与SQL一起加入2个表

Question

我有一个试图连接在一起的Customer和Customer_2表：

两个表中都有数据，但是在加入语句后，仅返回列名而没有数据。我正在尝试使用以下join语句：

select distinct * 
from Customer c 
join Customer_2 d on c.CUST_NUM = d.CUST_NUM  

这些是表格：

CREATE TABLE [Customer] 
(
    [CUST_NUM] [INT] NOT NULL,
    [CUST_LNAME] [VARCHAR](50) NULL,
    [CUST_FNAME] [VARCHAR](50) NULL,
    CUST_BALANCE [MONEY] NOT NULL, 
) ON [PRIMARY]

CREATE TABLE [INVOICE] 
(
    [Invoice_NUM] [INT] NOT NULL,
    [CUST_NUM] [INT] NOT NULL,
    [INV_DATE] [SMALLDATETIME] NOT NULL,
    [INV_AMOUNT] [MONEY] NOT NULL, 
) ON [PRIMARY]

CREATE TABLE [Customer_2]  
(
    [CUST_NUM] [INT] NOT NULL,
    [CUST_LNAME] [VARCHAR](50) NULL,
    [CUST_FNAME] [VARCHAR](50) NULL, 
) ON [PRIMARY]

每个表中的数据：

INSERT INTO Customer 
VALUES ('1000', 'Smith', 'Jeanne', '1050.11'),
       ('1001', 'Ortega', 'Juan', '840.92');

INSERT INTO INVOICE 
VALUES ('8000', '1000', '2016-03-23', '235.89'),
       ('8001', '1001', '2016-03-23', '312.82'),
       ('8002', '1001', '2016-03-30', '528.10'),
       ('8003', '1000', '2016-04-12', '194.78'),
       ('8004', '1000', '2016-04-23', '619.44');

INSERT INTO CUSTOMER_2 
VALUES ('2000', 'McPherson', 'Anne'),
       ('2001', 'Ortega', 'Juan'),
       ('2002', 'Kowalski', 'Jan'),
       ('2003', 'Chan', 'George');

预期的输出将把customer_2组合到客户表的底部，而对于customer 2表上的四个客户中的每一个，额外的列CUST_BALANCE为0或为空。

Answer 1

select c.[CUST_NUM] , c.[CUST_BALANCE]
FROM INVOICE I
JOIN (
     SELECT CUST_NUM, CUST_BALANCE FROM Customer c 
     UNION
     SELECT CUST_NUM, NULL as CUST_BALANCE FROM Customer_2 d
   ) tbl t on i.cust_num = t.cust_num

Answer 2

我想发生了什么事，就是您的联接条件从未得到满足，这意味着两个表都没有相同的cust_name尝试按以下左联接进行调试

   select distinct * 
   from Customer c 
   Left join Customer_2 d on 
   c.CUST_NUM =
   d.CUST_NUM

Answer 3

您可以将Customer和Customer_2与UNION ALL合并。您可以将此数据集与INNER JOIN合并以获取每张发票的客户数据：

SELECT * 
FROM INVOICE INNER JOIN (
  SELECT CUST_NUM, CUST_LNAME, CUST_FNAME, CUST_BALANCE FROM Customer
  UNION ALL
  SELECT CUST_NUM, CUST_LNAME, CUST_FNAME, NULL FROM Customer_2
) CUSTOMER ON INVOICE.CUST_NUM = CUSTOMER.CUST_NUM

demo on dbfiddle.uk

您还可以在SO上检查此问题：What is the difference between JOIN and UNION?