SQL query involving relation between multiple tables. SQlite3

Question

I have these three tables:

create table Nation ("nationkey" integer, 
                     "name" text, 
                     "regionkey" integer, 
                     "comment" text, 
                     "null" text,
                     foreign key (regionkey) references Region);

create table Supplier ("suppkey" integer, 
                       "name" text, 
                       "address" text, 
                       "nationkey" integer, 
                       "phone" text, 
                       "acctbal" real, 
                       "comment" text, 
                       "null" text,
                       foreign key (nationkey) references Nation);

create table Customer ("custkey" integer, 
                       "name" text, 
                       "address" text, 
                       "nationkey" integer, 
                       "phone" text, 
                       "acctbal" real, 
                       "mktsegment" text, 
                       "comment" text, 
                       "null" text, 
                       foreign key (nationkey) references Nation);

I have to write a sql query that returns the names of the nations that have more customers than suppliers. The query needs to be in Sqlite3. I am really new to sql and not sure how to go about doing this.

Answer 1

For a specific nation key, you can get the number of corresponding customers with COUNT:

SELECT COUNT(*)
FROM Customer
WHERE nationkey = ?

The same works for suppliers.

You can then use these COUNT queries as correlated subqueries to compare these values for each Nation record:

SELECT name
FROM Nation
WHERE (SELECT COUNT(*)
       FROM Customer
       WHERE nationkey = Nation.nationkey) >
      (SELECT COUNT(*)
       FROM Supplier
       WHERE nationkey = Nation.nationkey)

Answer 2

Another possible solution that uses explicit JOIN s:

SELECT n.name
FROM Nation n
JOIN Customer c ON n.nationkey = c.nationkey
JOIN Supplier s ON n.nationkey = s.nationkey
GROUP BY n.name
HAVING COUNT(c.nationkey) > COUNT(s.nationkey)