每个派生表在MySQL中都必须有自己的别名错误

I have the following query:

我有以下查询：

SELECT SUM( cost ) 
FROM (

SELECT s.cost
FROM sandwiches AS s
WHERE s.name =  "Cheese Steak"
)
UNION (

SELECT p.cost
FROM pizza AS p
WHERE TYPE =  "Plain"
AND SIZE =  "L"
)
SELECT