SQL aggregation with case statement and group by - sql

Question

I'm trying to understand the problem with this code:
SELECT COUNT(CASE liveIn.state
WHEN ("NY" OR "NJ") THEN "group1"
WHEN ("NC" or "SC") THEN "group2"
END) AS state_groups
FROM (SELECT DISTINCT user_guid, state
FROM users
WHERE country="US" AND country IS NOT NULL) AS liveIn
GROUP BY state_groups;
The error I get is: "Can't group on 'state_groups'"
I have other code that solve my problem which look like this (but I'm trying to understand the problem with the one above):
SELECT COUNT(DISTINCT user_guid),
CASE
WHEN (state="NY" OR state="NJ") THEN "group1"
WHEN (state="NC" OR state="SC") THEN "group2"
END AS state_groups
FROM users
WHERE country="US" AND country IS NOT NULL
GROUP BY state_groups;
my output should look like this:
TKS!
P.S.- this is part of coursera sql learning course, so I'm working on Jupyter.

Answer 1

Your case when returns group values but they are not useful when wrapped in a count, as they will all contribute as 1 to it. Furthermore, you cannot group by the aggregate you want to calculate based on a group. Note that you only have 1 column in your first SQL. You should have 2: one identifying the group, and the other the count. You merged the two into one, and that does not make sense.
This would be a valid alternative:
SELECT CASE state
WHEN "NY" THEN "group1"
WHEN "NJ" THEN "group1"
WHEN "NC" THEN "group2"
WHEN "SC" THEN "group2"
ELSE "others"
END AS state_group,
COUNT(DISTINCT user_guid) AS user_count
FROM USERS
WHERE country = "US"
GROUP BY state_group
SQL fiddle
Note that if you put an or in this variation of the when clause, the second term of that or will be evaluated separately, and make the condition true, so all values will then end up in group1.
You could use the other variant of the case when syntax, where you can use or or even better, in:
SELECT CASE WHEN state IN ("NY", "NJ") THEN "group1"
WHEN state IN ("NC", "SC") THEN "group2"
ELSE "others"
END AS state_group,
COUNT(DISTINCT user_guid) AS user_count
FROM USERS
WHERE country = "US"
GROUP BY state_group
Note that in this syntax, there is nothing between the case and the first when.