如果我自己联接，为什么INNER JOIN将所有行相乘？

Question

我有一个样本表，加入时打错了字。但这会产生一些我无法理解的结果。您能用简单的英文解释为什么SQL会乘行吗？

我知道如何解决问题：

INNER JOIN @temp tfo
ON t.id_StationDeparture = tfo.id_StationDeparture
    AND t.id_StationDestination = tfo.id_StationDestination
    AND t.id_City = tfo.id_City
    AND t.id_District = tfo.id_District
    AND t.id_Street = tfo.id_Street
    AND t.Amount = tfo.Amount

但是，我真的很想知道为什么会这样。

请不要关闭我的问题

样本数据：

DECLARE @temp TABLE
(   
    id_StationDeparture INT,
    id_StationDestination INT,  
    id_City INT,
    id_District INT,
    id_Street INT,
    Amount DECIMAL(15, 2)
)

INSERT INTO @temp
(
    id_StationDeparture,
    id_StationDestination,
    id_City,
    id_District,
    id_Street,
    Amount
)
VALUES  ( 10, 11, 1, 1, 1, 0)
, ( 12, 14, 1, 1, 0,0)
, ( 16, 18, 1, 1, 0,0)

而我的查询：

SELECT   
  t.id_StationDeparture
, t.id_StationDestination
, t.id_City
, t.id_District
, t.id_Street
, t.Amount
FROM @temp  t
INNER JOIN @temp tfo
ON t.id_StationDeparture = t.id_StationDeparture
    AND t.id_StationDestination = t.id_StationDestination
    AND t.id_City = t.id_City
    AND t.id_District = t.id_District
    AND t.id_Street = t.id_Street
    AND t.Amount = t.Amount

我期望得到以下结果，因为我要加入不同的行：

但是，我得到的是：

Answer 1

您需要更改表别名，因为对于不可为空的列t.id_StationDeparture = t.id_StationDeparture，您的条件始终为true，并且工作方式与CROSS JOIN：

SELECT   
  t.id_StationDeparture
, t.id_StationDestination
, t.id_City
, t.id_District
, t.id_Street
, t.Amount
FROM @temp  t
INNER JOIN @temp tfo
ON t.id_StationDeparture = tfo.id_StationDeparture
    AND t.id_StationDestination = tfo.id_StationDestination
    AND t.id_City = tfo.id_City
    AND t.id_District = tfo.id_District
    AND t.id_Street = tfo.id_Street
    AND t.Amount = tfo.Amount;

db <> fiddle演示

编辑：

结果集有9行，因为CROSS JOIN它是笛卡尔积，并且3乘以3等于9

集合的基数是集合中元素的数量。输出集的基数等于所有输入集的基数的乘积

| A×B | = | A | ·| B |