MySQL：使用兩個表表達完全聯接，這些表具有指向公共基表的外鍵

Question

假設我們需要在表1-3上執行兩個FULL JOIN （以PostgreSQL術語）的查詢，其中表2和表3都具有指向表1的外鍵。 這將在MySQL中起作用，而FULL JOIN不可用。 因此，必須找到一種解決方法。

從LEFT JOIN s開始，以下代碼如（1）中所述工作 ：

SELECT table1.ID, table1.x, table2.y, table3.z
   FROM (table1
      LEFT JOIN table2
         ON table1.ID = table2.FOREIGN_ID)
      LEFT JOIN table3
         ON table1.ID = table3.FOREIGN_ID

在（2）之后 ，我可以使用UNION語句為單個UNION模擬完整的外部UNION ：

SELECT table1.ID, table1.x, table2.y
   FROM table1
      LEFT JOIN table2
         ON table1.ID = table2.FOREIGN_ID
UNION
SELECT table1.ID, table1.x, table2.y
   FROM table1
      RIGHT JOIN table2
         ON table1.ID = table2.FOREIGN_ID

生產

和

SELECT table1.ID, table1.x, table3.z
   FROM table1
       LEFT JOIN table3
         ON table1.ID = table3.FOREIGN_ID
UNION
SELECT table1.ID, table1.x, table3.z
   FROM table1
       RIGHT JOIN table3
         ON table1.ID = table3.FOREIGN_ID
ORDER BY ID

生產

我需要有關如何從此處獲取此輸出的建議：

也就是說，通過外鍵引用來自table1的條目的結果行應合理地合並，而來自table2和table3的雜散條目（外鍵為NULL的那些條目）應在額外的行中列出（在示例中為y2和z4） 。

Answer 1

事實證明，在PostgreSQL中這將是一個簡單的兩步式FULL JOIN ：

SELECT table1.id, table1.x, table2.y, table3.z
  FROM table1
  FULL JOIN table2
    ON table2.Foreign_Id = table1.id
  FULL JOIN table3
    ON table3.Foreign_Id = table1.id
 ORDER BY table1.id, table2.id, table3.id;

從freenode #sql向大家大聲疾呼，指出這個等效的MySQL查詢，避免使用FULL JOIN ：

SELECT id1, x, y, z FROM (
SELECT table1.id as id1, table1.x, table2.id as id2, table2.y, table3.id as id3, table3.z
  FROM table1
  LEFT JOIN table2
    ON table2.Foreign_Id = table1.id
  LEFT JOIN table3
    ON table3.Foreign_Id = table1.id
 UNION ALL
SELECT NULL, NULL, table2.id, table2.y, NULL, NULL
  FROM table2 WHERE NOT EXISTS (SELECT 1 FROM table1 WHERE table1.id = table2.Foreign_Id)
 UNION ALL
SELECT NULL, NULL, NULL, NULL, table3.id, table3.z
  FROM table3 WHERE NOT EXISTS (SELECT 1 FROM table1 WHERE table1.id = table3.Foreign_Id)
 ORDER BY id1, id2, id3 ) AS T