set @userName = '刘德华';
SELECT @userName : = '刘青云';
SELECT @userName as '读取到的userName变量值';
set @x=5,@y=7;
SELECT @x + @y as '5+7的结果';
set @dx=0.55,@dy=2;
SELECT @dx + @dy;
set @result=(select @dx + @dy)
SELECT @result;
set @cityName1='Kabul';
SET @cityName2='Qandahar';
SET @cityName3='Maastricht';
SELECT * from city where `Name`=@cityName1;
SELECT * FROM city WHERE `Name` IN(@cityName1,@cityName2, @cityName3);
SELECT @cityName;