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

模擬SQLite中的FULL OUTER JOIN：使用UNION ALL

SQLite原生不支援FULL OUTER JOIN。但是，我們可以透過組合兩個LEFT JOIN並使用UNION ALL來達到相同的效果。

步驟一：建立左側連接基底表

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

步驟二：建立空值內部連接表

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

步驟三：聯合所有表

最後，使用UNION ALL組合兩個LEFT JOIN表以建立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不會刪除重複項，因此結果中可能包含一些重複值。

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