mysql - 三张表的全外连接

Question

如何在三个表上执行完全外部联接？

我已经在两个表上成功地进行了联接，但是我对如何将第三个表添加到我的查询一无所知。

这是我的（冗长的）查询：

SELECT MONTH(t1.datetime) AS month, COUNT(DISTINCT t1.column) AS t1Count, COUNT(DISTINCT t2.column) AS t2Count
FROM t1
LEFT OUTER JOIN t2 ON MONTH(t2.datetime) = MONTH(t1.datetime)
GROUP BY MONTH(t1.datetime)
UNION
SELECT MONTH(t1.datetime) AS month, COUNT(DISTINCT t1.column) AS t1Count, COUNT(DISTINCT t2.column) AS t2Count
FROM t1 
RIGHT OUTER JOIN t2 ON MONTH(t2.datetime) = MONTH(t1.datetime)
GROUP BY MONTH(t2.datetime)

非常感谢任何指导。

Answer 1

试试这个查询 -

（已编辑）

SELECT
  t.month,
  COUNT(DISTINCT t1.column) t1count,
  COUNT(DISTINCT t2.column) t2count,
  COUNT(DISTINCT t3.column) t3count
FROM 
  (SELECT MONTH(datetime) AS month FROM t1
    UNION
  SELECT MONTH(datetime) AS month FROM t2
    UNION
  SELECT MONTH(datetime) AS month FROM t3
  ) t
  LEFT JOIN t1
    ON t.month = MONTH(t1.datetime)
  LEFT JOIN t2
    ON t.month = MONTH(t2.datetime)
  LEFT JOIN t3
    ON t.month = MONTH(t3.datetime)
  GROUP BY
    month