feat(jdbc-sqlite): add DML operations (INSERT, DELETE, UPDATE) in Database example

What
- Enhanced `Database.java` under Section28JDBCusingSQLite with Data Manipulation Language (DML) examples.
- Demonstrates how to perform:
  - `INSERT` → add new department records into `dept` table.
  - `DELETE` → remove rows based on condition (`deptno >= 60`).
  - `UPDATE` → modify existing row values (e.g., update department name for `deptno=50`).
- Added inline comments explaining constraints (e.g., PRIMARY KEY uniqueness).

Why
- Provides hands-on examples of executing SQL write operations using JDBC.
- Highlights the importance of constraints like `PRIMARY KEY` and error handling.
- Shows how Java applications interact with relational databases to manipulate data.

How
- Setup:
  - Loaded SQLite JDBC driver with `Class.forName("org.sqlite.JDBC")`.
  - Established connection via `DriverManager.getConnection("jdbc:sqlite:.../univ.db")`.
- Execution:
  - Created `Statement` object to run SQL commands.
  - Used `executeUpdate()` for INSERT, DELETE, and UPDATE.
- Resource management:
  - Closed `Statement` and `Connection` to free resources.

Key Notes
- Running INSERT with duplicate primary key values results in `PRIMARY KEY constraint failed`.
- DELETE operation removes rows permanently; always test with WHERE conditions.
- UPDATE modifies specific records; ensure WHERE clause is accurate to avoid accidental bulk updates.
- JDBC auto-creates database file if it does not exist in the given path.

Real-life Applications
- Enterprise apps: employee/department management systems.
- Inventory management: inserting, updating, and removing stock entries.
- Banking apps: updating balances, deleting old records, inserting new transactions.
- Education systems: maintaining student and department records.

Future Improvements
- Switch to `PreparedStatement` to prevent SQL injection and improve performance.
- Add transaction management (`commit`, `rollback`) for multi-step operations.
- Implement try-with-resources for automatic resource closing.
- Extend example with `SELECT` queries to validate DML results.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section28JDBCusingSQLite/DMLUsingJDBC/src/Database.java

@@ -6,24 +6,65 @@ public class Database {
     public static void main(String[] args) throws Exception {
         Class.forName("org.sqlite.JDBC");
 
-        Connection con = DriverManager.getConnection("jdbc:sqlite:C:/sqlite/univ.db");
+        Connection con = DriverManager.getConnection("jdbc:sqlite:/Users/somesh/Java SE/JavaEvolution-Learning-Growing-Mastering/Section28JDBCusingSQLite/univ.db\"");
 
         Statement stm = con.createStatement();
 
-        //Inserting a value
-        //If you run again then it say, PRIMARY KEY constraint failed (UNIQUE constraint failed: dept.deptno).
-        //Primary key is Unique and cannot be null.
-        //stm.executeUpdate("insert into dept values(60,'Chem added using JDBC')");
+        /*
+        Inserting a value:
+        If you run again then it say, PRIMARY KEY constraint failed (UNIQUE constraint failed: dept.deptno).
+        The primary key is Unique and cannot be null.
+        stm.executeUpdate("insert into dept values(60,'Chem added using JDBC')");
 
-        //stm.executeUpdate("insert into dept values(70,'DML Done Using Insert Query')");
+        stm.executeUpdate("insert into dept values(70,'DML Done Using Insert Query')");
 
-        //Perform delete operation
-        //stm.executeUpdate("DELETE FROM dept WHERE deptno >= 60");
+        Perform delete operation
+        stm.executeUpdate("DELETE FROM dept WHERE deptno >= 60");
 
-        //Update into the database.
-        //stm.executeUpdate("update dept set dnmae = 'Ai set using JDBC' where deptno=50");
+        Update into the database.
+        stm.executeUpdate("update dept set dnmae = 'Ai set using JDBC' where deptno=50");
+        */
 
         stm.close();
         con.close();
     }
-}
+}
+
+/*
+1. JDBC Setup:
+   - `Class.forName("org.sqlite.JDBC");`
+     → SQLite JDBC driver ko load karta hai.
+   - `DriverManager.getConnection("jdbc:sqlite:.../univ.db")`
+     → SQLite database file `univ.db` ke saath connection establish karta hai.
+     → Agar file exist nahi karti toh SQLite automatically bana deta hai.
+
+2. Statement Object:
+   - `Statement stm = con.createStatement();`
+     → Isse SQL commands (INSERT, DELETE, UPDATE, SELECT) execute kar sakte ho.
+
+3. Insert Example:
+   - `stm.executeUpdate("insert into dept values(60,'Chem added using JDBC')");`
+     → `dept` table me ek naya record add karega.
+   - Agar firse wahi primary key (deptno = 60) insert karoge →
+     PRIMARY KEY constraint failed (UNIQUE constraint failed) error aayega.
+   - Reason: Primary key hamesha unique aur non-null hoti hai.
+
+4. Delete Example:
+   - `stm.executeUpdate("DELETE FROM dept WHERE deptno >= 60");`
+     → `deptno` 60 ya usse upar ke saare rows delete kar dega.
+
+5. Update Example:
+   - `stm.executeUpdate("update dept set dname = 'AI set using JDBC' where deptno=50");`
+     → Dept number 50 wale record ka naam update ho jayega.
+
+6. Close Resources:
+   - `stm.close(); con.close();`
+     → Database resources release karna important hai.
+
+
+✔ INSERT → naye records add karne ke liye.
+✔ DELETE → existing records remove karne ke liye.
+✔ UPDATE → existing records modify karne ke liye.
+✔ PRIMARY KEY constraint → duplicate values allow nahi karega.
+✔ Always close connection after queries.
+*/