mysql 查询结果为空时值时执行后面的sql语句

sql server支持变量所以一般使用方法如下：

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

mysql中执行语句时不支持定义变量及条件判断，所以想了个办法，也能达到相同目的

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;