LA_Customers(CustomerID)

I have already made sure InnoDB matches.

Any help would much appreciated.

Answer 1

This would occur if LA_Orders already has data and has invalid CustomerID columns.

You can find them using:

select lo.*
from LA_Orders lo
where lo.CustomerID not in (select lc.CustomerID from LA_Customers lc);

(Actually, this assumes that LA_Customers(CustomerID) is not NULL . It should be a primary key, so that should be true.)