如何过滤子查询？

Question

这是基于AdventureWorks sample database.

这是我的查询：

SELECT FirstName, LastName,

(SELECT COUNT(SalesOrderID)
FROM SalesOrderHeader
WHERE SalesOrderHeader.ContactID = Contact.ContactID) AS OrderCount

FROM Contact
ORDER BY OrderCount desc

问题：我应该在查询中添加些什么，以便它仅显示大于20的OrderCount？我的输出没有其他改变。我尝试了一下，但没有成功：

WHERE SalesOrderHeader.ContactID = Contact.ContactID AS OrderCount AND COUNT(SalesOrderID) > 20

Answer 1

重写为联接聚合查询，然后使用HAVING：

SELECT
    c.FirstName,
    c.LastName,
    COUNT(s.SalesOrderID) AD OrderCount
FROM Contacts c
INNER JOIN SalesOrderHeader s
    ON c.ContactID = s.ContactID
GROUP BY
    c.FirstName,
    c.LastName
HAVING
    COUNT(s.SalesOrderID) > 20;