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

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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