mysql - 如何对mysql行求和

Question

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

这会返回多行数字，我将如何将它们加在一起，我似乎做不到。

Answer 1

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

就这么容易。