I am new to MySQL and don't really know how to check if in the column "friendCode" the same value already exist and if that's the case it should look if in the same row in the "watched" column the value "0" is. I have already managed to check whether the code exists in a row. I just don't know how to look for the second condition.
Here is a picture how the table looks like:
And here that what I already managed to do:
public static boolean checkRow(String query) {
boolean ret = false;
try (Connection con = DriverManager.getConnection(url, user, password)) {
Statement stmt = con.createStatement();
stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(query);
if (rs.next() && !rs.getString("watched").equals(0)) {
ret = false;
} else {
ret = true;
}
rs.close();
stmt.close();
} catch (SQLException e) {
System.out.println(e.getMessage());
}
return ret;
}
I think you are not written any select query in this program. If you are calling from the main method just check and try the below steps.
In Given Table watched column have integer DataType data.
Your Compare to String type.
Try it my code it's working if you have any concern revert back to this comment.
public static boolean checkRow(String query) {
boolean ret = false;
try (Connection con = DriverManager.getConnection(url, user, password)) {
Statement stmt = con.createStatement();
stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(query);
if (rs.next() && rs.getString("watched")!=0) {
ret = false;
} else {
ret = true;
}
rs.close();
stmt.close();
} catch (SQLException e) {
System.out.println(e.getMessage());
}
return ret;
}
Related
I'm using a MySQL database to hold information for my reminder application in Java. I'm trying to pull the information out and store it in an array and the compare each element of the array to an updating current timestamp. The issue is the code I have gives a nullpointer exception and I can't figure out why. It works when the LocalDateTime isn't an array but the moment I turn it into an array it throws the error. It also demands I initialize it to null over anything else.
Thoughts on how I can fix this? Any help is appreciated.
Here's the method in question.
public static LocalDateTime[] getReminderTime()
{
String SQL = "SELECT r_dateTime FROM reminder_database.reminder;";
LocalDateTime reminderTime[] = null;
try
{
Connection conn = main.getConnection();
java.sql.Statement stmt;
stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(SQL);
if(rs.isBeforeFirst())
{
for(int i = 0; rs.next(); i++)
{
reminderTime[i] = rs.getTimestamp(1).toLocalDateTime();
}
}
rs.close();
stmt.close();
conn.close();
}
catch(Exception e)
{
System.out.println("Exception thrown with getReminderTime --> " + e + e.getStackTrace());
}
return reminderTime;
}
Heres the exception thrown
Exception thrown with getReminderTime --> java.lang.NullPointerException[Ljava.lang.StackTraceElement;#6dfc1e5f
Exception thrown with getReminderTime --> java.lang.NullPointerException[Ljava.lang.StackTraceElement;#3b2da18f
Found a solution.
I needed to initialize a size for the array in order to fill it. So I created another method that went through all the elements and gets the size.
Here's the code.
public static LocalDateTime[] getReminderTime()
{
String SQL = "SELECT r_dateTime FROM reminder_database.reminder;";
LocalDateTime reminderTime[] = null;
reminderTime = new LocalDateTime[getSizeOfRs()];
try
{
Connection conn = main.getConnection();
java.sql.Statement stmt;
stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(SQL);
if(rs.isBeforeFirst())
{
for(int i = 0; rs.next(); i++)
{
reminderTime[i] = rs.getTimestamp(1).toLocalDateTime();
}
}
rs.close();
stmt.close();
conn.close();
}
catch(Exception e)
{
System.out.println("Exception thrown with getReminderTime --> " + e + e.getStackTrace());
}
return reminderTime;
}
and the get rs size
public static int getSizeOfRs()
{
String SQL = "SELECT * FROM reminder_database.reminder;";
int size = 0;
try {
Connection conn = main.getConnection();
java.sql.Statement stmt;
stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(SQL);
for(int i = 0; rs.next(); i++)
size++;
}
catch(Exception e)
{
System.out.println("Exception thrown with getSizeofRS --> " + e + e.getStackTrace());
}
return size;
}
All of my database access methods have been working up until now. I have a string of n usernames separated by ";", and I want to make n separate select statements to check if the user is online or offline. However, when used, this method returns nothing (""). The database connection is working. Any solutions are welcome.
EDIT:
New method, still does not work:
public static String CheckOn(String users) {
String toreturn = "";
PreparedStatement stmt2 = null;
ResultSet rs2 = null;
try {
conn = Database.getDBConnection("test");
String sql = "SELECT name FROM sessions WHERE name=?";
stmt2 = conn.prepareStatement(sql);
String[] usarr = users.substring(1, users.length()).split(";");
for (int i = 0; i <usarr.length;i++){
if (usarr[i].equals("Empty")){
toreturn = toreturn + "-";
}
else{
stmt2.setString(1, usarr[i]);
rs2 = stmt2.executeQuery();
if (rs2.next()){
toreturn = toreturn + "y";
}
else{
toreturn = toreturn + "n";
}
rs2.close();
}
}
rs2.close();
stmt2.close();
conn.close();
} catch (SQLException se) {
se.printStackTrace();
} finally {
try {
if (stmt2 != null)
stmt2.close();
} catch (SQLException se2) {
}
try {
if (conn != null)
conn.close();
} catch (SQLException se) {
se.printStackTrace();
}
}
return toreturn;
}
i have a database with a table 'admin'.But presently just creating table not having any values.table columns are user_name and password.My motive is to check whether the table is empty or not.I am using mysql database.I tried following code and its fails.Please help me.
public void nullCheck() {
PreparedStatement stmt = null;
ResultSet rs = null;
String qry = "SELECT * From admin ";
try {
stmt = (PreparedStatement) conn.prepareStatement(qry);
rs = stmt.executeQuery();
boolean empty = true;
while( rs.next() ) {
// ResultSet processing here
empty = false;
}
if( empty ) {
Util.showWarningMessageDialog("null");
}
} catch (SQLException ex) {
Logger.getLogger(RemoveFaculty.class.getName()).log(Level.SEVERE, null, ex);
}
}
If you just need to check the table then you should use query:
String qry = "SELECT count(*) From admin ";
for better performance.
And get the row count from ResultSet to check the table is null or not.
int count=0;
while( rs.next() )
{
count=rs.getInt("count");
}
Try this code, I think this will do it. I updated it coz the first I post here is not working at all, my bad.
public void nullCheck() {
PreparedStatement stmt = null;
ResultSet rs = null;
String qry = "SELECT * From admin ";
try {
stmt = (PreparedStatement) conn.prepareStatement(qry);
rs = stmt.executeQuery();
int count = 0;
while(rs.next()){
count++;
}
if(count == 0){ // if equal to 0 then the table is null
bla bla bla
}
} catch (SQLException ex) {
Logger.getLogger(RemoveFaculty.class.getName()).log(Level.SEVERE, null, ex);
}
}
I am writing a program that will take in a student ID and verify if that ID exists in a mysql table. If it does exist, I would like to take the entire row that it exists in and copy that row to another table. Currently the program will just copy all rows in a table to the other. Any help appreciated. I have inserted a snippet of code below.
try {
String compareText = IDField.getText().trim();
if(compareText.length() > 0){
Class.forName("com.mysql.jdbc.Driver");
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/simlab","root","password");
System.out.println("Connected to database");
Statement stmt1 = conn.createStatement();
ResultSet rs1 = stmt1.executeQuery("select * from students where LUID='"+IDField.getText()+"' ");
boolean isPresent = rs1.next();
if (isPresent)
{
Class.forName("com.mysql.jdbc.Driver");
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/simlab","root","password");
System.out.println("Connected to database");
int rows = stmt1.executeUpdate("INSERT INTO skills(ID_Student,LUID_Student)SELECT ID, LUID FROM students");
if (rows == 0)
{
System.out.println("Don't add any row!");
}
else
{
System.out.println(rows + " row(s)affected.");
conn.close();
}
//System.out.println("Already exists!!");
}
You could all do that in a single SQL statement:
INSERT INTO <Dest-Table>
(SELECT * FROM <Src-Table> WHERE ID=?);
It will only copy rows that exist.
I suspect it's due to this line:
int rows = stmt1.executeUpdate("INSERT INTO skills(ID_Student,LUID_Student)SELECT ID, LUID FROM students");
As, if that line is parsed, the SELECT statement has no WHERE clause, and will therefore get every row, and therefore insert everything.
With Prepared statements
String sql = "INSERT INTO abc"
+ "(SELECT id1,id2 FROM pqr)";
ps1 = con.prepareStatement(sql);
int rs = ps1.executeUpdate();
if (rs > 0) {
update = true;
} else {
update = false;
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
try {
if (ps1 != null) {
ps1.close();
ps1 = null;
}
if (con != null) {
con.close();
con = null;
}
} catch (Exception e) {
}
}
return update;
Can we use two queries in one method while using prepared statement, I have tried using this but invalid column name exception is coming.
My code snippets is as follows.
public double getPayroll(){
ResultSet rs = null;
ResultSet rs2 = null;
Connection conn = null;
PreparedStatement pstmt = null;
try {
conn = getDBConnection();
int employeeId;
String q1 = "select e_salary,e_house_rent,e_conv_allow,e_id
from employee";
pstmt = conn.prepareStatement(q1);
rs = pstmt.executeQuery();
double dailyPay=0,basicPay=0,payroll2=0;
int houseRent=0,convAllow=0;
while (rs.next()) {
dailyPay = rs.getInt(1)*.03;
houseRent=rs.getInt(2);
convAllow=rs.getInt(3);
employeeId=rs.getInt(4);
}
String q2="select att_status from attendance where
e_id=employeeId";
pstmt = conn.prepareStatement(q2);
rs2 = pstmt.executeQuery();
int noOfPresents = 0;
while(rs2.next()){
noOfPresents+=1;
}
basicPay=dailyPay*noOfPresents;
payroll2+=basicPay+houseRent+convAllow;
return payroll2;
} catch (Exception e) {
e.printStackTrace();
return 0.0;
} finally {
try {
rs.close();
pstmt.close();
conn.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
Your problem is that the sql in q2 assumes that there is a column named employeeId, but I suspect you want to insert the value of the variable employeeId.
Change it to
select att_status from attendance where e_id=?
Then execute
pstmt.setString(1, employeeId);
before executing pstmt.executeQuery();