SQL中 IFnull 和 if函数

SELECT `one`.*, IFnull ( sum(two.class_two_qty), 0 ) AS `tnum` FROM `nb_goods_class_one` AS `one` LEFT JOIN `nb_goods_class_two` AS `two` ON `one`.`class_code` = `two`.`class_one_code` WHERE `one`.`st
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