如何将2个表与SQL连接在一起

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]

Data in each Table:
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customer_2组合到customer表的底部，而对于customer 2表的四个客户中的每一个，额外的列CUST_BALANCE为0或为空。

非常感谢您的帮助

Answer 1

我猜发生了什么事，您的连接条件从未得到满足，这意味着两个表具有相同的cust_name尝试调试为不相等

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

Answer 2

select c.[CUST_NUM] , c.[CUST_BALANCE]
from Customer c 
join Customer_2 d on c.CUST_NUM = d.CUST_NUM  

Answer 3

您可以使用以下内容。我假设您想合并Customer和Customer2并加入Invoice：

SELECT * 
FROM INVOICE i INNER JOIN (
  SELECT CUST_NUM, CUST_LNAME, CUST_FNAME, CUST_BALANCE FROM Customer
  UNION ALL
  SELECT CUST_NUM, CUST_LNAME, CUST_FNAME, NULL FROM Customer2
) Customer c ON i.CUST_NUM = c.CUST_NUM