06. How to retrieve data from the database into a JDBC client? By LN RAO 28-Oct-24 | Ashok IT

Summary

The lesson explains how to retrieve database records into a Java program using JDBC. Unlike data-changing operations—such as inserts, updates, and deletes—which use executeUpdate() and return an integer, a SELECT query uses executeQuery() and returns a ResultSet.

How Retrieval Works

1. The Java program calls executeQuery() on a Statement object and supplies a SELECT statement.
2. The JDBC driver translates the call into a database query and sends it to the DBMS.
3. The DBMS executes the query and returns zero or more matching records.
4. The driver makes the returned records available to the Java program through a ResultSet.
5. If an error occurs, the operation can throw an SQLException.

A ResultSet cursor starts before the first record. The cursor must be moved to a record before its values are read; attempting to read a value before reaching a record or after reaching the end can cause an exception.

Reading Records

A typical retrieval sequence is:

1. Create a database connection and a Statement.
2. Call executeQuery() with a SELECT statement, such as one that selects all columns from an account table.
3. Call ResultSet.next() to move forward one record at a time:
   • It returns true when it moves to a record.
   • It returns false when there are no more records.
4. Use next() in a while loop when the number of returned records is not known in advance.
5. Inside the loop, retrieve column values with suitable getter methods:
   • getInt() for integer columns
   • getString() for text columns
   • getDouble() for decimal-valued columns
6. Display or otherwise process the values.
7. Close the ResultSet, Statement, and Connection when finished.

The example retrieves an account number, account-holder name, and balance, then prints them for each returned account. The instructor notes that developers should know the table’s columns and data types, usually through documentation. JDBC metadata can also be used to inspect this information programmatically.

Additional Class Points

• Students are encouraged to download and configure the MySQL connector, then run the examples themselves. The instructor emphasizes that attending lectures without executing programs is unlikely to build lasting understanding.
• A “no suitable driver” error can indicate that the MySQL connector JAR is missing or incorrectly configured on the classpath.
• The driver makes database results available through JDBC objects, and ResultSet getter methods provide values for processing in Java.
• Questions about NoSQL access, dynamic columns, notes, and downloads are briefly raised. The instructor says NoSQL is outside the scope of the Java class and directs students to consult the administrator about download issues.

Speakers and Sources

• LN Rao — instructor and primary speaker.
• Students in the class — several ask questions during the Q&A; some names are difficult to identify from the auto-generated subtitles. “Manus” and “Rajkumar” are mentioned in connection with questions.