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getGeneratedKeys() returns an Empty ResultSet

Hello there and thanks for reading.
I'm trying to retrieve the ID of the newly inserted data, but I always get an empty ResultSet.
Connection con = main.getCon();
String sqlCommand = "Insert Into Relations(name,explanation) values(?,?)";
PreparedStatement state =
con.prepareStatement(sqlCommand,Statement.RETURN_GENERATED_KEYS);
state.setString(1,name.getText());
state.setString(2,explanation.getText());
int affectedRows = state.executeUpdate();
assert (affectedRows>0);
ResultSet rs = state.getGeneratedKeys();
assert rs.next();
int instertedID= rs.getInt("ID");
Not sure what's wrong with it. Checked different samples online, but couldn't figure out what's my mistake.
I also tried it with Statement, but no luck with that either.
Point 1: the code runs smoothly and my data in inserted into the database.
Point 2: there are examples online for this very case, you can check it here:
https://www.baeldung.com/jdbc-returning-generated-keys
I just realized that my ResultSet wasn't empty, I had problem with using my debugger and that's why I thought it was empty.
As Mark Rotteveel mentioned in a comment, the problem was with "assert" statement.

The problem is your use of assert rs.next(). Assertions in Java are intended for checking invariants (eg during testing), but when you normally run Java, assert statements are not executed, they are only executed when explicitly enabling this with the -ea commandline option.
As a result, rs.next() is not called, so your result set is still positioned before the first row when you call rs.getInt(1). Instead use if (rs.next()) { ... }.

This is DB engine dependent. Some tips:
JDBC is low-level and not appropriate to program with
It's a complicated API. Use something that makes it easier: JDBI, or JOOQ. They may have abstractions over insertion that takes care of this stuff for you.
Some DB engines require that you list the column name
Try:
con.prepareStatement(sqlCommand, new String[] {"UNID"});
Some DB engines will only return generated values as direct resultset
Don't call .executeUpdate(); instead, call .executeQuery() which returns a ResultSet; check that one.
Something else
Post the exact table structure and DB engine you're working with if the above doesn't help.
Your code is broken
You can't create resource objects (once that must be closed) unless you do so safely, and you're not doing so safely. Use try-with-resources:
String sql = "INSERT INTO relations(name, explanation) VALUES (?, ?)";
try (Connection con = main.getCon();
PreparedStatement ps = con.prepareStatement(sql, new String[] {"unid"})) {
state.setString(1, name.getText());
state.setString(2, explanation.getText());
try (ResultSet rs = state.executeQuery()) {
if (!rs.next()) throw new SQLException("insert didn't return autogen?");
System.out.println(rs.getInt(1));
}
}
ResultSets, Statements, PreparedStatements, and Connections are all resources (must be closed!) - if you want to store one of those things in a field, you can do that, but only if the class that contains this field is itself a resource: It must have a close() method, it must implement AutoClosable, and you can then only make instances of this class with try-with-resources as above.
Failure to adhere to these rules means your app seems to work, but is leaking resources as it runs, thus, if you let it run long enough, it will start crashing. Also, your DB engine will grind to a halt as more and more connections are left open, stuck forever.

change the last line of code to this because the DBMS you are using may not support the getting value by column name so pass the index of that column:
int instertedID = rs.getInt(1);

String sqlCommand = "Insert Into Relations (name, explanation) values(?, ?)";
PreparedStatement state = con.prepareStatement(sqlCommand, Statement.RETURN_GENERATED_KEYS);
state.setString(1,name.getText());
state.setString(2,explanation.getText());
state.executeUpdate();
ResultSet resultSet = state.getGeneratedKeys();
if(resultSet.next()) {
System.out.println(resultSet.getInt(1)); //Indicate the corresponding column index value.
}

PreparedStatement for several statements

Is it possible to use PreparedStatement for several statements?
E.g., I mean
if
String sql = "INSERT INTO OR INGNORE ... ; UPDATE ... ; INSERT INTO ...";
this
PreparedStatement pre = conn.prepareStatement(sql);
//...
pre.executeUpdate();
executes only first statement "INSERT INTO OR INGNORE ... (until semicolon).
Is it possible to execute all at once?

Is it possible to execute all at once?
this might depend on the implementation of the JDBC driver by the database vendor but in general I would not expect that to work.

String[] query = { "insert into Gericht (classification,date,name,preisExtern,preisIntern) values (?,?,?,?,?)",
"test" };
PreparedStatement stmt;
for (String str : query) {
stmt = c.prepareStatement(str);
stmt.addBatch();
}
stmt.executeBatch();
here is an example on how to use batch. if its not what you want, please tell me so.
as wished:
#XtremeBaumer how about when the parameters in prepared statement change? how can you change it dynamically?
Answer:
you can't. if you want several different queries to be executed at once, you can only use fixed statement, otherwise your code will be very large, and then you can do it manually by setting the parameters and adding it to a batch. batches are good if you have 1 query that gets different parameters and you want to add all at once

Execute multiple queries using a single JDBC Statement object

In JDBC, can I use single Statement object to call executeQuery("") multiple times? Is it safe? Or should I close the statement object after each query, and create new object for executing another query.
E.G:
Connection con;
Statement s;
ResultSet rs;
ResultSet rs2;
try
{
con = getConnection();
// Initially I was creating the Statement object in an
// incorrect way. It was just intended to be a pseudocode.
// But too many answerers misinterpretted it wrongly. Sorry
// for that. I corrected the question now. Following is the
// wrong way, commented out now.
// s = con.prepareStatement();
// Following is the way which is correct and fits for my question.
s = con.createStatement();
try
{
rs = s.executeQuery(".......................");
// process the result set rs
}
finally
{
close(rs);
}
// I know what to do to rs here
// But I am asking, should I close the Statement s here? Or can I use it again for the next query?
try
{
rs2 = s.executeQuery(".......................");
// process the result set rs2
}
finally
{
close(rs2);
}
}
finally
{
close(s);
close(con);
}

Yes you can re-use a Statement(specifically a PreparedStatement) and should do so in general with JDBC. It would be inefficient & bad style if you didn't re-use your statement and immediately created another identical Statement object. As far as closing it, it would be appropriate to close it in a finally block, just as you are in this snippet.
For an example of what you're asking check out this link: jOOq Docs

I am not sure why you are asking. The API design and documentation show it is perfectly fine (and even intended) to reuse a Statement object for multiple execute, executeUpdate and executeQuery calls. If it wouldn't be allowed that would be explicitly documented in the Java doc (and likely the API would be different).
Furthermore the apidoc of Statement says:
All execution methods in the Statement interface implicitly close a statment's [sic] current ResultSet object if an open one exists.
This is an indication that you can use it multiple times.
TL;DR: Yes, you can call execute on single Statement object multiple times, as long as you realize that any previously opened ResultSet will be closed.
Your example incorrectly uses PreparedStatement, and you cannot (or: should not) be able to call any of the execute... methods accepting a String on a PreparedStatement:
SQLException - if [...] the method is called on a PreparedStatement or CallableStatement
But to answer for PreparedStatement as well: the whole purpose of a PreparedStatement is to precompile a statement with parameter placeholders and reuse it for multiple executions with different parameter values.

I can't find anything in the API docs that would state, that you shouldn't call executeQuery() on a given PreparedStatement instance more than once.
However your code does not close the PreparedStatement - a call to executeQuery() would throw a SQLException in that case - but the ResultSet that is returned by executeQuery(). A ResultSet is automatically closed, when you reexecute a PreparedStatement. Depending on your circumstances you should close it, when you don't need it anymore. I would close it, because i think it's bad style not to do so.
UPDATE Upps, I missed your comment between the two try blocks. If you close your PreparedStatement at this point, you shouldn't be able to call executeQuery() again without getting a SQLException.

A Prepared Statement tells the database to remember your query and to be prepared to accept parameterized variables to execute in that query. It's a lot like a stored procedure.
Prepared Statement accomplishes two main things:
It automatically escapes your query variables to help guard against SQL Injection.
It tells the database to remember the query and be ready to take variables.
Number 2 is important because it means the database only has to interpret your query once, and then it has the procedure ready to go. So it improves performance.
You should not close a prepared statement and/or the database connection in between execute calls. Doing so is incredibly in-efficient and it will cause more overhead than using a plain old Statement since you instruct the database each time to create a procedure and remember it. Even if the database is configured for "hot spots" and remembers your query anyways even if you close the PreparedStatement, you still incur network overhead as well as small processing time.
In short, keep the Connection and PreparedStatement open until you are done with them.
Edit: To comment on not returning a ResultSet from the execution, this is fine. executeQuery will return the ResultSet for whatever query just executed.

Firstly I am confused about your code
s = con.prepareStatement();
Is it work well?I can't find such function in JAVA API,at least one parameter is needed.Maybe you want to invoke this function
s = con.createStatement();
I just ran my code to access DB2 for twice with one single Statement instance without close it between two operation.It's work well.I think you can try it yourself too.
String sql = "";
String sql2 = "";
String driver = "com.ibm.db2.jcc.DB2Driver";
String url = "jdbc:db2://ip:port/DBNAME";
String user = "user";
String password = "password";
Class.forName(driver).newInstance();
Connection conn = DriverManager.getConnection(url, user, password);
Statement statement = conn.createStatement();
ResultSet resultSet = statement.executeQuery(sql);
int count = 0;
while (resultSet.next()) {
count++;
}
System.out.println("Result row count of query number one is: " + count);
count = 0;
resultSet = statement.executeQuery(sql2);
while (resultSet.next()) {
count++;
}
System.out.println("Result row count of query number two is: " + count);

Which way is better for update database record?

I have two method for update:
String query = "update mytable set name = 'new_value' where id ='20' ";
Connection conn;
PreparedStatement pState;
try {
conn = DriverManager.getConnection(dbUrl, "root", "2323");
pState = conn.prepareStatement(query);
pState.executeUpdate();
} catch (SQLException sql) {
sql.printStackTrace();
}
OR:
String query = "update mytable set name = ?" + "where id = ?";
Connection conn;
PreparedStatement pState;
int s;
try {
conn = DriverManager.getConnection(dbUrl, "root", "2323");
pState = conn.prepareStatement(query);
pState.setStringt(1, "new_value");
pState.setString(2, "20");
s = pState.executeUpdate(); // if s = 1 then update done successfully
} catch (SQLException sql) {
sql.printStackTrace();
}
Both methods update database record correctly, Which is better?

Second approach is good practice to avoid SQL Injection attacks.
And following is enough to construct query String, another + concatenation is not required.
String query = "update mytable set name = ? where id = ?";

I would say the second approach.
You aren't returning anything, so why create a result set and go down that path?
Edit:
Even after your comment, I would still use the second template. It's more flexible. Additionally, it's faster. The PreparedStatement is pre-compiled in the database which allows the database to execute a parametric query using the statement faster than a normal query. This won't happen if you use string concatenation (like in your first example).
See: http://docs.oracle.com/javase/tutorial/jdbc/basics/prepared.html
Additionally, from that page:
The main feature of a PreparedStatement object is that, unlike a
Statement object, it is given a SQL statement when it is created. The
advantage to this is that in most cases, this SQL statement is sent to
the DBMS right away, where it is compiled. As a result, the
PreparedStatement object contains not just a SQL statement, but a SQL
statement that has been precompiled. This means that when the
PreparedStatement is executed, the DBMS can just run the
PreparedStatement SQL statement without having to compile it first.
Although PreparedStatement objects can be used for SQL statements with
no parameters, you probably use them most often for SQL statements
that take parameters. The advantage of using SQL statements that take
parameters is that you can use the same statement and supply it with
different values each time you execute it.

The second way is more faster if you use frequently the same query. Depends of the database vendor, the query is cached and the efficiency is higher than that using flat sentences. But all that depends on the implementation of the JDBC driver and the services provided by the database.
See more in Using Prepared Statements in the The Java Tutorials.

Reusing ResultSet

I need to run several queries in a row
Statement st = cnx.createStatement();
ResultSet rs = st.executeQuery( "SELECT [good stuff]");
// do something smart with rs
rs = st.execute( "SELECT [better stuff]");
// do something smarter with rs
rs = st.execute( "SELECT [best stuff]");
// you got it
try{ rs.close();} catch( SQLException ignore){};
try{ st.close();} catch( SQLException ignore){};
Is this a problem that the first two ResultSet are not properly closed or is it implicitely done during garbage collection?

As soon as you execute the 2nd query, the previous ResultSet is automatically closed. And as far as Garbage Collection is concerned, you don't have to worry about that. You can just have a stmt.close() at the end that's all. It will automatically close all the related ResultSet objects.
Take a look at : - ResultSet#close documentation, which says that: -
A ResultSet object is automatically closed by the Statement object
that generated it when that Statement object is closed, re-executed,
or is used to retrieve the next result from a sequence of multiple
results.
If you want to test, whether your resultset gets closed or not, you can use a while loop to iterate over the result set and inside the while loop, create another query and assign it to same result set. You will see that an Exception will be thrown..
ResultSet res = stmt.executeQuery("SELECT * FROM sometable");
while (res.next()) {
res.getString(1);
// Closes the previous `ResultSet`
res = stmt.executeQuery("SELECT * FROM othertable");
}
So, in the above code, on the 2nd iteration, you will get an Exception: - Cannot perform operation after ResultSet is closed

I don't know what's your problem, but if you have some problems to run this code, you can try to close connection and open other to make the second query. Some database products, like SQLite, only admit one open connection. If you have any problem with database access, you should try that.