SQL combine 2 rows into 1 row then delete 1 - sql

Question

I'm trying to complete an SQL query homework question and cannot figure out how to add 2 rows together from the same table, with the intention of deleting one after they are combined.
I have a table name 'country', in it, I have 241 rows of data with columns.
name region area population GDP
------------------------------------------------------------------------
Hong Kong Southeast Asia 1040 5542869 136100000000
China Asia 9596960 1203097268 2978800000000
Expected output:
name region area population GDP
-------------------------------------------------------------------
Hong Kong Southeast Asia 1040 5542869 136100000000
China Asia 9598000 1208640137 3114900000000
The goal is to keep the name of the row as "China" and the region as "Asia" but adding the numeric values from the "Hong Kong" row to the "China" row for the columns (area, population, and GDP).
I have tried UNION and MERGE but I'm not familiar with using them and couldn't get it to work.
I feel like it has to be something like the SQL query below:
update country
set area = area.HongKong + area.China
where name = 'China';
but I don't know the proper way to reference a specific row.

Answer 1

Ah yes, homework. I remember those days.
Here is some code to help out -
--create a temp table to hold sample data
select
*
into #country
from
(
values
('Hong Kong', 'Southeast Asia', 1040, 5542869, 136100000000),
('China', 'Asia', 9596960, 1203097268, 2978800000000),
('USA', 'North America', 1, 10, 100) --some other row in table
) d ([name], region, area, population, gdp);
select * from #country;
Here is what the data looks like in this table:
--update statement
with
Totals as
(
select
count(1) rows, --should be 2
sum(area) areaTotal,
sum(population) populationTotal,
sum(gdp) gdpTotal
from #country
where [name] in ('Hong Kong', 'China') --this filter may be insufficient; it would be better to select by a primary key such as a rowID but the table does not have one
)
update c
set c.area = t.areaTotal,
c.population = t.populationTotal,
c.gdp = t.gdpTotal
from #country c
inner join totals t
on c.[name] = 'China';
Here is what the data looks like after the update:
There are a few noteworthy things here:
The code above the update statement, with Totals as (..., is called a common table expression (also abbreviated as CTE). It is being used here to contain the query that calculates the totals needed to perform the update. A CTE is one approach to creating an intermediate data set. Alternative approaches to this include: creating a temp table or using a derived table.
To see the results of the query within the CTE, those lines of code can be highlighted in your editor and executed against the database.
The original post said there are 241 rows of data in the country table. This means it is important to have a reliable filter in the query that calculates the totals so the correct rows for summing are isolated. The preferred way to filter the rows is to use a primary key, such as a rowID. Since this table does not have a primary key column, I made a guess and used the [name] column.
The purpose of count(1) rows in the CTE is to make sure only 2 rows are being summed. If this number comes back as anything other than 2, it means there is a problem with the filter and the where clause needs to be modified. The number of rows will only be visible when you do #2 above.
Now the Hong Kong row needs to be cleaned up:
--delete old row
delete from #country where [name] = 'Hong Kong';
This is what the data looks like now:
Again, it would be preferable to use a primary key column in the where clause instead of the [name] column to be certain that the correct row is being deleted.