WHERE子句中条件T.STATUS NOT IN('Void', 'Failed')的存在有效地将事务表的左联接转换为内联接，因为如果WHERE子句中的条件不满足，则不能接受任何行。要将T.STATUS...条件视为左连接的一部分，它必须出现在连接本身的ON子句中：

SELECT C.CUSTOMER_ID, C.NAME, NVL(SUM(AMOUNT), 0) AS PURCHASE_AMOUNT
FROM CUSTOMER C
LEFT JOIN TRANSACTION T
  ON T.CUSTOMER_ID = C.CUSTOMER_ID AND
     T.STATUS NOT IN('Void', 'Failed')
GROUP BY C.CUSTOMER_ID, C.NAME;

您可以在SELECT PART中用CASE检查‘T.STATUS’，而不是在WHERE子句中检查它。

SELECT
    c.customer_id,
    c.name,
    SUM(
        CASE
            WHEN t.status NOT IN('Void', 'Failed') THEN
                amount
            ELSE
                0
        END
    ) AS purchase_amount
FROM
    customer    c
    LEFT JOIN transaction t ON t.customer_id = c.customer_id
GROUP BY
    c.customer_id,
    c.name;