问题
How would one carry out a full outer join on three tables?
I have already carried my join successfully on two tables, but I am clueless on how to add the third table to my query.
Here is my (long-winded) query:
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)
Any guidance is much appreciated.
回答1:
Try this query -
(edited)
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
来源:https://stackoverflow.com/questions/9226127/full-outer-join-on-three-tables