Question

SQLite语法文档显示在声明外键时可以指定“类型名称”。

https://www.sqlite.org/syntax/column-def.html

我无法找到任何地方，它与所指的键的“类型”有什么作用。当外键“type-name”与它所引用的值的“type-name”不匹配时会发生什么？它是否起任何作用，或只是为了可读性？

提前致谢。

Answer 1

外键约束定义不包含类型名称。它可以附加到列定义，该列定义可以具有类型名称，但该类型名称属于列本身：

#standardSQL
WITH yourTable AS (
  SELECT 1 AS user_id,'1a' AS visit_id, '2017-01-01 14:10:12' AS DATETIME, 
    'google cpc' AS channelgrouping, 0 AS transaction , 1 AS visit UNION ALL
  SELECT 1, '1b', '2017-01-01 20:10:12', 'email', 1, 1 UNION ALL
  SELECT 1, '1c','2017-01-03 08:10:12', 'direct', 0, 1 UNION ALL
  SELECT 1, '1d','2017-01-04 13:10:14', 'organic', 1, 1 UNION ALL
  SELECT 1, '1e','2017-03-04 13:10:14', 'organic', 0, 1 UNION ALL
  SELECT 1, '1f','2017-03-04 13:10:14', 'direct', 0, 1 
)
SELECT
  user_id, 
  STRING_AGG(channelgrouping, ' > ' ORDER BY DATETIME) AS channelgrouping_path,
  SUM(transaction) AS transaction,
  SUM(visit) AS visits,
  IF(SUM(transaction) = 0, NULL, 
    TIMESTAMP_DIFF(MAX(CAST(datetime AS TIMESTAMP)), MIN(CAST(datetime AS TIMESTAMP)), DAY)
  ) AS days_before_purchase
FROM (
  SELECT 
*, 
SUM(transaction) OVER(PARTITION BY user_id ORDER BY datetime 
            ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
) AS grp
  FROM yourTable
)
GROUP BY user_id, IFNULL(grp, 0)

上面显示的所有三列都有一个类型，但外键约束以CREATE TABLE Parent ( ID INTEGER PRIMARY KEY, Name TEXT, [...] ); CREATE TABLE Child ( [...], ParentID INTEGER REFERENCES Parent(ID) -- column def. -- ---- foreign key ---- );开头，并且不包含类型。

外键约束也可以写为表约束：

REFERENCES

这个表格很明显，约束没有单独的类型。

无论如何，SQLite使用dynamic typing并在大多数情况下忽略了类型名称，因此无论你在那里写什么，或者你根本不使用类型，都无关紧要。

SQLite FOREIGN KEY类型名称

1 个答案: