SQL - 返回数字乘以查询结果的值

Question

我对SQL服务器相当陌生（这是我在StackOverflow上的第二篇文章）。我目前正在使用SQL Server 2008 R2。我有，我已经用下面的查询加入了几个表：

SELECT pt.first_name, pt.last_name, pt.provider_id, 
pt.last_visit_date, pl.last_name 
FROM dbo.pm_patient pt 
INNER JOIN dbo.ProviderList pl 
ON pt.provider_id = pl.provider_id 
WHERE pt.last_visit_date >= '02/01/2012' AND pt.last_visit_date <= '02/01/2013' 
ORDER BY provider_id 

上述查询的结果看起来是这样的：

first_name last_name provider_id last_visit_date last_name 
Smith   John   1    04/25/2012  Johnson 
Doe   Jane   1    02/25/2012  Johnson 
Davies  Ann   1    03/15/2012  Johnson 
Dupree  David  1    11/20/2012  Johnson 
Jones   Becky  1    04/21/2012  Smith 
Diaz   Mike   1    02/12/2012  Smith 
Williams  Allison  1    08/05/2012  Smith 
Taylor  Joe   1    10/01/2012  Smith 

我宁愿简单地得到以下结果：

last_name NoOfPatients 
Johnson  4 
Smith  4 

有人能帮我吗？

Answer 1

尝试：

SELECT pl.last_name, count(*) NoOfPatients 
FROM dbo.pm_patient pt 
INNER JOIN dbo.ProviderList pl 
ON pt.provider_id = pl.provider_id 
WHERE pt.last_visit_date >= '02/01/2012' AND pt.last_visit_date <= '02/01/2013' 
GROUP BY pl.last_name 

Answer 2

你需要了解如何使用组querys，基本上你需要做的是算的患者通过供应商的数量和组。

SELECT pt.last_name, COUNT(pt.*) NoOfPatients 
FROM dbo.pm_patient pt 
INNER JOIN dbo.ProviderList pl 
ON pt.provider_id = pl.provider_id 
WHERE pt.last_visit_date >= '02/01/2012' AND pt.last_visit_date <= '02/01/2013' 
ORDER BY provider_id 
GROUP BY pt.last_name