mysql查询字段为null时，返回空字符串

原始数据：

第一种：使用case  when判断

SELECT
	username AS virtualCardNum,
	realname AS realName,
	authenticated AS SMZ,
	(
		CASE
		WHEN idnumber IS NULL THEN
			''
		WHEN idnumber = "1" THEN
			"0"
		ELSE
			idnumber
		END
	) AS encIdNumber
FROM
	appuser
WHERE
	realname = "测试王宁"SELECT
	usernam