what’s the difference between an inner and outer join?

• A left outer join retains all of the rows of the left table, regardless of whether there is a row that matches on the right table.
• A right outer join is pretty much the same thing as a left outer join, except that the rows that are retained are from the right table.

The Definition of A Key in ?

• According to the SQL Standard, a key is a subset of columns in a table that allow a row to be uniquely identified. So, a key can be more than just one column. And, every row in the table will have a unique value for the key – or a unique combination of values if the key consists of more than just one column.

What is Foreign key? Can a table have multiple unique, foreign, and/or primary keys?

• Foreign keys are used to reference unique columns in another table. So, for example, a foreign key can be defined on one table A, and it can reference some unique column(s) in another table B.
• A table can have multiple unique and foreign keys. However, a table can have only one primary key.