SQLITE - combine a couples names (first and last name included) - sqlite

Question

I have two tables: People and Clients. Here is a basic sample.
'People' columns:
cliend_id|first_name|last_name
------------------------------
1 Dan Black
1 Jenn Black
2 Rob White
2 Lisa Green
'Clients' columns:
client_id|address
1 43 address
2 89 address
In the case where there is two partners (people) living in the same house (which in combination make a client), I am trying to combine the two to output something like "Black, Jenn & Dan"
I have the following
SELECT
clients.address_google Address,
group_concat(people.last_name || ", " || people.first_name, " & ") Name
FROM clients
INNER JOIN people ON people.client_id = clients.client_id
Which outputs a single line - Address and Name. My question is specific to the Name part.
If the last name is the same I would want "Black, Dan & Jenn"
If the last name is not the same, I would want "White, Rob & Green, Lisa"
Is there a way to test if the last name is the same, and if so omit it for the second person?

Answer 1

As a starter: you need to add a group by clause to the query to make it a valid aggregation query.
Then: if there always is 1 or 2 personn at a given address (not more), as shown in your sample data, then an option is a conditional expression:
select
c.address_google address,
case when min(p.last_name) <> max(p.last_name)
max(p.last_name) || ', ' || group_concat(p..first_name, ' & ')
else group_concat(p.last_name || ', ' || p.first_name, ' & ')
end as name
from clients c
inner join people p on p.client_id = c.client_id
group by c.address_google

Answer 2

You can use ROW_NUMBER() window function to get the 1st of the clients that has a last_name same as others:
SELECT
c.address_google Address,
GROUP_CONCAT(CASE WHEN p.rn = 1 THEN p.last_name || ", " ELSE '' END || p.first_name, " & ") Name
FROM clients c
INNER JOIN (
SELECT *, ROW_NUMBER() OVER (PARTITION BY client_id, last_name ORDER BY first_name) rn
FROM people
) p ON p.client_id = c.client_id
GROUP BY c.client_id, c.address_google;
See the demo.
Results:
| Address | Name |
| ---------- | ------------------------ |
| 43 address | Black, Dan & Jenn |
| 89 address | Green, Lisa & White, Rob |