Spelling corrections (#349)

Fixes some spelling errors in three different episodes.

_episodes/00-sql-introduction.md

@@ -120,7 +120,7 @@ Using a relational database serves several purposes.
 ## Database Management Systems
 
 There are different database management systems to work with relational databases
-such as SQLite, MySQL, Potsgresql, MSSQL Server, and many more. Each of them differ
+such as SQLite, MySQL, PostgreSQL, MSSQL Server, and many more. Each of them differ
 mainly based on their scalability, but they all share the same core principles of
 relational databases. In this lesson, we use SQLite to introduce you to SQL and
 data retrieval from a relational database.

_episodes/02-sql-aggregation.md

@@ -14,7 +14,7 @@ objectives:
 keypoints:
 - "Use the `GROUP BY` keyword to aggregate data."
 - "Functions like `MIN`, `MAX`, `AVG`, `SUM`, `COUNT`, etc. operate on aggregated data."
-- "Aliases can help shorten long queries. To write clear and readible queries, use the `AS` keyword when creating aliases."
+- "Aliases can help shorten long queries. To write clear and readable queries, use the `AS` keyword when creating aliases."
 - "Use the `HAVING` keyword to filter on aggregate properties."
 - "Use a `VIEW` to access the result of a query as though it was a new table."
 ---

_episodes/03-sql-joins.md

@@ -137,7 +137,7 @@ This is because, by default, SQL only returns records where the joining value
 is present in the joined columns of both tables (i.e. it takes the _intersection_
 of the two join columns). This joining behaviour is known as an `INNER JOIN`.
 In fact the `JOIN` keyword is simply shorthand for `INNER JOIN` and the two
-terms can be used interchangably as they will produce the same result.
+terms can be used interchangeably as they will produce the same result.
 
 We can also tell the computer that we wish to keep all the records in the first
 table by using a `LEFT OUTER JOIN` clause, or `LEFT JOIN` for short.