mysql 让空值用0补充的函数IFNULL(a,0)

记录，防止遗忘！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;