SQL Update Query - An aggregate may not appear in the set list of an UPDATE statement

Question

I'm trying to write a query that updates tbl8_update_transactions HID field (where it's null) with the primary key value (HID) that's highest in HOLIDAY_DATE_TABLE. I get the following error

"An aggregate may not appear in the set list of an UPDATE statement"

I've read that I need to accomplish this using a subquery, but need help. Thanks

USE BillingUI;
UPDATE tbl8_update_transactions
SET tbl8_update_transactions.HID = MAX(HOLIDAY_DATE_TABLE.HID)
FROM HOLIDAY_DATE_TABLE
WHERE tbl8_update_transactions.HID = NULL;

Update: Tried the proposed solution

UPDATE tbl8_update_transactions
SET HID = h.maxHID
FROM (select max(HOLIDAY_DATE_TABLE.HID) as maxHID from HOLIDAY_DATE_TABLE) h
WHERE tbl8_update_transactions.HID IS NULL;

Unfortunately this affects 0 rows/doesn't work. I think this is because HID is a foreign key (in tbl8_update_transactions). The real issue seems to be my C# methodology for inserting the records into the table (it inserts the row without populating the foreign key). I'd like to handle it with triggers rather than C# code. My tables are as follows.

USE BillingUI;
CREATE TABLE HOLIDAY_DATE_TABLE
(
HID INT IDENTITY PRIMARY KEY,
TABLE_NUMBER nchar(2) NOT NULL,
HOLIDAY_DATE nchar(8) NOT NULL,
FIELD_DESCRIPTION nVARchar(43) NULL,
);

USE BillingUI;
CREATE TABLE tbl8_update_transactions
(
TID INT IDENTITY PRIMARY KEY,
TABLE_NUMBER nchar(2) NOT NULL,
HOLIDAY_DATE nchar(8) NOT NULL,
FIELD_DESCRIPTION nVARchar(43) NULL,
HID int,
FOREIGN KEY (HID) REFERENCES HOLIDAY_DATE_TABLE (HID)
);

I think this might solve the null foreign key issue if I can get help with it

CREATE TRIGGER tbl8_ins
ON HOLIDAY_DATE_TABLE
FOR INSERT
AS
BEGIN
INSERT INTO tbl8_update_transactions
SELECT * FROM HOLIDAY_DATE_TABLE
WHERE HID = MAX(HID);
END

In case you want to see my C# code that performs the insert successfully, but doesn't populate the foreign key

 public ActionResult Create()
    {
        return View();
    }

    //
    // POST: /Table8/Create

    [HttpPost]
    public ActionResult Create(HOLIDAY_DATE_TABLE holiday_date_table, tbl8_update_transactions tbl8_update_transaction)
    {
        if (ModelState.IsValid)
        {
            db.HOLIDAY_DATE_TABLE.Add(holiday_date_table);
            db.SaveChanges();
            db.tbl8_update_transactions.Add(tbl8_update_transaction);
            db.SaveChanges();
            return RedirectToAction("../Billing/HolidayDateTable");
        }
        return View(holiday_date_table);
    }

Answer 1

YOu can write the query like this:

UPDATE tbl8_update_transactions
    SET HID = h.maxHID
    FROM (select max(HOLIDAY_DATE_TABLE.HID) as maxHID from HOLIDAY_DATE_TABLE) h
    WHERE tbl8_update_transactions.HID IS NULL;

I find it confusing to use a from clause and not have the main table mentioned there. I prefer writing this as:

UPDATE ut
    SET HID = h.maxHID
    FROM tbl8_update_transactions ut CROSS JOIN
         (select max(HID) as maxHID from HOLIDAY_DATE_TABLE) h
    WHERE ut.HID IS NULL;