样本数据：

SQL> with
  2  customers (customerid) as
  3    (select 1 from dual union all
  4     select 2 from dual union all
  5     select 3 from dual
  6    ),
  7  orders (orderid, customerid, orderdate) as
  8    (select 100, 1, date '1997-12-02' from dual union all  --> customer 1 counts as "1"
  9     select 101, 1, date '2000-03-28' from dual union all
 10     select 102, 2, date '1997-05-29' from dual union all  --> customer 2 counts as "1" as well
 11     select 103, 2, date '1997-06-12' from dual            --> because both orders were made in 1997
 12    )

质询：

13  select count(distinct c.customerid) numero_clientes
 14  from customers c join orders o on o.customerid = c.customerid
 15  where extract(year from o.orderdate) = 1997;

NUMERO_CLIENTES
---------------
              2

SQL>

DISTINCT是关键字，不是函数。
您可以使用一个日期范围，并将DISTINCT放入聚合中，以便获得不同公司名称的计数：

SELECT COUNT(DISTINCT companyname) AS numero_clientes
FROM   customers c
       JOIN orders o
       ON c.customerid = o.customerid
WHERE  orderdate >= DATE '1997-01-01'
AND    orderdate <  DATE '1998-01-01'

您还可以使用以下任一项：

• WHERE EXTRACT(YEAR FROM orderdate) = 1997
• WHERE TO_CHAR(orderdate, 'YYYY') = '1997'

它会起作用的但是，在这些情况下，Oracle不会使用orderdate列上的任何索引，而需要分别在EXTRACT(YEAR FROM orderdate)或TO_CHAR(orderdate, 'YYYY')上使用基于函数的索引。如果使用日期范围，则Oracle可以在orderdate列上使用索引。