mysql 查詢結果為空時值時執行后面的sql語句

sql server支持變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中執行語句時不支持定義變量及條件判斷，所以想了個辦法，也能達到相同目的

 

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;