Unpivot multiple columns in Snowflake - sql

Question

I have a table that looks as follows:
I need to unpivot the Rating and the Comments as follows:
What is the best way to do this in Snowflake?
Note: there are some cells in the comment columns that are NULL
Adding details:
create or replace table reviews(name varchar(50), acting_rating int, acting_comments text, comedy_rating int, comedy_comments text);
insert into reviews values
('abc', 4, NULL, 1, 'NO'),
('xyz', 3, 'some', 1, 'haha'),
('lmn', 1, 'what', 4, NULL);
select * from reviews;
select name, skill, skill_rating, comments
from reviews
unpivot(skill_rating for skill in (acting_rating, comedy_rating))
unpivot(comments for skill_comments in (acting_comments,comedy_comments))
--Following where clause is added to filter the irrelevant comments due to multiple unpivots
where substr(skill,1,position('_',skill)-1) = substr(skill_comments,1,position('_',skill_comments)-1)
order by name;
will produce produce the desired results, but with data that has NULLs, the unpivoted rows that have NULLs go missing from the output:
NAME SKILL SKILL_RATING COMMENTS
abc COMEDY_RATING 1 NO
lmn ACTING_RATING 1 what
xyz ACTING_RATING 3 some
xyz COMEDY_RATING 1 haha

Answer 1

If all you need to solve is for the table specified in the question - you can do it manually with a set of UNION ALL:
select NAME
, 'ACTING_RATING' as SKILL, ACTING_RATING as SKILL_RATING, ACTING_COMMENTS as SKILL_COMMENTS
from DATA
union all
select NAME
, 'COMEDY_RATING', COMEDY_RATING, COMEDY_COMMENTS
from DATA
union all
select NAME
, 'MUSICAL_PERFORMANCE_RATING', MUSICAL_PERFORMANCE_RATING, MUSICAL_PERFORMANCE_COMMENTS
from DATA

Answer 2

This is a basic script and should give the desired output
create or replace table reviews(name varchar(50), acting_rating int, acting_comments text, comedy_rating int, comedy_comments text);
insert into reviews values
('abc', 4, 'something', 1, 'NO'),
('xyz', 3, 'some', 1, 'haha'),
('lmn', 1, 'what', 4, 'hahaha');
select * from reviews;
select name, skill, skill_rating, comments
from reviews
unpivot(skill_rating for skill in (acting_rating, comedy_rating))
unpivot(comments for skill_comments in (acting_comments,comedy_comments))
--Following where clause is added to filter the irrelevant comments due to multiple unpivots
where substr(skill,1,position('_',skill)-1) = substr(skill_comments,1,position('_',skill_comments)-1)
order by name;

Answer 3

If the goal is to store the unpivoted result as a table then INSERT ALL could be used to unpivot mutliple columns at once:
Setup:
create or replace table reviews(
name varchar(50), acting_rating int,
acting_comments text, comedy_rating int, comedy_comments text);
insert into reviews values
('abc', 4, NULL, 1, 'NO'),
('xyz', 3, 'some', 1, 'haha'),
('lmn', 1, 'what', 4, NULL);
select * from reviews;
Query:
CREATE OR REPLACE TABLE reviews_transposed(
name VARCHAR(50)
,skill TEXT
,skill_rating INT
,skill_comments TEXT
);
INSERT ALL
INTO reviews_transposed(name, skill, skill_rating, skill_comments)
VALUES (name, 'ACTING_RATING', acting_rating, acting_comments)
INTO reviews_transposed(name, skill, skill_rating, skill_comments)
VALUES (name, 'COMEDY_RATING', comedy_rating, comedy_comments)
SELECT *
FROM reviews;
SELECT *
FROM reviews_transposed;
Before:
After:
This approach has one significant advantage over UNION ALL approach proposed by Felippe, when saving into table (the number of table scans and thus partition read is growing for each UNION ALL wheareas INSERT ALL scans source table only once.
INSERT INTO reviews_transposed
select NAME
, 'ACTING_RATING' as SKILL, ACTING_RATING as SKILL_RATING, ACTING_COMMENTS as SKILL_COMMENTS
from reviews
union all
select NAME
, 'COMEDY_RATING', COMEDY_RATING, COMEDY_COMMENTS
from reviews;
vs INSERT ALL

Answer 4

Back in TSQL days i'd just use a CROSS APPLY. The nearest equivalent in snowflake would be something like:
create or replace TEMPORARY table reviews(name varchar(50), acting_rating int, acting_comments text, comedy_rating int, comedy_comments text);
insert into reviews values
('abc', 4, NULL, 1, 'NO'),
('xyz', 3, 'some', 1, 'haha'),
('lmn', 1, 'what', 4, NULL);
SELECT R.NAME
,P.VALUE:SKILL::VARCHAR(100) AS SKILL
,P.VALUE:RATING::NUMBER AS RATING
,P.VALUE:COMMENTS::VARCHAR(1000) AS COMMENTS
FROM reviews R
,TABLE(FLATTEN(INPUT => ARRAY_CONSTRUCT(
OBJECT_CONSTRUCT('SKILL','COMEDY','RATING',R.COMEDY_RATING,'COMMENTS',R.COMEDY_COMMENTS),
OBJECT_CONSTRUCT('SKILL','ACTING','RATING',R.ACTING_RATING,'COMMENTS',R.ACTING_COMMENTS)
)
)) AS P;
This only hits the source table once and preserves NULLs.
ResultSet

Answer 5

I've had same problem,
Here is my solution for unpivoting by two categories AND keeping nulls:
First you replace NULL's with some string, for example: 'NULL'
Then brake the two unpivots into two separate cte's and create common category column to join them again later, 'skill' in your case.
Lastly, join the two cte's by name and skill category, replace the 'NULL' string with actual NULL
create or replace table reviews(name varchar(50), acting_rating int, acting_comments text, comedy_rating int, comedy_comments text);
insert into reviews values
('abc', 4, 'something', 1, 'NO'),
('xyz', 3, 'some', 1, 'haha'),
('lmn', 1, 'what', 4, 'hahaha');
WITH base AS (SELECT name
, acting_rating
, IFNULL(acting_comments, 'NULL') AS acting_comments
, comedy_rating
, IFNULL(comedy_comments, 'NULL') AS comedy_comments
FROM reviews
)
, skill_rating AS (SELECT name
, REPLACE(skill, '_RATING', '') AS skill
, skill_rating
FROM base
UNPIVOT (skill_rating FOR skill IN (acting_rating, comedy_rating))
)
, comments AS (SELECT name
, REPLACE(skill_comments, '_COMMENTS', '') AS skill
, comments
FROM base
UNPIVOT (comments FOR skill_comments IN (acting_comments,comedy_comments))
)
SELECT s.name
, s.skill
, s.skill_rating
, NULLIF(c.comments, 'NULL') AS comments
FROM skill_rating AS s
JOIN comments AS c
ON s.name = c.name
AND s.skill = c.skill
ORDER BY name;
The result:
name skill skill_rating comments
abc ACTING 4 <null>
abc COMEDY 1 NO
lmn ACTING 1 what
lmn COMEDY 4 <null>
xyz ACTING 3 some
xyz COMEDY 1 haha