How to FULL OUTER JOIN multiple tables in MySQL - mysql

Question

I need to FULL OUTER JOIN multiple tables. I know how to FULL OUTER JOIN two tables from here. But I have several tables, and I can't apply it over them. How can I achieve it?
My SQL code, below:
INSERT INTO table
(
customer_id
,g01
,g02
,g03
,has_card
,activity
)
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
LEFT JOIN s_category sc
ON sc.customer_id = sgd.customer_id
UNION
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
RIGHT JOIN s_category sc
ON sc.customer_id = sgd.customer_id
UNION
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
LEFT JOIN s_activity a
ON a.customer_id = sgd.customer_id
UNION
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
RIGHT JOIN s_activity a
ON a.customer_id = sgd.customer_id
Also I tried this query:
INSERT INTO reportls.table
(
customer_id
,g01
,g02
,g03
,has_card
,activity
)
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
LEFT JOIN s_category sc
ON sc.customer_id = sgd.customer_id
LEFT JOIN s_activity a
ON sc.customer_id = sgd.customer_id
UNION
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
LEFT JOIN s_category sc
ON sc.customer_id = sgd.customer_id
RIGHT JOIN s_activity a
ON a.customer_id = sgd.customer_id
UNION
SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
FROM s_geo_data sgd
RIGHT JOIN s_category sc
ON sc.customer_id = sgd.customer_id
LEFT JOIN s_activity a
ON a.customer_id = sgd.customer_id
Last query executes very long time, I need faster query.

Answer 1

I think to have a FULL OUTER JOIN over 3 tables, you need to do it like this:
SELECT t1.value, t2.value, t3.value
FROM t1 LEFT JOIN t2 ON t1.value = t2.value
LEFT JOIN t3 ON t1.value = t3.value
UNION ALL
SELECT t1.value, t2.value, t3.value
FROM t2 LEFT JOIN t1 ON t1.value = t2.value
LEFT JOIN t3 ON t2.value = t3.value
WHERE t1.value IS NULL
UNION ALL
SELECT t1.value, t2.value, t3.value
FROM t3 LEFT JOIN t1 ON t1.value = t3.value
LEFT JOIN t2 ON t2.value = t3.value
WHERE t1.value IS NULL AND t2.value IS NULL
As an alternative for this:
SELECT t1.value, t2.value, t3.value
FROM t1 FULL OUTER JOIN t2 ON t1.value = t2.value
FULL OUTER JOIN t3 ON t1.value = t3.value
I suggest you to create some temporary tables like t1, t2 and t3 for storing results of your queries, then use above query over those.