用嵌套查询和连接查询查询供应了'W1'和'W2'号物资的公司编号

嵌套查询：

SELECT DISTINCT s.sid
FROM suppliers s
WHERE s.sname IN (SELECT sp.sname
FROM supply sp
WHERE sp.wid IN ('W1', 'W2'));

连接查询：

SELECT DISTINCT s.sid
FROM suppliers s
JOIN supply sp ON s.sname = sp.sname
WHERE sp.wid IN ('W1', 'W2');