如何SUM mysql行

Question

SELECT `contract_fine` AS TotalFine FROM `contract` 
NATURAL JOIN `rented_vehicle` WHERE `contractID` = 3;

这会返回数字行，我如何将它们添加到一起，我似乎无法做到。

Answer 1

SELECT SUM(`contract_fine`) AS TotalFine FROM `contract` 
NATURAL JOIN `rented_vehicle` WHERE `contractID` = 3;

就这么简单。