They have given you the correct answers. Based on your posts it seems you do not know much about Relational databases.
1, HP Potion
2, MP Potion
3, XP Potion
1,1,5 - Row 0 (Character 1 has HP Potion x5)
1,2,10 - Row 1 (Character 1 has MP Potion x10)
2,3,15 - Row 2 (Character 2 has XP Potion x15)
2,1,3 - Row 3 (Character 2 has HP Potion x3)
Executing this SQL:
SELECT * From Character will bring back information for all characters (rows 0 and 1 in Character table)
SELECT * FROM Character WHERE character_id=1 will bring back row 0 in Character Table "Juicebox"
SELECT * FROM Item will bring back all rows in t he item table
SELECT * FROM Item WHERE name='HP Potion' will bring back 1 row for every item named HP Potion
SELECT * FROM Item WHERE item_id=1 will bring back 1 row for item_id 1 (Row 0 - HP Potion)
SELECT * FROM Character_Item will bring back all rows
SELECT * FROM Character_Item WHERE character_id=1 will only bring back row 0 and 1.
SELECT * FROM Character_Item WHERE character_id=2 will only bring back row 2 and 3.
SELECT * FROM Character_Item WHERE character_id=2 AND item_id=3 will only bring back row 2 (you can run something like this to see if a character has a certain item ID)
Here is some basic SQL:
Note: SQL Server and MySQL both use SQL - there may be slight differences in syntax / keywords, however, the concepts are the same.
I advise you to pickup mysql and mysql workbench and work on creating tables and writing queries such as:
INSERT queries (http://www.techonthenet.com/sql/insert.php)