Put a MySQL query into an if Statement in Java

Question

I'm having the below issue. I already have the database in MySQL and now I want to let the user choose the data, if it exists. The program runs to the "true" case and the "false" case for the rest. I put a MySQL query into an if Statement in Java, but it causes a problem when I put it in. No matter what I choose the data, it always returns to "true" and does not run into the "false" case. When I try to use.isNullOrEmpty it always returns to "false". Any other solution in this situation?

Here is my code

try {
    Connection conn = null;
    conn = DriverManager.getConnection(DB_URL, USER_NAME, PASSWORD);
    Statement stmt = null;
    stmt = conn.createStatement();
    String MSV = scanner.nextLine().toUpperCase();
    String COUNT = "SELECT COUNT(MSV) FROM project.student where MSV = '"+MSV+"';";
    stmt.executeQuery(COUNT);               
    ResultSet resultSet = stmt.executeQuery(COUNT);                             
    if (stmt.executeQuery(COUNT) != null)  {                    
        System.out.println("true");
     } else {                   
        System.out.println("false");
     } conn.close();                
} catch (SQLException e) {
    e.printStackTrace();
}

Answer 1

ResultSet resultSet = stmt.executeQuery(COUNT); if (stmt.executeQuery(COUNT) != null) {

This runs the executeQuery method, which returns a resultSet . You store this resultset in a local variable named resultSet and then proceed to do nothing with it . That first line ( ResultSet resultSet = stmt.executeQuery(COUNT); ) doesn't do anything useful - worse, it creates a resource that you don't close.

You then run the executeQuery method a second time, and check if it returns null . It never will. It always returns a resultset.

The resultset is a 'cursor' - a tool that you can use to traverse the excelsheet. Because all queries on a database always return a result set - a thing with rows and columns (or, if you prefer SQL style thinking, a query itself returns a table: A thing with rows and columns).

This is a peculiar excel sheet - it has just one column (namely: COUNT(MSV) ) and given that you're aggregating over the entire result, it has just the one row. You may therefore think: Nono, this query doesn't return an excel sheet, it returns a single result.

No, it doesn't - the way the ResultSet API works is that you always get a result set. Perhaps with just one cell filled in (one column, one row), but always 'an excel sheet'.

Therefore, you must [A] forward this 'cursor' to the next (first) row, given that they always start before the first row, and then [B] invoke the right get method to obtain the contents of the cell, and then [C] presumably, check if that value is 0 or not.

Thus, instead of the above quoted snippet, you want instead:

int value;
try (var resultSet = stmt.executeQuery(COUNT)) {
  if (!resultSet.next()) {
    value = 0; // no row somehow
  } else {
    value = resultSet.getInt(1); // get first (and only) column value
  }
}

if (value != 0) {

Yeah that looks a bit unwieldy. JDBC isn't meant to be nice to use. It's a low level API. You don't program in assembler either. Consider using an abstraction built on top of JDBC that makes it nice to use, such as JDBI or JOOQ.

Answer 2

count wont give result of null even though query result is null/nothing. The count will return 0. Trying adding 0 to your if condition, it might work:)

String COUNT = "SELECT COUNT(MSV) as count_name FROM project.student where MSV = '"+MSV+"';";
stmt.executeQuery(COUNT);               
ResultSet resultSet = stmt.executeQuery(COUNT);                         
if (resultSet.next()) {
    int count= rs.getInt("count_name");
if (count > 0)  {                    
                System.out.println("true");
             } else {                   
                System.out.println("false");
             }
       }

Answer 3

if condition should be this

if (stmt.executeQuery(COUNT) > 0) {
  //
}