mysql 查询结果为空时值时执行后面的sql语句

DECLARE @Val varchar(50)
select @Val = param_value where t_param where param_name = ‘log_type‘
if(@Val is null)
begin
select @Val = param_default_value  where t_param_info where param_name = ‘log_type‘
end
select @Val

SELECT IFNULL(
(select param_value from t_param where param_name = ‘log_type‘),
(select param_default_value from t_param_info where param_name = ‘log_type‘)
) as param_value;