SQL Server returns the value from the dynamic query sql to the method to pass to the variable

declare @count int, @sqlDnamic nvarchar(max)   --此处务必是nvarchar类型，varchar报错  
set @sqlDnamic ='select @a=count(*) from tableName ' 
exec sp_executesql @sqlDnamic ,N'@a int output',@count output 


--带输入带输出示例  参数后带OUT为输出其它为输入
declare @count int, @sqlDnamic nvarchar(max)   --此处务必是nvarchar类型，varchar报错  
set @sqlDnamic ='select @a=count(*) from S_menu where menuid=@menuid ' 
exec sp_executesql @sqlDnamic ,N'@a int output,@menuid int',@count OUTPUT,2
SELECT @count