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如何使用 UNION ALL 在 SQLite 中模拟 FULL OUTER JOIN?

DDD
DDD原创
2025-01-11 09:02:42594浏览

How to Simulate a FULL OUTER JOIN in SQLite Using UNION ALL?

模拟SQLite中的FULL OUTER JOIN:使用UNION ALL

SQLite原生不支持FULL OUTER JOIN。但是,我们可以通过组合两个LEFT JOIN并使用UNION ALL来实现相同的效果。

步骤一:创建左侧连接基表

<code class="language-sql">SELECT employee.*, department.*
FROM employee LEFT JOIN department
ON employee.DepartmentID = department.DepartmentID</code>

步骤二:创建空值内部连接表

<code class="language-sql">SELECT employee.*, department.*
FROM department LEFT JOIN employee
ON employee.DepartmentID = department.DepartmentID
WHERE employee.DepartmentID IS NULL</code>

步骤三:联合所有表

最后,使用UNION ALL组合两个LEFT JOIN表以创建FULL OUTER JOIN:

<code class="language-sql">SELECT *
FROM (
  SELECT employee.*, department.*
  FROM employee LEFT JOIN department
  ON employee.DepartmentID = department.DepartmentID
) UNION ALL (
  SELECT employee.*, department.*
  FROM department LEFT JOIN employee
  ON employee.DepartmentID = department.DepartmentID
  WHERE employee.DepartmentID IS NULL
)</code>

注意:UNION ALL不会删除重复项,因此结果中可能包含一些重复值。

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