Update tech_docs/database/sql_guide.md

tech_docs/database/sql_guide.md

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+### SQL Basics
+
+#### 1. **Database and Table Structure**
+- **Database:** A collection of related data organized for easy access and management.
+- **Table:** A structured set of data organized in rows and columns within a database.
+
+#### 2. **Basic SQL Commands**
+- **SELECT:** Used to fetch data from a database.
+- **INSERT:** Used to add new data into a table.
+- **UPDATE:** Used to modify existing data in a table.
+- **DELETE:** Used to remove data from a table.
+
+### Writing SQL Queries
+
+#### 1. **SELECT Statement**
+The `SELECT` statement is used to retrieve data from a database.
+
+```sql
+SELECT column1, column2, ...
+FROM table_name;
+```
+
+- **Example:**
+
+```sql
+SELECT Name, Composer
+FROM Track;
+```
+
+This query retrieves the `Name` and `Composer` columns from the `Track` table.
+
+#### 2. **WHERE Clause**
+The `WHERE` clause is used to filter records.
+
+```sql
+SELECT column1, column2, ...
+FROM table_name
+WHERE condition;
+```
+
+- **Example:**
+
+```sql
+SELECT Name, Composer
+FROM Track
+WHERE GenreId = 1;
+```
+
+This query retrieves the names and composers of tracks in the genre with `GenreId` 1.
+
+#### 3. **INSERT INTO Statement**
+The `INSERT INTO` statement is used to add new records to a table.
+
+```sql
+INSERT INTO table_name (column1, column2, ...)
+VALUES (value1, value2, ...);
+```
+
+- **Example:**
+
+```sql
+INSERT INTO Artist (Name)
+VALUES ('New Artist');
+```
+
+This query adds a new artist named 'New Artist' to the `Artist` table.
+
+#### 4. **UPDATE Statement**
+The `UPDATE` statement is used to modify existing records.
+
+```sql
+UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition;
+```
+
+- **Example:**
+
+```sql
+UPDATE Track
+SET Composer = 'New Composer'
+WHERE TrackId = 1;
+```
+
+This query updates the composer of the track with `TrackId` 1 to 'New Composer'.
+
+#### 5. **DELETE Statement**
+The `DELETE` statement is used to remove existing records from a table.
+
+```sql
+DELETE FROM table_name
+WHERE condition;
+```
+
+- **Example:**
+
+```sql
+DELETE FROM Artist
+WHERE ArtistId = 1;
+```
+
+This query removes the artist with `ArtistId` 1.
+
+### Advanced SQL Concepts
+
+#### 1. **JOINs**
+JOINs are used to combine rows from two or more tables based on a related column.
+
+- **INNER JOIN:** Returns records with matching values in both tables.
+
+  ```sql
+  SELECT columns
+  FROM table1
+  INNER JOIN table2
+  ON table1.common_column = table2.common_column;
+  ```
+
+- **Example:**
+
+  ```sql
+  SELECT a.Name, t.Title
+  FROM Album a
+  INNER JOIN Artist ar ON a.ArtistId = ar.ArtistId
+  WHERE ar.Name = 'AC/DC';
+  ```
+
+  This query returns the names of albums by the artist 'AC/DC'.
+
+- **LEFT JOIN:** Returns all records from the left table and the matched records from the right table.
+
+  ```sql
+  SELECT columns
+  FROM table1
+  LEFT JOIN table2
+  ON table1.common_column = table2.common_column;
+  ```
+
+- **RIGHT JOIN:** Returns all records from the right table and the matched records from the left table.
+
+  ```sql
+  SELECT columns
+  FROM table1
+  RIGHT JOIN table2
+  ON table1.common_column = table2.common_column;
+  ```
+
+- **FULL OUTER JOIN:** Returns all records when there is a match in either left or right table.
+
+  ```sql
+  SELECT columns
+  FROM table1
+  FULL OUTER JOIN table2
+  ON table1.common_column = table2.common_column;
+  ```
+
+#### 2. **GROUP BY and HAVING**
+- **GROUP BY:** Used to arrange identical data into groups.
+
+  ```sql
+  SELECT column1, COUNT(*)
+  FROM table_name
+  GROUP BY column1;
+  ```
+
+- **Example:**
+
+  ```sql
+  SELECT GenreId, COUNT(*)
+  FROM Track
+  GROUP BY GenreId;
+  ```
+
+  This query counts the number of tracks in each genre.
+
+- **HAVING:** Used to filter groups.
+
+  ```sql
+  SELECT column1, COUNT(*)
+  FROM table_name
+  GROUP BY column1
+  HAVING COUNT(*) > 1;
+  ```
+
+- **Example:**
+
+  ```sql
+  SELECT GenreId, COUNT(*)
+  FROM Track
+  GROUP BY GenreId
+  HAVING COUNT(*) > 10;
+  ```
+
+  This query returns genres with more than 10 tracks.
+
+#### 3. **ORDER BY**
+The `ORDER BY` statement is used to sort the result set.
+
+```sql
+SELECT column1, column2, ...
+FROM table_name
+ORDER BY column1 ASC|DESC;
+```
+
+- **Example:**
+
+```sql
+SELECT Name, Composer
+FROM Track
+ORDER BY Name ASC;
+```
+
+This query retrieves track names and composers sorted by the track name in ascending order.
+
+### Practical Example
+
+Consider the following database schema in the Chinook database:
+
+- **Artist Table:**
+
+| ArtistId | Name       |
+| -------- | ---------- |
+| 1        | AC/DC      |
+| 2        | Accept     |
+
+- **Album Table:**
+
+| AlbumId | Title         | ArtistId |
+| ------- | ------------- | -------- |
+| 1       | For Those...  | 1        |
+| 2       | Balls to...   | 2        |
+
+#### Query Examples
+
+- **Fetch all tracks in the 'Rock' genre:**
+
+  ```sql
+  SELECT Name, Composer
+  FROM Track
+  WHERE GenreId = 1;
+  ```
+
+- **Fetch all artists and the number of albums they have:**
+
+  ```sql
+  SELECT ar.Name, COUNT(al.AlbumId) AS album_count
+  FROM Artist ar
+  LEFT JOIN Album al ON ar.ArtistId = al.ArtistId
+  GROUP BY ar.Name;
+  ```
+
+This should give you a solid foundation to start with SQL using the Chinook database. 
+
+---
+
 # Complete Guide to Getting Started with SQL and SQLite3
 
 ## Introduction to SQL