Any advice on how to read auto-incrementing identity field assigned to newly created record from call through java.sql.Statement.executeUpdate?
I know how to do this in SQL for several DB platforms, but would like to know what database independent interfaces exist in java.sql to do this, and any input on people's experience with this across DB platforms.
The following snibblet of code should do ya':
PreparedStatement stmt = conn.prepareStatement(sql,
Statement.RETURN_GENERATED_KEYS);
// ...
ResultSet res = stmt.getGeneratedKeys();
while (res.next())
System.out.println("Generated key: " + res.getInt(1));
This is known to work on the following databases
Derby
MySQL
SQL Server
For databases where it doesn't work (HSQLDB, Oracle, PostgreSQL, etc), you will need to futz with database-specific tricks. For example, on PostgreSQL you would make a call to SELECT NEXTVAL(...) for the sequence in question.
Note that the parameters for executeUpdate(...) are analogous.
ResultSet keys = statement.getGeneratedKeys();
Later, just iterate over ResultSet.
I've always had to make a second call using query after the insert.
You could use an ORM like hibernate. I think it does this stuff for you.
#ScArcher2 : I agree, Hibernate needs to make a second call to get the newly generated identity UNLESS an advanced generator strategy is used (sequence, hilo...)
#ScArcher2
Making a second call is extremely dangerous. The process of INSERTing and selecting the resultant auto-generated keys must be atomic, otherwise you may receive inconsistent results on the key select. Consider two asynchronous INSERTs where they both complete before either has a chance to select the generated keys. Which process gets which list of keys? Most cross-database ORMs have to do annoying things like in-process thread locking in order to keep results deterministic. This is not something you want to do by hand, especially if you are using a database which does support atomic generated key retrieval (HSQLDB is the only one I know of which does not).
Related
I'm trying to create a table in mysql through java. I'm using putty for this by the way. Here is a bit of the code I have so far but it doesn't work.
rs=s.executeQuery("CREATE TABLE test(id CHAR(2),name VARCHAR(3),PRIMARY KEY(id)); ");
while(rs.next())
{
System.out.println(rs.getString(1));
}
catch (SQLException ex)
{
System.out.println("SQLException:"+ ex.getMessage());
}
executeQuery() is for quires (usually SELECT) that return a ResultSet.
With DML (and DDL) queries you need to use executeUpdate() method.
For more information and examples use Setting Up Tables tutorial.
See this post here: Data Manipulation Statements
You should be using executeUpdate() if you wish to actually modify the database.
Your query is ok! But you don't get a result set! the CREATE TABLE won't give an rows or columns.
You have been tricked be the documentation:
Returns:
a ResultSet object that contains the data produced by the given query; never null
however
Throws:
SQLException - if a database access error occurs,... the given SQL statement produces anything other than a single ResultSet object, ...
In my opinion a call of "execute" would be the proper way.
I don't think its ever a good idea to generate your database schema via Java. Use the utility tool that comes with your database to create your schema. This way, you or anyone (such as a DBA) can create your tables, views, indexes, constraints, grant permissions, etc without having to know Java. You can even have your database utility generate an SQL script that you can run to re-generate the schema from scratch. Last point: I believe you will be better off calling your primary key test_id and making it type numberic, long, or int. this way, when you refer to it as a foreign key in another table, you will immediately know it refers back to the test table.
sorry, if the question title is misleading or not accurate enough, but i didn't see how to ask it in one sentence.
Let's say we have a table where the PK is a String (numbers from '100,000' to '999,999', comma is for readability only).
Let's also say, the PK is not sequentially used.
Now i want to insert a new row into the table using java.sql and show the PK of the inserted row to the User. Since the PK is not generated by default (e.g. insert values without the PK didn't work, something like generated_keys is not available in the given environment) i've seen two different approaches:
in two different statements, first find a possible next key, then try to insert (and expect that another transaction used the same key in the time between the two statements) - is it valid to retry until success or could any sql trick with transaction-settings/locks help here? how can i realize that in java.sql?
for me, that's a disappointing solution, because of the non-deterministic behaviour (perhaps you could convince me of the contrary), so i searched for another one:
insert with a nested select statement that looks up the next possible PK. looking up other answers on generating the PK myself I came close to a working solution with that statement (left out the casts from string to int):
INSERT INTO mytable (pk,othercolumns)
VALUES(
(SELECT MIN(empty_numbers.empty_number)
FROM (SELECT t1.pk + 1 as empty_number
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON t1.pk + 1 = t2.pk
WHERE t2.pk IS NULL
AND t1.pk > 100000)
as empty_numbers),
othervalues);
that works like a charm and has (afaik) a more predictable and stable solution than my first approach, but: how can i possibly retrieve the generated PK from that statement? I've read that there is no way to return the inserted row (or any columns) directly and most of the google results i've found, point to returning generated keys - even though my key is generated, it's not generated by the DBMS directly, but by my statement.
Note, that the DBMS used in development is MSSQL 2008 and the productive system is currently a DB2 on AS/400 (don't know which version) so i have to stick close to SQL standards. i can't change the db-structure in any way (e.g. use generated keys, i'm not sure about stored procedures).
DB2 for i allows generated keys, stored procedures, user defined functions - pretty much all of the things SQL Server can do. The exact implementation is different, but that's what manuals are for :-) Ask your admin what version of IBM i they're running, then hit up the Infocenter for specifics.
The constraining factor is that you can't alter the database design; you are stuck with apparently multiple processes trying to INSERT while backfilling 'holes' in the existing keyspace. That's a very tough nut to crack. Because you can't change the DB design, there's nothing to be done except to allow for and handle PK collisions. There's no SQL trick that'll help - the SQL way is to have the DB generate the PK, not the application.
There are several alternatives to suggest, in the event that some change is allowed. All have issues needing a workaround, but that is unavoidable at this point due to the application design.
Create a UDF that all INSERT clients use to retrieve the next available PK. Use a table of 'available numbers' and delete them as they are issued.
Pre-INSERT all the available numbers. Force clients to do an UPDATE. Make them FETCH...FOR UPDATE where (rest of data = not populated). This will lock the row, avoiding collisions as well as make the PK immediately available.
Leave the DB and the other application programs using this table as-is, but have your INSERT process draw from a block of keys that's been set aside for your use. Keep the next available number in an SQL SEQUENCE or an IBM i data area. This only works if there's a very large hole in the keyspace that's not yet used.
Iam trying to implement memcache in my web application and just wanted to get suggestions that whether what iam doing is right in terms of design.
I have a class SimpleDataAccessor which run all my insert, update and select sql queries. So any query that has to be performed is executed inside the method of this class.
So inside the method where I have my select query implementation i have a method which stores the resultset in memcache like this.
storeinMC(resultset.getJSON(),sqlquery);
the sqlquery here is my key.
Also before running the selectquery i check in memcache that whether I have a resultset already for that query.
if((String res=getRSFromMC(sqlquery)==null)
So i've tried to keep it plain and simple.
Do you see any issues with this.?
As rai.skumar rightfully pointed out your SQL statements could be constructed differently (e.g. WHERE clause could contain same conditions in diff order, etc.)
So to overcome above mentioned issues, you need to parse your SQL and get all the relevant pieces from it. Then you can combine these pieces into a cache key.
You can take a look at SQL parsers: ZQL, JSqlParser, General SQL Parser for Java that return you java classes out of your SQL.
Another option would be to use JPA instead of straight JDBC. For example Hibernate has great JPA support and fully capable of caching your queries.
If you feel closer to JDBC you could use MyBatis that has very JDBC like syntax and caching support.
Consider below queries:
String k1 = "Select * from table"; //Query1
String k2 = "Select * from TABLE"; // Query2 ; notice TABLE is in caps
Both of above SQL queries are same and will fetch same data. But if above queries are used as keys in Memchached they will get stored at different places ( as k1.equals(k2) will return false).
Also if somehow you can ensure that there are no typos or extra spaces, it won't be very efficient as keys/queries could be very big.
I am writing a DAO layer IN Java for my Tomcat server application,
I wish to use Prepared Statement wrapping my queries (1. parsing queries once, 2. defend against SQL injections),
My db design contains a MyISAM table per data source system. And most of the queries through DBO are selects using different table names as arguments.
Some of this tables may be created on the fly.
I already went though many posts that explain that i may not use table name as an argument for Prepared statement.
I have found solutions that suggest to use some type of function (e.g. mysql_real_escape_string) that may process this argument and append the result as a string to the query,
Is there any built in Jave library function that may do it in the best optimized way, or may be you may suggest to do something else in the DAO layer (i do not prefer to add any routines to the DB it self)?
Are you able to apply restrictions to the table names? That may well be easier than quoting. For example, if you could say that all table names had to match a regex of [0-9A-Za-z_]+ then I don't think you'd need any quoting. If you need spaces, you could probably get away with always using `table name` - but again, without worrying about "full" quoting.
Restricting what's available is often a lot simpler than handling all the possibilities :)
If you want to be extra safe than you can prepare a query and call it with supplied table name to check if it really exists:
PreparedStatement ps = conn.prepareStatement("SHOW TABLES WHERE tables = ?");
ps.setString(1, nameToCheck);
if(!ps.executeQuery().next())
throw new RuntimeException("Illegal table name: " + nameToCheck);
(The WHERE condition might need some correction because I don't have mysql under my fingers at the moment).
I am new to MySql database. I've large table(ID,...). I select ID frequently with java code and.And that make a heavy load on transaction
select from tableName where ID=someID
notes:
1.Database could be 100,000 records
2.I can't cache result
3.ID is a primary key
4.I try to optimize time needed to return result from query.
Any ideas for optimization ?
thanks in advance
I fail to see the need to optimize. This is a simple query against a very tiny table in database terms and the item inthe where clause is a PK and thus indexed. This should run very fast.
Have you considere partitioning? Improving Database Performance with Partitioning.
If you change the query to use a parameter, it might be a bit more efficient. The server would not have to parse and semantic check the statement each time.
select * from tableName where ID = #someID
Then assign the parameter value for each execution. Here is an explanation of using prepared statements.