Hi I currently have a database that has 5 questions and answers stored in it. I want to randomly output three of the questions each time the app runs. I have the below code but my app crashes when it loads
public List<Question> getAllQuestions() {
List<Question> quesList = new ArrayList<Question>();
int nQuestions = 3; //select COUNT(*) from questions
Random random = new Random();
int id = random.nextInt(nQuestions);
String selectQuery = "SELECT id FROM " + TABLE_QUEST;
dbase=this.getReadableDatabase();
Cursor cursor = dbase.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
Question quest = new Question();
quest.setID(cursor.getInt(0));
quest.setQUESTION(cursor.getString(1));
quest.setANSWER(cursor.getString(2));
quest.setOPTA(cursor.getString(3));
quest.setOPTB(cursor.getString(4));
quest.setOPTC(cursor.getString(5));
quesList.add(quest);
} while (cursor.moveToNext());{
id = random.nextInt(nQuestions);
}
}
// return quest list
return quesList;
}
Sorry that I write this as an answer, but I don't have permissions to write comments to a question, so I want to tell you that first of all you will always become one of the first 3 added questions as a random question, because for the random id you use as limitations 3 id = random.nextInt(nQuestions); instead of nQuestions (which is 3 for you, you should use questList.size() and do it 3 times for the numbers from 0 to questList.size()
The issue is, you are only selected the id from the table, for all entries.
Your random will also select only questions 0, 1 or 2. Your random should be random.nextInt(5) since you have 5 questions.
Finally, this should be in a loop and then add multiple wheres to your query to get multiple questions.
String query = "SELECT * FROM " + TABLE_QUEST + " WHERE ";
for (int x = 0; x < nQuestions; x++) {
if (x > 0) {
query += " OR ";
}
query += "id=" + random.nextInt(5);
}
This will make your query look like this:
SELECT * FROM TABLE_QUEST WHERE id=4 OR id=2 OR id=3
Finally, change the way your cursor.moveToNext() is. Your code should look something like this:
public List<Question> getAllQuestions() {
List<Question> quesList = new ArrayList<Question>();
int nQuestions = 3; //select COUNT(*) from questions
Random random = new Random();
String query = "SELECT * FROM " + TABLE_QUEST + " WHERE ";
for (int x = 0; x < nQuestions; x++) {
if (x > 0) {
query += " OR ";
}
query += "id=" + random.nextInt(5);
}
dbase=this.getReadableDatabase();
Cursor cursor = dbase.rawQuery(query, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
Question quest = new Question();
quest.setID(cursor.getInt(0));
quest.setQUESTION(cursor.getString(1));
quest.setANSWER(cursor.getString(2));
quest.setOPTA(cursor.getString(3));
quest.setOPTB(cursor.getString(4));
quest.setOPTC(cursor.getString(5));
quesList.add(quest);
} while (cursor.moveToNext());
}
// return quest list
return quesList;
}
There are a couple of mistakes in your code.
First, your SQL query is wrong:
String selectQuery = "SELECT id FROM " + TABLE_QUEST;
gives you all the values of the "id" column in the table TABLE_QUEST, not the column with the id you determined with your call to Random.nextInt.
String selectQuery = "SELECT * FROM " + TABLE_QUEST + " WHERE id=" + id;
The "*" stands for "all columns in this table" and the "WHERE" enables you to filter your rows according to the conditions that follow (i.e. id=3).
Note while it's not a problem in this case, passing unescaped values (i.e., the WHERE id=" + id part is very bad form because it might make you vulnerable to SQL injections as soon as you use user input for this).
Second: the rest of the code doesn't make much sense for various reasons. I'll try to list a few:
if your code would work (i.e. with * instead of id in the SELECT, but without the WHERE clause), you'd just add all of the questions in your database to the list, because there's no condition.
if the condition would work, you'd get just one row with a certain id. your do-while loop would run only once.
if you had a loop around the select, then it might work, but it'd probably return the same rows TWICE or even more often.
you'd still get occasional crashes for various reasons, among them
primary keys - the id attribute which is probably auto-increment - is not necessarily in the range 0..n-1. it might be in your case, but that's not exactly typical. auto-incrementing primary keys usually start at 1.
if your question-answers records are CRUD (create, read, update, delete), the ids might have holes, i.e. "1, 2, 6, 8, 12".
i'd rewrite the whole method; there are a couple of ways of doing it. just a few hints:
if there are very few records in the table (e.g. less than a couple hundreds) you can just load all the questions into list A.
create list A
add all records from the database to list A
create a list B
as long as there are fewer elements in list B than you want (i.e. listB.size() < 3) remove a random element from List A and add it to List B.
return listB. // this seems wasteful, but would probably be ok in your case.
let the database do the randomizing:
SELECT * FROM table ORDER BY RANDOM() LIMIT 3;
read all the records into a list
return the list
done! (note: this works with sqlite, but is, as far as i know, not universally accepted by different database systems).
there are countless other ways of achieving your goal. but i'd just use the ORDER BY RANDOM() LIMIT x trick, it's probably the easiest way.
Related
I'm trying to write an sql query which runs over a set and sees if the id is in the set but it gives the error that only 1000 items can be in the array. I'm trying to solve it but I got stuck here:
for (int i = 0; i < e.getEmployeeSet().size(); i+=1000) {
sqlQuery.append("AND employee.id");
if(!e.incudeEmployee()){
sqlQuery.append("NOT ");
}
sqlQuery.append("IN (");
for(Employee employee: e.getEmployeeSet().){
sqlQuery.append(employee.getEmployeeId())
.append(",");
}
sqlQuery.deleteCharAt(sqlQuery.length()-1)
.append(") ");
}
I still have to figure out that the first time it has to be AND id.., the other times it has to be OR ... and I have to go over the set in a way that the first time I only go over the first 1000 employee's and so on. Any clean way to fix this?
Sql allows upto 1000 list values in SQL statements. And it is not an efficient way of including the list in IN clause.
Better store data in a temporary table and add join in your query.
Temporary table creation :
create table temp_emprecords as
select * from a, b,c
where clause...;
Now add the temp_emprecords table in your query and join with employee id.
select *
from employee emp,
temp_emprecords tmp
where emp.id = tmp.id;
You can modify your sql to be like:
SELECT /* or UPDATE (whatever you do) */
...
WHERE
employee.id IN (... first thousand elements ...)
OR
employee.id IN (... next thousand elements ...)
OR
... and so on ...
Your Java code will be slightly different to produce "OR employee.id IN" block for each thousand of ids
UPD: to make it just introduce another counter to do like (pseudo code):
counter=0;
for each employeeId {
if counter equals 1000 {
complete current IN block;
counter=0;
if not first thousand {
start new OR block;
}
start new IN block;
}
add employeeId into IN block;
counter++;
}
but important: I do not recommend go the way as you do either with or without OR blocks
It is because construct SQL as you do is direct way to SQL injection.
To avoid it just follow simple rule:
No any actual data must be inline in SQL String. All data must be passed to Query as parameters
You have to use prepared SQL statement with parameters for employee.id values.
Also: Simple way is to run separate query for each 1000 ids in the loop
So the solution that works is like this:
int counter = 1;
String queryString= "WHERE (employeeId IN ( ";
Iterator<Long> it = getEmployeeSet().iterator();
while (it.hasNext()) {
if (counter % 999 == 0)
queryString= queryString.substring(0, queryString.length() - 1) + " ) or
employeeId IN ( '" + it.next()+ "',";
else
queryString+= "'" + it.next() + "',";
counter++;
}
append(queryString.substring(0, queryString.length() - 1) + " )) ");
This is my JPA ENTITY
#Entity
#NamedQueries({
#NamedQuery(name = "Question.randQuestion", query = "SELECT q FROM Question AS q ORDER BY RANDOM")
})
#Table(name = "questions")
public class Question implements Serializable {
.....
}
The problem is:
eclipse gives me an error for this namedQuery. It says:
"The identification variable 'RANDOM' is not defined in the FROM clause"
I've tried also with RAND() instead of RANDOM and also NEWID().
Thanks.
To get a Random Row, first get list of total question and get any one.
public Question getRandomQuestion(EntityManager em) {
Query countQuery = em.createNativeQuery("select count(*) from Question");
long count = (Long)countQuery.getSingleResult();
Random random = new Random();
int number = random.nextInt((int)count);
Query selectQuery = em.createQuery("select q from Question q");
selectQuery.setFirstResult(number);
selectQuery.setMaxResults(1);
return (Question)selectQuery.getSingleResult();
}
Note: You may need to implement a logic to avoid duplicates while calling method more than once.
I had to solve a specific case of this problem where I had to select random records from a set of records matching a certain input criteria. The solution also had to support limit. I describe my solution below starting with assumptions.
Assumptions:
Given the set of criteria as input, it is possible to count number of records that match a selection criteria, as supported by the org.springframework.data.querydsl.QueryDslPredicateExecutor<T>.count(Predicate predicate) method.
Pages are zero indexed.
It is possible to request specific page as supported by the org.springframework.data.domain.PageRequest(int page, int size) method.
Algorithm
Count all records matching the input criteria.
Calculate total number of pages based on the count and specified limit.
Generate a random page index in range [0, total pages).
Request the page with index generated in previous step.
Shuffle elements in the returned page.
Code
Long totalRecords = someRepository.count(somePredicate);
Long totalPages =
(totalRecords % someLimit == 0)
? (totalRecords / someLimit)
: ((totalRecords / someLimit) + 1);
int pageIndex = (int) (Math.random() * totalPages);
PageRequest pageRequest = new PageRequest(pageIndex, someLimit);
Page<T> somePage = someRepository.findAll(somePredicate, pageRequest);
List<T> someList;
if (somePage.getTotalElements() > 0) {
someList = new ArrayList<>(somePage.getContent());
} else {
someList = new ArrayList<>();
}
Collections.shuffle(someList);
The second shuffle is to ensure records within the page are also randomized. The general case of this solution is that there is no criteria and so the count() has to be invoked with no predicate thus getting a count of all rows in the table.
As far as I understand you want to select random question from the table. You'd rather use WHERE clause, with providing some parameter from your code, like:
SELECT q FROM Question AS q WHERE id = :id
Then in the code, which creates your query you must generate random id to be selected:
query.setParam("id", getRandomId());
And to get random id, you may want to query number of rows from DB and use java.util.Random.nextInt(rowsCount) (if all ids are there, of course).
Btw, something similar is described here: http://www.shredzone.de/cilla/page/53/how-to-fetch-a-random-entry-with-hibernate.html
I am developing a portlet in which I access a database quite often. I have to specify the query in a way, that offers the possibility of filtering as a reaction on a user input. The parameters used for filtering are two at the moment, but this number can grow in the future.
At the moment, my construction works pretty well for all inputs, however, I dont think that I am doing it in a right/effective way, since I do not use prepared statement and just construct the query manually.
This is example of my code (serviceFilter is an arrayList and typeFlag is a String)
private String prepareQuery() {
String query = "SELECT * from messages ";
// check filters
if (!typeFlag.equals("ALL")) {
if (typeFlag.equals("XML")) {
query += "WHERE type='" + TYPE_XML + "'";
} else {
query += "WHERE type='" + TYPE_JAVA + "'";
}
}
// lets see if user specifies some service filtering
if (serviceFilter.size() > 0) {
if (!typeFlag.equals("ALL")) {
query += " AND (";
} else {
query += " WHERE (";
}
for (int i = 0; i < serviceFilter.size(); i++) {
if (i>0) {
query += " OR ";
}
String service = serviceFilter.get(i);
System.out.println("Filter: " + service);
query += "sender='" + service + "' OR receiver='" + service + "'";
}
query += ")";
}
query += " ORDER BY id DESC LIMIT " + String.valueOf(limit);
System.out.println(query);
return query;
}
First problem is, that this has no way to prevent SQL injection (which would not be such a big problem since all the inputs come from checkBoxes and scrollbars, so the user does not actually type anything). I am not sure how to use a prepared statement here, because the population of my arrayList can be quite long and changes for every query.
The query itself, due to this fact can get really long. Here is an example of a query just for two arguments (imagine this for 20 items):
SELECT * from messages WHERE (sender='GreenServiceESB#GreenListener' OR receiver='GreenServiceESB#GreenListener' OR sender='queue/DeadMessageQueue' OR receiver='queue/DeadMessageQueue') ORDER BY id DESC LIMIT 50
So basically, my question is: Is this an effective way of constructing my query (propably not, right)? What approach would you suggest?
PS: I am using JDBC to connect to db and execute the query, if it is important in any way...
Thanks for any tips!
If you want to use something like
create.selectFrom(BOOK)
.where(PUBLISHED_IN.equal(2011))
.orderBy(TITLE)
instead of
SELECT * FROM BOOK
WHERE PUBLISHED_IN = 2011
ORDER BY TITLE
you can look to http://www.jooq.org/. It will simplify your code and you can avoid things like "if (something) { sql += " WHERE ..." }". This is antipattern and should not be used when possible.
First of all, you hinted at one of your issues - not using a PreparedStatement. Taking user input and using it directly in a SQL statement opens a site to SQL injection attacks.
I think what you're wanting is this:
select * from (
select *, row_number over (order by id_desc) as rowNum
from messages
where sender in (?,?,?,?,?,?,?,?) --8, 16 or however many ?'s you'll need
or receiver in (?,?,?,?,?,?,?,?)
) results
where rowNum between (1 and ?)
order by rowNum
Now, you bind the parameters with whatever the user input is and if you have extra spots in you IN operator left over, you bind them with some value that can't (or likely won't) be in your table such as null or HiMom#2$#. If you need to support an arbitrary number of values, you run the query multiple times and sort the results in memory yourself.
As far as the row_number function, that may not work in MySQL (I'm not a MySQL guy) but it does have an equivalent (or it could be that limit may be parameterizable, I don't know.
I'm having a problem with a java OutOfMemoryError. The program basically looks at mysql tables that are running on mysql workbench, and queries them to get out certain information, and then puts them in CSV files.
The program works just fine with a smaller data set, but once I use a larger data set (hours of logging information as opposed to perhaps 40 minutes) I get this error, which to me says that the problem comes from having a huge data set and the information not being handled too well by the program. Or it not being possible to handle this amount of data in the way that I have.
Setting Java VM arguments to -xmx1024m worked for a slightly larger data set but i need it to handle even bigger ones but it gives the error.
Here is the method which I am quite sure is the cause of the program somewhere:
// CSV is csvwriter (external lib), sment are Statements, rs is a ResultSet
public void pidsforlog() throws IOException
{
String[] procs;
int count = 0;
String temp = "";
System.out.println("Commence getting PID's out of Log");
try {
sment = con.createStatement();
sment2 = con.createStatement();
String query1a = "SELECT * FROM log, cpuinfo, memoryinfo";
rs = sment.executeQuery(query1a);
procs = new String[countThrough(rs)];
// SIMPLY GETS UNIQUE PROCESSES OUT OF TABLES AND STORES IN ARRAY
while (rs.next()) {
temp = rs.getString("Process");
if(Arrays.asList(procs).contains(temp)) {
} else {
procs[count] = temp;
count++;
}
}
// BELIEVE THE PROBLEM LIES BELOW HERE. SIZE OF THE RESULTSET TOO BIG?
for(int i = 0; i < procs.length; i++) {
if(procs[i] == null) {
} else {
String query = "SELECT DISTINCT * FROM log, cpuinfo, memoryinfo WHERE log.Process = " + "'" + procs[i] + "'" + " AND cpuinfo.Process = " + "'" + procs[i] + "'" + " AND memoryinfo.Process = " + "'" + procs[i] + "' AND log.Timestamp = cpuinfo.Timestamp = memoryinfo.Timestamp";
System.out.println(query);
rs = sment.executeQuery(query);
writer = new CSVWriter(new FileWriter(procs[i] + ".csv"), ',');
writer.writeAll(rs, true);
writer.flush();
}
}
writer.close();
} catch (SQLException e) {
notify("Error pidslog", e);
}
}; // end of method
Please feel free to ask if you want source code or more information as I'm desperate to get this fixed!
Thanks.
SELECT * FROM log, cpuinfo, memoryinfo will sure give a huge result set. It will give a cartesian product of all rows in all 3 tables.
Without seeing the table structure (or knowing the desired result) it's hard to pinpoint a solution, but I suspect that you either want some kind of join conditions to limit the result set, or use a UNION a'la;
SELECT Process FROM log
UNION
SELECT Process FROM cpuinfo
UNION
SELECT Process FROM memoryinfo
...which will just give you all distinct values for Process in all 3 tables.
Your second SQL statement also looks a bit strange;
SELECT DISTINCT *
FROM log, cpuinfo, memoryinfo
WHERE log.Process = #param1
AND cpuinfo.Process = #param1
AND memoryinfo.Process = #param1
AND log.Timestamp = cpuinfo.Timestamp = memoryinfo.Timestamp
Looks like you're trying to select from all 3 logs simultaneously, but ending up with another cartesian product. Are you sure you're getting the result set you're expecting?
You could limit the result returned by your SQL queryes with the LIMIT estatementet.
For example:
SELECT * FROM `your_table` LIMIT 100
This will return the first 100 results
SELECT * FROM `your_table` LIMIT 100, 200
This will return results from 100 to 200
Obviously you can iterate with those values so you get to all the elements on the data base no matter how many there are.
I think your are loading too many data at the same in the memory. try to use offset and limit in your sql statement so that you can avoid this problem
Your Java code is doing things that the database could do more efficiently. From query1a, it looks like all you really want is the unique processes. select distinct Process from ... should be sufficient to do that.
Then, think carefully about what table or tables are needed in that query. Do you really need log, cpuinfo, and memoryinfo? As Joachim Isaksson mentioned, this is going to return the Cartesian product of those three tables, giving you x * y * z rows (where x, y, and z are the row counts in each of those three tables) and a + b + c columns (where a, b, and c are the column counts in each of the tables). I doubt that's what you want or need. I assume you could get those unique processes from one table, or a union (rather than join) of the three tables.
Lastly, your second loop and query are essentially doing a join, something again better and more efficiently left to the database.
Like others said, fetching the data in smaller chunks might resolve the issue.
This is one of the other threads in stackoverflow that talks about this issue:
How to read all rows from huge table?
I made a SQLite highscores that has 3 columns; rank(int), score(long) and percentage(int). I am trying to pull all the data from the columns and compare them to the score the user just got to see if their new score made the highscores list that holds the top 10.
I want the highscores to be prioritized by percentage first and then score to break any ties. Below is where I am starting to do this. Am I on the correct track? I have never used SQLite even outside of Java so this is all brand new to me.
Thank you in advance for your help.
//Check if new record makes the top 10.
public boolean check(int rank, long score, int percentage) {
//SQL query to get last score/percentage if highscores has 10 records...
long[] scores = new long[9];
int[] percentages = new int[9];
scores = db.execSQL("SELECT " + SCORE + " FROM " + TABLE + ";"); //Error: Type mismatch
percentages = db.execSQL("SELECT " + PERCENTAGE + " FROM " + TABLE + ";"); //Error: Type mismatch
//Algorithms to compare scores and percentages go here...
}
scores = db.execSQL("SELECT " + SCORE + " FROM " + TABLE + ";");
execSQL returns void.
As per javadoc
Execute a single SQL statement that is NOT a SELECT or any other SQL statement that returns data.
You may need to use either query (or) rawQuery mechanisms to do select.