mysql 讓空值用0補充的函數IFNULL(a,0)

記錄，防止遺忘！sql如下：

SELECT IFNULL((SELECT sum(EO_using_value) FROM cq_energy_other as eo WHERE eo.EO_type = #{energyType} and DATE_FORMAT(EO_alt,'%Y-%m-%d') = curdate()),0) as vals;