Let's say I have a program that puts email addresses into a database where the email attribute is a primary key.
If I have a duplicate email address, I could deal with it in two ways.
1) run a "select email from table" query. If the email is currently in there, don't add it.
2) don't check if email is in the table. catch(SQLException e), but don't print the stack trace, simply skip over it. This way, if I'm inserting a duplicate it effectively ignores it.
Granted with method 1, I'm only executing a simple select query (no joins or anything fancy) so performance isn't really a huge issue. But if I wanted to optimize performance, would method 2 be a viable, safe way of doing this?
So instead of running a "select ..." every time, I just add it.
Are there any safety issues with skipping over the exception?
Java Example (with JDBC):
try {
String sql = "insert into emails values(?)";
PreparedStatement pstmt = conn.prepareStatement(sql);
pstmt.setString(1, email);
pstmt.execute();
return true;
}
catch(SQLException e) {
// e.printStackTrace(); // skip; don't print out error
return false;
}
Related
In my database contain about 2 millions rows. That contains collection of sentences in each rows(Not English language).
![Image of database][1]
As a example if user give 'abcd' ,that matched with both row 1,3. Therefor both should select.
Because of that huge time taken to data retrieve. I'm using following code to retrieve data. If any given word match with the database those rows should be select.
sb.append("SELECT Sentence FROM corpus Where ");
for(int k=0;k<wordList.size();k++){
sb.append( " Sentence like '%" + wordList.get(k) + "%' OR ");
}
sb.append(" 1=0");
rs2 = dbc.sqlExecute(sb.toString());
That selection word may be anywhere in the row. That may be front or middle or end. My problem is when user give some word phrase, huge time taken to select matched rows. Is there any proper way to speed my database retrieves? I think indexing may not work because of I'm not searching only the starting point of the rows. That matched words may contain anywhere in the rows. What is the efficient way to do this? Huge time taken to retrieve. Huge problem to me.Can anyone know proper way to do this?
You don't need for loop, just do like this:
sb.append("SELECT Sentence FROM corpus Where Sentence like '%");
sb.append(wordList);
sb.append("%'");
I believe the efficient solution is to use a PreparedStatement with a bind parameter and execute it in a loop. You could use a For-Each Loop, and for each word in wordList bind the parameter and then get a ResultSet (using a try-with-resources) and add the returned sentences to your List with something like
List<String> sentences = new ArrayList<>();
String sql = "SELECT Sentence FROM corpus WHERE Sentence LIKE ?";
PreparedStatement ps = null;
try {
ps = conn.prepareStatement(sql);
for (String word : wordList) {
ps.setString(1, String.format("%%%s%%", word));
try (ResultSet rs = ps.executeQuery()) {
sentences.add(rs.getString(1));
} catch (Exception e) {
e.printStackTrace();
}
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if (ps != null) {
try {
ps.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
While a FULLTEXT index on the Sentence column is probably advisable, this method has the benefit using the query cache and reducing the size of your query. The OR clauses just muddy the situation, since you want the matching rows of any of them. You might also consider a Set<String> sentences.
I am getting an exception using PreparedStatement to select.
Got an exception accessing TestCase data! null
Problem to connect.
com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException:
Too many connections
Here is my code:
public Integer getTypeByInputAndProblemId(String inputTestCase, Long problemId) {
String sql = "SELECT type FROM test_case where problem_id= ? and input= ?";
Integer type = 0;
try {
PreparedStatement ps = connection.prepareStatement(sql);
ps.setLong(1, problemId);
ps.setString(2, inputTestCase);
ResultSet rs = ps.executeQuery();
if (rs.next()) {
type = new Integer(rs.getInt("type"));
}
rs.close();
ps.close();
} catch (Exception e) {
System.err.println("Got an exception accessing TestCase data! " + e.getMessage());
}
return type;
}
In line PreparedStatement ps = connection.prepareStatement(sql);
my problem is because connection sometimes is Null (the debug shows this).
I'm guessing this is because of many connections, but I don't know why this is happening.
I would like some help, please!
Yes, the issue happens because your server is reaching the max number of multiple connections accepted by your MySQL Server.
First, you need to see if you have a proper number configured in MySQL for multiple connections: max_connections. If this look low to you, you can increase this number in order to "fix" this issue.
Secondly, if the number makes sense, you're probably using more connections than you think you are. Probably because you're opening connections in your application and not closing them.
Check how many multiple connections your server have used so far.
show status like 'Max_used_connections';
This number is reset when you restart your database service.
I have the following work on my application, in which I am trying to update the value total on my mysql database table called "porcobrar2012". However, the only value that gets updated is the last one generated in the while loop. Why? all values are been printout on the screen with no problem, but those values are not getting updated in the database.
Here is the code:
BigDecimal total = new BigDecimal("0");
try
{
//Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
//Connection connection=DriverManager.getConnection("jdbc:odbc:db1","","");
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection connection=DriverManager.getConnection("jdbc:mysql://localhost/etoolsco_VecinetSM?user=etoolsco&password=g7Xm2heD41");
Statement statement=connection.createStatement();
String query;
query="SELECT * FROM porcobrar2012";
ResultSet resultSet=statement.executeQuery(query);
while(resultSet.next())
{
out.println(resultSet.getString(2)+"");out.println(resultSet.getBigDecimal(3)+"");out.println(resultSet.getBigDecimal(4)+"");out.println(resultSet.getBigDecimal(5)+"");out.println(resultSet.getBigDecimal(6)+"");out.println(resultSet.getBigDecimal(7)+"");out.println(resultSet.getBigDecimal(8)+"");out.println(resultSet.getBigDecimal(9)+"");out.println(resultSet.getBigDecimal(10)+"");out.println(resultSet.getBigDecimal(11)+"");out.println(resultSet.getBigDecimal(12)+"");out.println(resultSet.getBigDecimal(13)+"")out.println(resultSet.getBigDecimal(14)+"");out.println(resultSet.getBigDecimal(15)+"");
total = resultSet.getBigDecimal(3).add(resultSet.getBigDecimal(4)).add(resultSet.getBigDecimal(5)).add(resultSet.getBigDecimal(6)).add(resultSet.getBigDecimal(7)).add(resultSet.getBigDecimal(8)).add(resultSet.getBigDecimal(9)).add(resultSet.getBigDecimal(10)).add(resultSet.getBigDecimal(11)).add(resultSet.getBigDecimal(12)).add(resultSet.getBigDecimal(13)).add(resultSet.getBigDecimal(14)).add(resultSet.getBigDecimal(15));
String query1;
query1="UPDATE porcobrar2012 SET total=total";
PreparedStatement ps = connection.prepareStatement(query1);
ps.executeUpdate();
out.println(total);
}
connection.close();
statement.close();
}
catch (Exception e)
{
//e.printStackTrace();
out.println(e.toString());
}
It's because the update closes the existing result set. But I would ask why you aren't doing the addition in a single UPDATE statement without any prior query, at the database, no loops, no BigDecimals. Rule one of database programming is 'don't move the data further than you need to'. It would be many times as efficient to just write "UPDATE porcobrar2012 SET a=b+c+d+...". And you can remove the Class.forName() call too: it hasn't been required for years.
hello guys i am try to validate username from the database with the username that the user entered in the html from, assume
un//be the variable where username entered now from html form is stored
now how to retrieve all the columns of the uname from user table
uname //column name in mysql for usernames
user //table name in mysql
and check weather the username i.e,un entered now is present or not in the database
i am using
Connection con = DriverManager.getConnection("jdbc:mysql://localhost/mebps","root","admin");
Statement stmt = (Statement) con.createStatement();
ResultSet rs = stmt.executeQuery("select un from userinfo");
while(rs.next())
{
if(rs.getString("uname") == un)
{
out.println("user is present");
}
}
There are at least two major mistakes:
You're comparing string instances by == instead of comparing their values by equals() method. The proper line would be if (rs.getString("uname").equals(un)).
You're not letting the DB do the job of returning the right row, instead you're copying the entire DB table into Java's memory and doing the comparison in Java. This is very inefficient. Make use of SQL powers the smart way so that it always returns exactly the information you need. There's for example a WHERE clause.
On an unrelated note, you seem not to be closing DB resources properly after use. This will result in resource leaking which is also a bad idea as it may cause your application to crash on long term. Further, the column name uname and un are not the same. But I'll assume it to be careless oversimplifying of the example.
Here's a minor rewrite:
public boolean exist(String username) throws SQLException {
Connection connection = null;
PreparedStatement statement = null;
ResultSet resultSet = null;
boolean exist = false;
try {
connection = database.getConnection();
statement = connection.prepareStatement("SELECT uname FROM userinfo WHERE uname=?");
statement.setString(1, username);
resultSet = statement.executeQuery();
exist = resultSet.next();
} finally {
if (resultSet != null) try { resultSet.close(); } catch (SQLException ignore) {}
if (statement != null) try { statement.close(); } catch (SQLException ignore) {}
if (connection != null) try { connection.close(); } catch (SQLException ignore) {}
}
return exist;
}
You see, if there's a match, then it returns true (at least one record), otherwise false (no one record). No need to copy the entire table into Java's memory and crawl through it in Java.
Last but not least, this code doesn't belong in a JSP file, but in a normal Java class, starting with a servlet. See also our servlets wiki page to learn more about it.
I'm building a REST WebService with JAX-RS and Tomcat to consume a MySQL Database.
I'm following this model:
#Path("/login")
public class Login {
String username;
String password;
// This method is called if POST is requested
#POST
#Produces(MediaType.APPLICATION_XML)
public String loginResponseXML(#FormParam("username") String user, #FormParam("password") String pass) {
//Connection to MySQL Database
try {
Class.forName("com.mysql.jdbc.Driver");
Connection con = DriverManager.getConnection("jdbc:mysql://localhost/sakila", "root","larcom");
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("Select first_name, last_name From actor where first_name='" +
user + "' and last_name='" + pass + "'");
while (rs.next()){
System.out.println(rs.getString("first_name") + " " + rs.getString("last_name"));
username = rs.getString("first_name");
password = rs.getString("last_name");
}
}
catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
if (user.equals(username) && pass.equals(password)) {
return ("<?xml version=\"1.0\"?>" + "<auth>200" + "</auth>"); //Success
//return "Success!";
} else {
return ("<?xml version=\"1.0\"?>" + "<auth>404" + "</auth>"); //Damn
//return "Damn!";
}
}
}
I call this method with:
HttpPost httppost = new HttpPost("http://192.168.15.245:8080/org.jersey.andre/rest/login");
Now, my question is:
If I want to query the DB for another table I have to create a new class like Login and make the JDBC connection again?
A new class and a new JDBC connection for each class that make a query to the DB? Performance issues?
Hope you can understand.
Thanks in advance.
A few tips are in order here: Please isolate the DB based code to a "data layer" so to speak...only perform dispatching/business logic within your resource classes.
Now If you are querying a different table, you WILL have a different query! You could either use the same connection (bad) or create a new one and fire a different query(s).
Now whether each resource hits a different table or the same table with a different query depends on your choice of 'representation' for that resource. There is a reason a RDB schema has multiple tables and it's quite common you'll have a different query involving multiple tables or to mutually independent tables.
Performance issues: For 'fresh data' you ARE always going to hit the DB so to speak. If you want to optimize that either develop your own cache (extremely hard) or use approaches like memcached or ehcache to boost performance - before you decide to do that make sure you verify if it's worth it.
Are you going to be having about 1000 DB hits per second? You probably need some performance boosting/handling. Per day...maybe not. Per 2-3 days...YAGNI (You ain't gonna need it, so don't worry for now)
So, for every 'resource' that you design in your application (Login is NOT a resource: See related post: Why is form based authentication NOT considered RESTful?) choose the representation. It may involve different queries etc., for you to return json/xml/xhtml (whatever you choose). Each 'DB related call' should be isolated into it's own 'data layer' - I suggest go with Spring JDBC to make your life easier. It'll take the burden of JDBC plumbing off your shoulders so you can focus on creating your DAOs (Data Access Objects - a patter for Data Access classes. All DAOs logically belong in the data layer)
Hope this helps