How do I use a select Subquery in MYSQL

Question

I have two databases that I'm trying to transfer data between.

I'm working on getting orders from one to the other right now but I've run into something that I can't figure out the correct way to do it.

Here's the query thus far...

INSERT INTO `NEWTEST`.`Order_LineDetails`

( OrderLineItem_ID, Customer_ID, Order_ID )

SELECT

OrderDetailID,

(    
SELECT o.CustomerID
FROM `OLDTEST`.`Order_Details` od
JOIN `OLDTEST`.`Orders` o ON  o.OrderID = od.OrderID
),

OrderID

FROM `OLDTEST`.`Order_Details`

This is returning an error of '#1242 - Subquery returns more than 1 row'.

The result I'm going for would be to get...

OLDTEST.Order_Details           -> NEWTEST.Order_LineDetails
OLDTEST.Order.CustomerID        -> NEWTEST.Customer_ID
OLDTEST.Order_Details.OrderID   -> NEWTEST.Order_ID

What am I missing?

:::::: EDIT :::::::

This is now correct and works fine.

SELECT

  od.OrderDetailID,
  o.CustomerID,
  od.OrderID

FROM `OrderProcessing`.`Order_Details` od

JOIN `OrderProcessing`.`Orders` o ON o.OrderID = od.OrderID

LIMIT 100

Answer 1

When you use a subquery in the SELECT clause as you did, it must return a single row, hence the error you're getting.

Actually you don't seem to need the subquery here:

INSERT INTO `NEWTEST`.`Order_LineDetails`
SELECT
    od.OrderDetailID,
    o.CustomerID,
    od.OrderID
FROM `OLDTEST`.`Order_Details` od
JOIN `OLDTEST`.`Orders` o ON  o.OrderID = od.OrderID

(Before running that, comment out the INSERT line to make sure it's giving the results you expect).

Answer 2

MySQL subquerys can only contain one result (and apparently yours is returning more). You can add a limit to fix this:

SELECT o.CustomerID
FROM `OLDTEST`.`Order_Details` od
JOIN `OLDTEST`.`Orders` o ON  o.OrderID = od.OrderID LIMIT 1