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Distinct不仅可用在SELECT中，也可用在COUNT内
如：查询出所有在一线、二线部门都出过单的客户
方法：按客户ID分组，对成单的部门进行DISTINCT,次数大于2的即是
SELECT a.CustomerID
FROM bdOrder a(NOLOCK)
JOIN frmuser f ON a.SalesStaff = f.Account
JOIN mdDepartment md ON f.DepartmentId = md.ID
WHERE md.DepartType IN('first','second')
AND a.OrderDate >='2011-01-01'
GROUP BY a.CustomerID
HAVING COUNT(DISTINCT md.DepartType) >=2