JDBC : MCQs & PYQs with Explanation

Option B

In JDBC, a Statement or PreparedStatement is used to send SQL commands to a database. When a SQL query contains parameters (placeholders) whose values are supplied at runtime, PreparedStatement is generally used. A parameterized query uses the ? symbol as a placeholder.
PreparedStatement ps = con.prepareStatement("SELECT * FROM Student WHERE rollNo = ?");
ps.setInt(1, 101);
ResultSet rs = ps.executeQuery();
Here, ? is the parameter placeholder, and setInt() supplies its value.
PreparedStatement is specifically designed for precompiled/parameterized SQL statements. It makes it easier to supply different values to the same SQL statement and, when parameters are bound properly, helps prevent SQL injection.
SQL Injection is a security attack in which an attacker provides specially crafted input that changes the meaning of an SQL query executed by an application. It usually happens when user input is directly concatenated into an SQL query instead of being safely treated as data.

Important JDBC Objects:

Option D

In JDBC, a SELECT query is used to retrieve data from a database. The executeQuery() method is generally used to execute a SELECT statement and returns a ResultSet containing the rows returned by the query.
E.g.:
PreparedStatement ps = con.prepareStatement("SELECT * FROM Student");
ResultSet rs = ps.executeQuery();
Here, executeQuery() executes the SELECT statement, and the returned ResultSet contains the retrieved records.

Important JDBC Methods:

Option B

In JDBC, INSERT, UPDATE, and DELETE statements are used to modify data in a database rather than retrieve rows. 
The executeUpdate() method is generally used for these statements. It returns an int value representing the number of rows affected by the operation.
E.g.:
PreparedStatement ps = con.prepareStatement("UPDATE Student SET name = ? WHERE rollNo = ?");
ps.setString(1, "Suresh");
ps.setInt(2, 101);
int rows = ps.executeUpdate();
If one student record is updated, rows will generally contain 1.

Option B

JDBC (Java Database Connectivity) is used by Java programs to communicate with databases. ODBC (Open Database Connectivity) is another database connectivity standard. 
The old JDBC-ODBC Bridge acted as a translator between JDBC calls from a Java program and ODBC calls understood by an ODBC driver. It was a Type 1 JDBC driver.
The traditional Java JDBC-ODBC Bridge did not support concurrent access from different threads. It used synchronized methods to serialize the calls made to ODBC. 
In simple terms, even if a Java program had multiple threads, the Bridge processed its ODBC calls one at a time rather than allowing them to proceed concurrently.

Option B

In JDBC, the SQL data type REAL represents a single-precision floating-point number. It is generally used to store floating-point values with approximately seven digits of precision.
JDBC provides the java.sql.Types class, which contains constants representing SQL data types. The REAL type is represented by Types.REAL.

Option B

In JDBC (Java Database Connectivity), DML (Data Manipulation Language) statements are used to modify data in database tables. Common DML statements include INSERT, UPDATE, and DELETE.
JDBC provides different methods for executing SQL statements, and the method depends on the type of SQL statement being executed. The executeUpdate() method is specifically used for DML statements because it returns an int value representing the number of rows affected by the operation.
Syntax:
int executeUpdate(String sql) throws SQLException
E.g.:
Statement stmt = connection.createStatement();
int rows = stmt.executeUpdate("UPDATE Student SET name = 'Suresh' WHERE id = 101");

Option D

In JDBC, a transaction is a group of one or more database operations that are treated as a single unit of work. 
For example, transferring money from one account to another may involve two operations, deducting money from one account and adding it to another.
The commit() method is used to make all changes performed in the current transaction permanent in the database.
E.g.:
Connection con = DriverManager.getConnection(...);
con.setAutoCommit(false);
Statement stmt = con.createStatement();
stmt.executeUpdate("UPDATE Account SET balance = balance - 100 WHERE id = 1");
stmt.executeUpdate("UPDATE Account SET balance = balance + 100 WHERE id = 2");
con.commit();
Here, setAutoCommit(false) allows the operations to be controlled as a transaction. The commit() call permanently applies the pending changes.
Note:
The getConnection() method of DriverManager class is used to establish a connection between a Java application and a database. It generally receives a database URL, username, and password, and returns a Connection object when the connection is successfully established.

The commit() vs rollback():

Option C

In JDBC, when a SQL query such as a SELECT statement retrieves data from a database, the returned rows are represented by a ResultSet object.
A ResultSet provides a cursor that allows the Java program to read the retrieved records row by row.
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM Student");
Here, executeQuery() executes the SELECT query, and the returned ResultSet contains the query result. The records can then be accessed using next():
while (rs.next())
{
    System.out.println(rs.getString("name"));
}