Add dropping and truncating

src/data/courses/sql/chapters/data-definition-language/lessons/dropping-or-truncate.md

@@ -0,0 +1,148 @@
+---
+title: Dropping and Truncating
+description: Learn about dropping tables and truncating data in PostgreSQL.
+order: 180
+type: lesson-challenge
+---
+
+There are times when we need to perform destructive operations on our tables such as removing a table (aka dropping a table) or removing all the data from a table (aka truncating a table).
+
+In this final lesson of the chapter, we will look at different ways to perform these operations.
+
+## Dropping Tables
+
+Dropping a table removes the table and all its data from the database. The syntax for dropping a table is as follows:
+
+```sql
+DROP TABLE table_name;
+```
+
+Given below is a sample query to drop the `books` table:
+
+```sql
+DROP TABLE books;
+```
+
+### Error Handling with DROP TABLE
+
+If you try to drop a table that doesn't exist, you'll get an error. To prevent this, you can use the `IF EXISTS` clause:
+
+```sql
+DROP TABLE IF EXISTS books;
+```
+
+## Truncating Tables
+
+The `TRUNCATE` statement removes all rows from a table but keeps the table structure intact. For example:
+
+```sql
+TRUNCATE TABLE orders;
+```
+
+This will remove all the rows from the `orders` table but the table structure will remain intact.
+
+### RESTART IDENTITY
+
+If you have a table with an auto-incrementing column e.g. in the `books` table below, we have an `id` column that is set to auto-increment (i.e. `SERIAL`).
+
+```sql
+CREATE TABLE books (
+  id SERIAL PRIMARY KEY,
+  title VARCHAR(255),
+  author VARCHAR(255)
+);
+
+-- Inserting 3 rows into the books table
+INSERT INTO books (title, author)
+VALUES ('The Great Gatsby', 'F. Scott Fitzgerald'),
+       ('1984', 'George Orwell'),
+       ('To Kill a Mockingbird', 'Harper Lee');
+
+-- Selecting all the rows from the books table
+SELECT * FROM books;
+```
+
+The `books` table will look like this after the above queries:
+
+| id  | title                 | author              |
+| --- | --------------------- | ------------------- |
+| 1   | The Great Gatsby      | F. Scott Fitzgerald |
+| 2   | 1984                  | George Orwell       |
+| 3   | To Kill a Mockingbird | Harper Lee          |
+
+Because the `id` column is set to auto-increment, the next time we insert a new book, the `id` column will start from 4 and so on.
+
+If we now truncate the `books` table, and start inserting new books, the `id` column will still start from 4 and so on.
+
+```sql
+-- Truncating the books table
+TRUNCATE TABLE books;
+
+-- Inserting 3 new rows into the books table
+INSERT INTO books (title, author)
+VALUES ('The Catcher in the Rye', 'J.D. Salinger'),
+       ('Pride and Prejudice', 'Jane Austen'),
+       ('The Hobbit', 'J.R.R. Tolkien');
+
+-- Selecting all the rows from the books table
+SELECT * FROM books;
+```
+
+The output will be:
+
+| id  | title                  | author         |
+| --- | ---------------------- | -------------- |
+| 4   | The Catcher in the Rye | J.D. Salinger  |
+| 5   | Pride and Prejudice    | Jane Austen    |
+| 6   | The Hobbit             | J.R.R. Tolkien |
+
+Notice that even though we truncated the `books` table, the `id` column still started from 4 and so on. This is because by default, `TRUNCATE` doesn't reset the auto-increment counter.
+
+To reset the auto-increment counter, you can use the `RESTART IDENTITY` clause. This will reset the auto-increment counter to 1.
+
+```sql
+-- Reset auto-increment counter
+TRUNCATE TABLE books RESTART IDENTITY;
+
+INSERT INTO books (title, author)
+VALUES ('The Catcher in the Rye', 'J.D. Salinger'),
+       ('Pride and Prejudice', 'Jane Austen'),
+       ('The Hobbit', 'J.R.R. Tolkien');
+
+SELECT * FROM books;
+```
+
+The output will be:
+
+| id  | title                  | author         |
+| --- | ---------------------- | -------------- |
+| 1   | The Catcher in the Rye | J.D. Salinger  |
+| 2   | Pride and Prejudice    | Jane Austen    |
+| 3   | The Hobbit             | J.R.R. Tolkien |
+
+The opposite of `RESTART IDENTITY` is `CONTINUE IDENTITY`. This will keep the current auto-increment counter value.
+
+```sql
+TRUNCATE TABLE books CONTINUE IDENTITY;
+
+INSERT INTO books (title, author)
+VALUES ('The Catcher in the Rye', 'J.D. Salinger'),
+       ('Pride and Prejudice', 'Jane Austen'),
+       ('The Hobbit', 'J.R.R. Tolkien');
+
+SELECT * FROM books;
+```
+
+The output will now be:
+
+| id  | title                  | author         |
+| --- | ---------------------- | -------------- |
+| 4   | The Catcher in the Rye | J.D. Salinger  |
+| 5   | Pride and Prejudice    | Jane Austen    |
+| 6   | The Hobbit             | J.R.R. Tolkien |
+
+Notice how the `id` column started from 4 and so on.
+
+---
+
+Remember, both `DROP` and `TRUNCATE` are destructive operations that cannot be undone. Always double-check before executing these commands in a production environment!

src/data/courses/sql/chapters/data-definition-language/lessons/primary-key-constraint.md

@@ -5,7 +5,7 @@ order: 160
 type: lesson-challenge
 ---
 
-In our previous lessons, we learned about various constraints that help maintain data integrity. In this lesson, we'll look at two of the most important constraints in SQL: Primary Keys and Foreign Keys.
+In our previous lessons, we learned about various constraints that help maintain data integrity. In this lesson we will learn about primary keys and how to use them effectively.
 
 ## Primary Keys