for example in `Customers` table, I will update the column `TotalItemsObtained` every time the customer make a sales transaction, `Sales` table saves every transaction of a customer.

Customers Table Sales Table
CustomerID CustID
TotalItemsObtained QtyOfSales

Example,

Customer 001 bought 2 items today

INSERT INTO `Sales` VALUES(’001′, ’2′);

UPDATE `Customers` INNER JOIN `Sales` ON `CustomerID` = `CustID` SET `TotalItemsObtained` = `QtyOfSales`;

(this is fine as long as you are sure that’s the very first transaction of Customer 001)

Customer 001 bought another 4 items on the second day

INSERT INTO `Sales` VALUES(’001′, ’4′);

UPDATE `Customers` INNER JOIN `Sales` ON `CustomerID` = `CustID` SET `TotalItemsObtained` = `QtyOfSales`;

(this is where this blog enters, what `QtyOfSales` will you get? 2 or 4? so this blog suggests..)

UPDATE `Customers` INNER JOIN `Sales` ON `CustomerID` = `CustID` SET `TotalItemsObtained` = SUM(`QtyOfSales`);

will it be like this?

UPDATE tb1 SET col1 = (SELECT col1 FROM tb2 WHERE tb1.id = tb2.id), col2 = (SELECT col2 FROM tb2 WHERE tb1.id = tb2.id);

Thanks