Variable disappears when I filter the database table

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I am making a library System where the user can search and filter the data in the database. My code is now filtering the table however when I start to type the variable txtNo disappears. I cannot identify what is the problem.

What I have tried:

Java

private void txtSearchKeyReleased(java.awt.event.KeyEvent evt) {                                      
        try { 
            String sql = "SELECT txtNo, txtTitle, txtAuthor, txtGenre, txtLexile, txtPoints FROM LibrarySystemDatabase WHERE txtNo like '%"+txtSearch.getText()+"%' "
                    + "OR txtTitle like '%"+txtSearch.getText()+"%'"
                    + "OR txtAuthor like '%"+txtSearch.getText()+"%'"
                    + "OR txtGenre like '%"+txtSearch.getText()+"%'"
                    + "OR txtLexile like '%"+txtSearch.getText()+"%'"
                    + "OR txtPoints like '%"+txtSearch.getText()+"%'";
            pst = conn.prepareStatement (sql);
            rs = pst.executeQuery();
            tblTable.setModel(DbUtils.resultSetToTableModel(rs));
            tblTable.removeColumn(tblTable.getColumnModel().getColumn(0));
        } catch(Exception e) {
             JOptionPane.showMessageDialog(null, e);
        }
        
    }

Posted 22-Feb-20 19:52pm

Darwin Ramos

Updated 22-Feb-20 20:14pm

Patrice T

v2

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Jon McKee 23-Feb-20 3:02am

Maybe I'm being dense but doesn't tblTable.removeColumn(tblTable.getColumnModel().getColumn(0)); delete txtNo? What exactly do you mean by "when I start to type the variable txtNo disappears"?

Richard MacCutchan 23-Feb-20 4:44am

Why are you using exactly the same text field for every database column in your SELECT statement? The chances of it finding any valid records are not high. Also, you have not explained what you mean by "when I start to type the variable txtNo disappears". Disappears from where?

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OriginalGriff · Answer 1 · 2020-02-22T19:56:00

You have a bigger problem than that: Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

Patrice T · Answer 2 · 2020-02-22T20:14:00

Quote:
when I start to type the variable txtNo disappears.

technically, txtNo is not a variable (Java), from query, it is a database field, so this means nothing to us.

Java

String sql = "SELECT txtNo, txtTitle, txtAuthor, txtGenre, txtLexile, txtPoints FROM LibrarySystemDatabase WHERE txtNo like '%"+txtSearch.getText()+"%' "
+ "OR txtTitle like '%"+txtSearch.getText()+"%'"
+ "OR txtAuthor like '%"+txtSearch.getText()+"%'"
+ "OR txtGenre like '%"+txtSearch.getText()+"%'"
+ "OR txtLexile like '%"+txtSearch.getText()+"%'"
+ "OR txtPoints like '%"+txtSearch.getText()+"%'";

Not a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]
SQL Injection Attacks by Example[^]
PHP: SQL Injection - Manual[^]
SQL Injection Prevention Cheat Sheet - OWASP[^]
How can I explain SQL injection without technical jargon? - Information Security Stack Exchange[^]

Variable disappears when I filter the database table

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