Develop Reference

Retrieve and update field java

Using Java I want to obtain all IDs from my database and select GW_STATUS if it is equal to 0. I used the following SQL statement to achieve this.
PreparedStatement get_id = con.prepareStatement("SELECT ID from SF_MESSAGES where GW_STATUS = 0");
Once the IDs have been obtained, I want to update GW_STATUS to 1 according to their ID as demonstrated in the code below but only one field is being updated when I execute the code.
PreparedStatement update = con.prepareStatement("update SF_MESSAGES set GW_STATUS=? where ID = ?");
update.setInt(1,1);
ResultSet x = get_id.executeQuery();
while(x.next()){
int uber = x.getInt(1);
int array[] = new int[] {uber};
for (int value : array) {
System.out.println("Value = " + value); //Successfully obtains and prints each ID from the databse table
update.setInt(2,value); // Only one ID is updated therefore only field updated
}
}
int result = update.executeUpdate();
System.out.println(result + " Records updated");
I've tried using another update statement within the for loop to update every ID obtained but that doesn't work too. How can I successfully update every field according to their ID?

You can make the whole processing much simple. It turns out that you just want to update SF_MESSAGES which have GW_STATUS equals to 0, so your query can look like the following:
update SF_MESSAGES set GW_STATUS=1 where GW_STATUS=0
Therefore, you do not have to fetch IDs, loop over them so it is more efficient solution.

Java/MySQL - How to retrieve data from specific row

I really can't find a solution for this problem:
Here I have two ResultSets, one which always shows me the number of items stored in my database and one that retrieves all the data from it.
I would like to generate a random number and then generate a random item based on the row number/id in my database. Since I'm fairly new I'm not sure if this is an efficient approach. It doesn't look very clean to retrieve all the data and then iterate over it every time. Especially if I had like 1000 items and the randomly generated number is 999.
PreparedStatement randomSelection = con.prepareStatement("SELECT * FROM items ORDER BY RAND() LIMIT 1"); {
String name = ((ResultSet) randomSelection).getString(2);
System.out.println(name);
}
Tried calling the column itemname with the last line. However I just can't look for a good solution for this problem. Would highly appreciate any help since I'm fairly new to databases.
Thank you
EDIT: This is what I tried now and there is no output somehow
Same for
ResultSet numberOfItemsInDataBase = stmt.executeQuery("SELECT count(*) FROM items;");
// this will return a number between 0 and the number of rows - 1
int id = new Random().nextInt(numberOfItemsInDataBase.getInt(1));
ResultSet itemsInDataBase = stmt.executeQuery("select * from items order by id limit 1 offset " + id);
if (itemsInDataBase.next()) {
String item = itemsInDataBase.getString(2);
System.out.println(item);
}

If you just need a random row of the table then you can do it with plain SQL with the function RAND():
ResultSet itemsInDataBase = stmt.executeQuery("select * from items order by rand() limit 1");
if (itemsInDataBase.next()) {
item = new Item(itemsInDataBase.getString(2));
}
If you want to use the generated random number, then use it in the OFFSET clause of the sql statement:
ResultSet numberOfItemsInDataBase = stmt.executeQuery("SELECT count(*) FROM items;");
// the above query will return exactly 1 row
numberOfItemsInDataBase.next();
// this will return a number between 0 and the number of rows - 1
int id = new Random().nextInt(numberOfItemsInDataBase.getInt(1));
ResultSet itemsInDataBase = stmt.executeQuery("select * from items order by id limit 1 offset " + id);
if (itemsInDataBase.next()) {
item = new Item(itemsInDataBase.getString(2));
}

Use ORDER BY RAND() and limit the result to 1. This circumvents you having to query for the count and then ultimately iterate through the ResultSet until you find the random entry.
try (ResultSet randomSelection = connection
.preparedStatement("SELECT * FROM items ORDER BY RAND() LIMIT 1")) {
if (randomSelection.next()) {
String name = randomSelection.getString(2);
}
}

You can use the limit function to get the item.
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1). So in your case the offset can be the the random generated id minus one and maximum number of rows is 1:
select * from items LIMIT {id-1},1; # Retrieve row (id-1)

PreparedStatement.execute() returns false even there is resultset in SQL server?

My sql query consists of 5 part which are highly connected to each other. First part creates a temporary table, second part uses that temporary table and creates another temporary table, third part uses the temporary table that created in second part and again creates another temporary table. And the 4th part select some data from 3rd temporary table and 5th part selects the count of 3th temporary table.
Since temporary tables are only usable within a preparedStatement (what I mean is that a temporary table which created by a preparedStatement are not usable from another preparedStatement, I tried that before it is okey) I need to do that within a prepare statement.
So the first 3 part creates temporary tables because of that after setting the parameters to preparedStatement I run preparedStatement.execute() 3 times(I also tried 1....x times) and then I run the preparedStatement.execute() but it returns false which means that there is no resultset. Why is that?
PreparedStatement preparedStatement = conn.prepareStatement("select * into #tmp from tablex where ...\n" +
" select * into #tmp2 from #tmp where ...\n" +
" select * into #tmp3 from #tmp2 where ...\n" +
" select * from #tmp3\n" +
" select count(*) from #tmp3");
Above, I added a simple illustration. Here I need to get the result of 4th and 5th query with prepared statement. How can I do that?

The statements you're executing produce the following results:
An update count
An update count
An update count
A result set
A result set
The meaning of the boolean false returned by execute(String) is:
true if the first result is a ResultSet object; false if it is
an update count or there are no results
This means that you need to use getUpdateCount() to obtain the (first) update count, and getMoreResults() to get the next result (again, this returns a boolean with the same meaning). Only if execute() or getMoreResults() returns false and getUpdateCount() returns -1 are there no more results.
You need to do something like:
boolean nextResultSet = statement.execute(...);
int resultSetCount = 0;
while (true) {
if (nextResultSet) {
resultSetCount++;
try (ResultSet rs = statement.getResultSet()) {
// Do something with result set
}
} else {
int updateCount = statement.getUpdateCount();
if (updateCount == -1) {
// no more results
break;
}
// do something with update count
}
nextResultSet = statement.getMoreResults();
}
You can probably skip part of this complexity by adding SET NOCOUNT ON as the first statement you execute; then you'll not get the update counts and only need to handle the two result sets.

Foreign Key Not Outputting Query Data

try {
Statement s = conn.createStatement();
ResultSet result2 = s.executeQuery("Select Distinct * From Poem p,Recording r Where r.PoemTitle = p.PoemTitle AND r.poemTitle = 'poem1'");
System.out.print("Result (Select with Join): ");
while (result2.next()) {
System.out.println(result2.getString(1)+ " " + result2.getString(2)+ result2.getString(3));
}
} catch(Exception e) {
System.out.print(e.getMessage());
}
I am trying to output the poemtitle and the date it was recorded. When this runs it outputs the poemtitle and then gives the date the poem was created instead of recorded? Is this because of the relationship?

Most likely it's because of the * in the SELECT list.
Specify the columns that you want returned, in the order you want them returned.
We're just guessing at the name of the column that contains "date recorded" and which table it's in:
SELECT p.PoemTitle
, r.dateRecorded
, r.readBy
FROM Poem p
JOIN Recording r
ON r.PoemTitle = p.PoemTitle
WHERE r.poemTitle = 'poem1'
GROUP
BY p.PoemTitle
, r.dateRecorded
, r.readBy
ORDER
BY p.PoemTitle
, r.dateRecorded DESC
, r.readBy
Notes:
Ditch the old-school comma syntax for the join operation and use the JOIN keyword instead, and relocate the join predicates from the WHERE clause to an ON clause.
Avoid using * in the SELECT list. Explicitly list the columns/expressions to be returned. When we read the code, and that SQL statement, we don't know how many columns are being returned, what order the columns are in, or what the datatypes are. (We'd have to go look at the table definitions.)
Explicitly listing the columns/expressions being returned only takes a little bit of work. If code was only ever written, then it would be fine, save the time writing. But code is READ ten times more than it is written. (And the SQL statement with the * makes the SQL statement virtually indecipherable in terms of which column is being referenced by getString(1).
Listing the columns columns can also make it more efficient on the database, to prepare a resultset with a few columns vs a resultset of dozens of columns, and we also transfer a smaller resultset from the database to the client. With a subset of columns, its more likely we can use a covering index for the query.

Trying to pass a Java variable into a sql string

I've had a look around on the web but can't seem to find a definite answer to my question.
Basically, I have a database and table that are successfully working. Now I want to read each line from my table one by one and store the result into a array and I am trying to use a for loop to be more professional rather then using repetition.
I have this code
for (int i=1; i<=8; i++)
{
String query = "Select * FROM Table1 WHERE ID = i";
Rs = St.executeQuery(query);
COL1Title[i] = Rs.getString("CO1Name");
COL2Age[i] = Rs.getString("CO2Rating");
}
The for loop is in a try catch statement and it's complaining with the error "Unknown column 'i' in 'where clause'"
Im guessing there's a certain way for how variable i is to be inserted in the the query.
I should point out ID is a column that has the auto increment feature added on and is primary key if that helps
Could anyone help me out here?

First, we can simplify the task be executing a single query. Note the addition of the range limit and the ORDER BY - without an ORDER BY the results have an unspecified order!
PreparedStatement stmt = "Select ID, CO1Name, CO2Rating"
+ " FROM Table1"
+ " WHERE ID >= ? AND ID <= ?"
+ " ORDER BY ID";
And bind in placeholders (unless there is good reason otherwise, always use placeholders when injecting data into a query). The values could have been hard-coded above in this case, just as they are hard-coded in the for-loop, but the binding is shown here for future reference:
stmt.setInt(1, 1);
stmt.setInt(2, 8);
Then execute the query:
ResultSet rs = stmt.executeQuery();
And iterate the results. Note that rs.next() must be invoke once before any column is read (the cursor starts before any records) and, in this case, it makes it easy to handle a bunch of results.
while (rs.next()) {
int id = rs.getInt("ID");
String title = rs.getString("CO1Name");
String name = rs.getString("CO2Rating");
// do stuff with this record
}
Note that even though the ORDER BY guarantees that the results are iterated in order of ID, assuming a database cardinality rule ensures each result has a unique ID, there may be 0 to 8 records returned - that is, non-existent records may need to be detected/handled separately.
Also (but not shown), make sure to cleanup (close) the ResultSet when done: use a try/finally or try-with-resources construct.

You need to pass i in string as integer, Replace line by:
String query = String.format("Select * FROM Table1 WHERE ID = %d",i);