sql: comparting two tables with different values produces duplicated results - sql

Question

I want to join/union two tables that have primary key, category, and a score, in such a way, that results will show the primary key and all categories and scores present in both tables together, and, if a given category is in only one table, then with a null for the score from the second table.
The tables are as follow:
opinion_1
fruit category score
apple color 15
apple sweet 50
apple scent 35
orange color 40
orange sweet 60
opinion_2
fruit category score
apple color 28
apple sweet 12
orange color 29
orange sweet 50
orange scent 31
I've tried full outer joining and double left joining with union, getting the same results with categories incorrectly multiplied.
WITH opinion_1 AS (
SELECT 'apple' as fruit, 'color' as category, 15 as score UNION ALL
SELECT 'apple', 'sweet', 50 UNION ALL
SELECT 'apple', 'scent', 35 UNION ALL
SELECT 'orange', 'color', 40 UNION ALL
SELECT 'orange', 'sweet', 60
), opinion_2 AS (
SELECT 'apple' as fruit, 'color' as category, 28 as score UNION ALL
SELECT 'apple', 'sweet', 12 UNION ALL
SELECT 'orange', 'color', 29 UNION ALL
SELECT 'orange', 'sweet', 50 UNION ALL
SELECT 'orange', 'scent', 31
)
SELECT
opinion_1.fruit,
opinion_1.category as category,
opinion_1.score as score1,
opinion_2.score as score2
FROM opinion_1
full outer join opinion_2
on opinion_1.fruit = opinion_2.fruit
I expect the following result of the operation:
fruit category score1 score2
apple color 15 28
apple sweet 50 12
apple scent 35 null
orange color 40 29
orange sweet 60 50
orange scent null 31
but I'm getting this:
fruit category score1 score2
apple color 15 12
apple color 15 28
apple sweet 50 12
apple sweet 50 28
apple scent 35 12
apple scent 35 28
orange color 40 50
orange color 40 31
orange color 40 29
orange sweet 60 50
orange sweet 60 31
orange sweet 60 29

Answer 1

I think you are missing a condition on your join to get the result you expect. Moreover, selecting opinion_1.fruit and opinion_1.category will produce nulls if there is no records for some fruit on opinion_1 whilst there are on opinion_2. The following query will produce the expected result :
WITH opinion_1 AS (
SELECT 'apple' as fruit, 'color' as category, 15 as score UNION ALL
SELECT 'apple', 'sweet', 50 UNION ALL
SELECT 'apple', 'scent', 35 UNION ALL
SELECT 'orange', 'color', 40 UNION ALL
SELECT 'orange', 'sweet', 60
), opinion_2 AS (
SELECT 'apple' as fruit, 'color' as category, 28 as score UNION ALL
SELECT 'apple', 'sweet', 12 UNION ALL
SELECT 'orange', 'color', 29 UNION ALL
SELECT 'orange', 'sweet', 50 UNION ALL
SELECT 'orange', 'scent', 31
)
SELECT
coalesce(opinion_1.fruit, opinion_2.fruit) as fruit,
coalesce(opinion_1.category, opinion_2.category) as category,
opinion_1.score as score1,
opinion_2.score as score2
FROM opinion_1
full outer join opinion_2
on opinion_1.fruit = opinion_2.fruit and opinion_1.category = opinion_2.category

Answer 2

Below is for BigQuery Standard SQL
#standardSQL
WITH opinion_1 AS (
SELECT 'apple' AS fruit, 'color' AS category, 15 AS score UNION ALL
SELECT 'apple', 'sweet', 50 UNION ALL
SELECT 'apple', 'scent', 35 UNION ALL
SELECT 'orange', 'color', 40 UNION ALL
SELECT 'orange', 'sweet', 60
), opinion_2 AS (
SELECT 'apple' AS fruit, 'color' AS category, 28 AS score UNION ALL
SELECT 'apple', 'sweet', 12 UNION ALL
SELECT 'orange', 'color', 29 UNION ALL
SELECT 'orange', 'sweet', 50 UNION ALL
SELECT 'orange', 'scent', 31
)
SELECT
IFNULL(a.fruit, b.fruit) fruit,
IFNULL(a.category, b.category) AS category,
a.score AS score1,
b.score AS score2
FROM opinion_1 a
FULL OUTER JOIN opinion_2 b
USING(fruit, category)
with result
Row fruit category score1 score2
1 apple color 15 28
2 apple sweet 50 12
3 apple scent 35 null
4 orange color 40 29
5 orange sweet 60 50
6 orange scent null 31