mysql - using if in subquery to CONCAT

Question

I trying to write a query to correct an error in a DATETIME field in a table, but my subquery can return NULL as transactiondetails and CreateDate field needs to have a valid value.

My current query is this one:

UPDATE transactionsheaders H SET CreateDate = 
    CONCAT(
    DATE((SELECT CreateDate FROM transactionsdetails WHERE TransactionUID=H.UID GROUP BY TransactionUID)),
    ' ',
    TIME(CreateDate)) 
WHERE UID>0;

and I want to use the result of

SELECT CreateDate FROM transactionsdetails WHERE TransactionUID=H.UID GROUP BY TransactionUID

if it isn't NULL and DATE(CreateDate) otherwise.

I've been trying to use IF for this purpose, but I am having problems writing something that works...

One more thing, transactionsdetails refers to transactionsheaders through TransactionUID (1 to many). Is GROUP BY guaranteed to return only one result?

Answer 1

Here is the correct thing

UPDATE transactionsheaders H
SET CreateDate = CONCAT(DATE(ifnull((SELECT
                       CreateDate
                     FROM transactionsdetails
                     WHERE TransactionUID = H.UID
                     GROUP BY TransactionUID)),'2012-02-02'), ' ', TIME(CreateDate))
WHERE UID > 0;

You can set any default value insted of '2012-02-02' or even date(now())