Unable to know how to link data between two tables - rest

Question

I'm learning RESTful webservices from javabrains website. Here there is a section named Comments and this is related to a message, But I'm unable to know How Can I link these both.
Below is my SQL Tables for Messages and comments.
Messages
Comments
Here Basically, both look pretty same(The table design), but the values differ. And I'm using the below method to send the data.
public Comment addComment(long messageId, Comment comment) throws Exception {
Properties properties = new Properties();
properties.load(getClass().getClassLoader().getResourceAsStream("/config.properties"));
String userName = properties.getProperty("user");
String password = properties.getProperty("pass");
String url = properties.getProperty("Sqldburl");
int key = 0;
try {
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
Connection conn = DriverManager.getConnection(url, userName, password);
String query = "select count(*) from Comments";
PreparedStatement ps = conn.prepareStatement(query);
ResultSet rs = ps.executeQuery();
rs.next();
key = rs.getInt(1);
} catch (Exception e) {
System.out.println(e);
}
try {
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
Connection conn = DriverManager.getConnection(url, userName, password);
String query = "insert into Comments(id, message, author) values(?,?,?)";
PreparedStatement ps = conn.prepareStatement(query);
ps.setInt(1, key);
ps.setString(2, comment.getMessage());
ps.setString(3, comment.getAuthor());
ps.executeQuery();
} catch (Exception e) {
System.out.println(e);
}
try {
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
Connection conn = DriverManager.getConnection(url, userName, password);
String query = "select * from Comments where messageId=?";
PreparedStatement ps = conn.prepareStatement(query);
ps.setLong(1, messageId);
ResultSet rs = ps.executeQuery();
while (rs.next()) {
comment.setAuthor(rs.getString("Author"));
comment.setId(rs.getInt("Id"));
comment.setMessage(rs.getString("message"));
}
} catch (Exception e) {
System.out.println(e + "b3");
}
return comment;
}
After writing this code, I've realized that Here I'm adding a comment for sure into Comments table, But it is no where linked to the Messages.
I know a way, that is I've create a new column in the Comments table and using join operation, I need to update the same messageId in comments table. But I want to know if there is a better way of getting this done, without using the concept of joins.
In MessageBean, there is a map declared as below.
private Map<Long, Comment> comments = new HashMap<>();
#XmlTransient
public Map<Long, Comment> getComments() {
return comments;
}
public void setComments(Map<Long, Comment> comments) {
this.comments = comments;
}
can I take any advantage of this to avoid join?