Add lesson about set operation queries

src/data/courses/sql-mastery/chapters/multi-table-queries/lessons/set-operator-queries.md

@@ -1,25 +1,25 @@
 ---
-title: Combining Query Results
+title: Set Operator Queries
 description: Learn how to use UNION, INTERSECT, and EXCEPT to combine results from multiple queries
 order: 150
 type: lesson-challenge
 setup: |
   ```sql
-  CREATE TABLE fiction_books (
+  CREATE TABLE fiction_book (
       id INT PRIMARY KEY,
       title VARCHAR(255),
       author VARCHAR(255),
       price DECIMAL(10, 2)
   );
 
-  CREATE TABLE non_fiction_books (
+  CREATE TABLE non_fiction_book (
       id INT PRIMARY KEY,
       title VARCHAR(255),
       author VARCHAR(255),
       price DECIMAL(10, 2)
   );
 
-  INSERT INTO fiction_books (id, title, author, price)
+  INSERT INTO fiction_book (id, title, author, price)
   VALUES 
       (1, 'The Great Gatsby', 'F. Scott Fitzgerald', 24.99),
       (2, '1984', 'George Orwell', 19.99),
@@ -27,7 +27,7 @@ setup: |
       (4, 'The Hobbit', 'J.R.R. Tolkien', 29.99),
       (5, 'The Doors of Perception', 'Aldous Huxley', 12.99);
 
-  INSERT INTO non_fiction_books (id, title, author, price)
+  INSERT INTO non_fiction_book (id, title, author, price)
   VALUES 
       (1, 'A Brief History of Time', 'Stephen Hawking', 29.99),
       (2, 'The Art of War', 'Sun Tzu', 19.99),
@@ -52,11 +52,7 @@ setup: |
 
 Sometimes you need to combine results from multiple queries into a single result set. SQL provides several set operations for this purpose: `UNION`, `UNION ALL`, `INTERSECT`, and `EXCEPT`.
 
-## UNION and UNION ALL
+Let' take a look at some examples to understand how these set operations work. I have setup the following two tables `fiction_book` and `non_fiction_book` with the following data:
-
-`UNION` combines results from two or more queries and removes duplicates. `UNION ALL` does the same but keeps duplicates.
-
-I have setup the following two tables `fiction_books` and `non_fiction_books` with the following data:
 
 | id  | title                   | author              | price |
 | --- | ----------------------- | ------------------- | ----- |
@@ -74,16 +70,20 @@ I have setup the following two tables `fiction_books` and `non_fiction_books` wi
 | 4   | Pride and Prejudice: A Study Guide | John Smith        | 12.99 |
 | 5   | The Doors of Perception            | Aldous Huxley     | 12.99 |
 
+Let's start with the `UNION` operations.
+
+`UNION` combines results from two or more queries and removes duplicates. `UNION ALL` does the same but keeps duplicates.
+
 ### UNION Example
 
-Let's find all books across fiction and non-fiction categories:
+Let's find all books across `fiction_book` and `non_fiction_book` tables using a `UNION` operation:
 
 ```sql
 SELECT title, author, price
-FROM fiction_books
+FROM fiction_book
 UNION
 SELECT title, author, price
-FROM non_fiction_books
+FROM non_fiction_book
 ORDER BY title;
 ```
 
@@ -101,16 +101,16 @@ The query above will combine the results from both the queries and remove any du
 | The Great Gatsby                   | F. Scott Fitzgerald | 24.99 |
 | The Hobbit                         | J.R.R. Tolkien      | 29.99 |
 
-Notice how the `The Doors of Perception` book is only listed once even though it appears in both the `fiction_books` and `non_fiction_books` tables.
+Notice how the `The Doors of Perception` book is only listed once even though it appears in both the `fiction_book` and `non_fiction_book` tables.
 
 ### UNION ALL Example
 
 If you want to keep duplicates, use UNION ALL:
 
 ```sql
-SELECT title, author, price FROM fiction_books
+SELECT title, author, price FROM fiction_book
 UNION ALL
-SELECT title, author, price FROM non_fiction_books
+SELECT title, author, price FROM non_fiction_book
 ORDER BY title;
 ```
 
@@ -133,12 +133,12 @@ Notice how the `The Doors of Perception` book is listed twice.
 
 ## INTERSECT
 
-`INTERSECT` returns only the rows that appear in both result sets. Let's find books that are in both the `fiction_books` and `non_fiction_books` tables:
+`INTERSECT` returns only the rows that appear in both result sets. Let's find books that are in both the `fiction_book` and `non_fiction_book` tables:
 
 ```sql
-SELECT title, author FROM fiction_books
+SELECT title, author FROM fiction_book
 INTERSECT
-SELECT title, author FROM non_fiction_books;
+SELECT title, author FROM non_fiction_book;
 ```
 
 This will return:
@@ -149,12 +149,12 @@ This will return:
 
 ## EXCEPT
 
-`EXCEPT` returns rows from the first query that don't appear in the second query. Let's find the books in the `fiction_books` table that aren't in the `non_fiction_books` table:
+`EXCEPT` returns rows from the first query that don't appear in the second query. Let's find the books in the `fiction_book` table that aren't in the `non_fiction_book` table:
 
 ```sql
-SELECT title, author FROM fiction_books
+SELECT title, author FROM fiction_book
 EXCEPT
-SELECT title, author FROM non_fiction_books
+SELECT title, author FROM non_fiction_book
 ORDER BY title;
 ```
 
@@ -177,24 +177,48 @@ For example, below is an invalid query:
 
 ```sql
 -- ERROR: each UNION query must have the same number of columns
-SELECT title, author FROM fiction_books
+SELECT title, author FROM fiction_book
 UNION
-SELECT title, author, price FROM non_fiction_books;
+SELECT title, author, price FROM non_fiction_book;
 ```
 
-Because the `fiction_books` table has 2 columns and the `non_fiction_books` table has 3 columns, the query will return an error.
+The above query will return an error because the `fiction_book` table has 2 columns and the `non_fiction_book` table has 3 columns.
 
 Similary if we try to combine the following two queries:
 
 ```sql
 -- ERROR: UNION types numeric and character varying cannot be matched
-SELECT title, price FROM fiction_books
+SELECT title, price FROM fiction_book
 UNION
-SELECT title, author FROM non_fiction_books;
+SELECT title, author FROM non_fiction_book;
 ```
 
 This will also return an error because the `price` column from the first query doesn't have the same data type as the `author` column from the second query.
 
+Also, another important thing to note for the set operations is that the column names in the result set will be the column names from the first query. For example, if we run the following query:
+
+```sql
+SELECT title, author as 'writer_name' FROM fiction_book
+UNION
+SELECT title, author FROM non_fiction_book;
+```
+
+The output will be:
+
+| title                              | writer_name         |
+| ---------------------------------- | ------------------- |
+| A Brief History of Time            | Stephen Hawking     |
+| The Hobbit                         | J.R.R. Tolkien      |
+| The Great Gatsby                   | F. Scott Fitzgerald |
+| Sapiens                            | Yuval Noah Harari   |
+| The Art of War                     | Sun Tzu             |
+| Pride and Prejudice: A Study Guide | John Smith          |
+| 1984                               | George Orwell       |
+| The Doors of Perception            | Aldous Huxley       |
+| Pride and Prejudice                | Jane Austen         |
+
+Notice how it took the `writer_name` column alias from the first query for the name of the column in the result set.
+
 ## Challenge Time! 🎯
 
 Using the provided tables, write queries to:
@@ -209,22 +233,22 @@ Using the provided tables, write queries to:
 ```sql
 -- 1. Books that cost $19.99
 SELECT title, author, 'Fiction' as category
-FROM fiction_books
+FROM fiction_book
 WHERE price = 19.99
 UNION
 SELECT title, author, 'Non-Fiction' as category
-FROM non_fiction_books
+FROM non_fiction_book
 WHERE price = 19.99;
 
 -- 2. Authors in both categories
-SELECT author FROM fiction_books
+SELECT author FROM fiction_book
 INTERSECT
-SELECT author FROM non_fiction_books;
+SELECT author FROM non_fiction_book;
 
 -- 3. Summer reads not in fiction
 SELECT title, author FROM summer_reads
 EXCEPT
-SELECT title, author FROM fiction_books;
+SELECT title, author FROM fiction_book;
 ```
 
 </details>