SQL从多个表中选择以显示甚至空值[重复]

Question

我有 3 个具有外键 ACCOUNT_NO 的表，第一个表是 CREDIT_LIST，其中包含从银行提取的所有信用，第二个表是 CUSTOMERS 保存客户信息，最后一个是 ACCOUNTS 保存有关帐户本身的所有信息。当我尝试选择时

SELECT 
    B.CUSTOMER_NO AS CUSTOMER_NO,
    B.CREDIT_TYPE AS CREDIT_TYPE,
    B.ACCOUNT_NO AS CREDIT_ACCOUNT_NUMBER,
    A.BRANCH_CODE AS BRANCH_CODE,
    C.EXTERNAL_ACCOUNT_NO AS EXTERNAL_ACCOUNT_NUMBER
FROM 
    CREDIT_LIST B,
    CUSTOMERS A,
    ACCOUNTS C
WHERE   
    B.STATUS = 'A' -- ACTIVE
    AND A.CUSTOMER_NO = B.CUSTOMER_NO
    AND C.ACCOUNT_NO = B.ACCOUNT_NO
    ;

我得到零结果，因为 ACCOUNTS 中没有 EXTERNAL_ACCOUNT_NO，它具有 c.account_no = b.account_no。问题是即使没有 EXTERNAL_ACCOUNT_NO 我也想显示信息并用 null 填充它，例如：

| CUSTOMER_NO | CREDIT_TYPE | CREDIT_ACCOUNT_NUMBER | BRANCH_CODE | EXTERNAL_ACCOUNT_NUMBER
+-------------+-------------+-----------------------+-------------+------------------------
|     1       | some_type   | 123456                | 01          | 
|     2       | some_type   | 654321                | 01          | 111111111111

我觉得这非常愚蠢，但无法弄清楚我到底做错了什么

Answer 1

我想，这就是你需要的

SELECT 
    B.CUSTOMER_NO AS CUSTOMER_NO,
    B.CREDIT_TYPE AS CREDIT_TYPE,
    B.ACCOUNT_NO AS CREDIT_ACCOUNT_NUMBER,
    A.BRANCH_CODE AS BRANCH_CODE,
    C.EXTERNAL_ACCOUNT_NO AS EXTERNAL_ACCOUNT_NUMBER
FROM 
    CREDIT_LIST B
    JOIN     CUSTOMERS A on A.CUSTOMER_NO = B.CUSTOMER_NO
    LEFT JOIN ACCOUNTS C on C.ACCOUNT_NO = B.ACCOUNT_NO
WHERE   
    B.STATUS = 'A' -- ACTIVE
;

左连接将确保您从 A 和 B 获得详细信息，即使 C 中没有可用数据。

另外，AS 是多余的，并不是必须的，您可以在查询的第 4 行写 B.ACCOUNT_NO CREDIT_ACCOUNT_NUMBER。

左外连接 - Reference

不建议，但是，如果有原因，你需要使用旧语法，下面是你需要的

SELECT 
    B.CUSTOMER_NO AS CUSTOMER_NO,
    B.CREDIT_TYPE AS CREDIT_TYPE,
    B.ACCOUNT_NO AS CREDIT_ACCOUNT_NUMBER,
    A.BRANCH_CODE AS BRANCH_CODE,
    C.EXTERNAL_ACCOUNT_NO AS EXTERNAL_ACCOUNT_NUMBER
FROM 
    CREDIT_LIST B,
    CUSTOMERS A,
    ACCOUNTS C
WHERE   
    B.STATUS = 'A' -- ACTIVE
    AND A.CUSTOMER_NO = B.CUSTOMER_NO
    AND C.ACCOUNT_NO(+) = B.ACCOUNT_NO -- (+) will do a outer join for you
    ;