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Adding custom fields in my application

I have a SAAS product, which is build by Spring MVC and Hibernate. Generally SAAS products allow user's to customize the product like adding extra fields to the table. So i want to give the flexibility to users, to create custom fields in the tables for themselves. Please provide all the viable solutions to achieve it. Thank you so much for your help.

I'm guessing your trying to back this to a Relational database. The primary problem is that relational databases store things in tables, and tables don't really handle free form data well.
So one solution is to use a document structure that is flexible, like XML (and perhaps ditch the database) but databases have features which are nice, so let's also consider the database-using approaches.
You could create a "custom field" table which would have columns (composite primary key) for
ExtendedTable
ColumnName
but you'd also have to store the data somewhere
(ExtendedKey)
DataItem
And now we get into the really nasty bits. How would you apply constraints to this data? I mean, what would the type be of a DataItem? A general solution would be quite complex (being a type of free form database). Hopefully you could limit the solution to solve only the problems you require solved.
Another approach is to use a single "extra" column that contains an XML record which embeds it's own "column and value" extensions, but if you wanted to display a table of the efficiently, you'd have to parse out every XML document in every field, which is not ideal.
Neither one of these approaches will work well with the existing SQL query language, so you'll then start building your own query language.
I suggest you go back and look at real data requirements, instead of sweeping them under the table with a "and anything else one might want" set of columns on your table.

Your requirement is best suited use case for NoSQL databases (like MongoDB).
Dynamically creating relational database tables & columns (modifying schemas) upon user requests in an application is not a best practice as these involve DDL operations, which are very powerful and in case if you don't handle them carefully, the whole application's database goes to the inconsistent state.

Selecting individual data_types across mutliple tables using SQL and HQL from Oracle

Hello World, Here is the situation,
Basically instead of generating values from a table like like so
34.324, 09/13/2011, thankyou,
I would like to generate the type of that specific value
e.g
VarChar2, Date, varchar2(char 30)
Just to add another layer of difficulty a Java application pulls data from the multiple Oracle database tables using HQL. Thus I would need an equivalent HQL statement to the SQL statement (if it exists)..
I understand the DESC keyword lists the column data types for an entire table however as mentioned above I require specificity.
In Summary
I would like to reverse engineer this application to generate a report of the data_types of the data instead of the actual data itself. I could easily just manually walk through the code however they are over 200 entries and this will be a real pain.
Any help is truly appreciated. If this question is unclear please let me know and I will provide more details and examples.

Insert Query Builder for java

I have a use case where in I need to read rows from a file, transform them using an engine and then write the output to a database (that can be configured).
While I could write a query builder of my own, I was interested in knowing if there's already an available solution (library).
I searched online and could find jOOQ library but it looks like it is type-safe and has a code-gen tool so is probably suited for static database schema's. In the use case that I have db's can be configured dynamically and the meta-data is programatically read and made available for write-purposes (so a list of tables would be made available, user can select the columns to write and the insert script for these column needs to be dynamically created).
Is there any library that could help me with the use case?

If I understand correctly you need to query the database structure, display the result to via a GUI and have the user map data from a file to that structure?
Assuming this is the case, you're not looking for a 'library', you're looking for an ETL tool.
Alternatively, if you're set on writing something yourself, the (very) basic way to do this is:
the structure of a database using Connection.getMetaData(). The exact usage can vary between drivers so you'll need to create an abstraction layer that meets your needs - I'd assume you're just interested in the table structure here.
the format of the file needs to be mapped to a similar structure to the tables.
provide a GUI that allows the user to connect elements from the file to columns in the table including any type mapping that is needed.
create a parametrized insert statement based on file element to column mapping - this is just a simple bit of string concatenation.
loop throw the rows in the file performing a batch insert for each.
My advice, get an ETL tool, this sounds like a simple problem, but it's full of idiosyncrasies - getting even an 80% solution will be tough and time consuming.

jOOQ (the library you referenced in your question) can be used without code generation as indicated in the jOOQ manual:
http://www.jooq.org/doc/latest/manual/getting-started/use-cases/jooq-as-a-standalone-sql-builder
http://www.jooq.org/doc/latest/manual/sql-building/plain-sql
When searching through the user group, you'll find other users leveraging jOOQ in the way you intend

The setps you need to do is:
read the rows
build each row into an object
transform the above object to target object
insert the target object into the db
Among the above 4 steps, the only thing you need to do is step 3.
And for the above purpose, you can use Transmorph, EZMorph, Commons-BeanUtils, Dozer, etc.

"Select or create" from database, in Java

I've done all my database development for the past few years in Ruby, mostly using ActiveRecord. Now I'm stuck using Java for a project, and it feels so verbose and hamfisted, I'm wondering if I'm doing things wrong.
In an ORM paradigm, if I want to insert into related tables, I'd so something like
# Joe Bob got a new car
p = Person.find_or_create_by_name("Joe Bob");
Car.new({:make=>"Toyota", :plate=>"ABC 123", :owner=>p});
In Java, at least using JDBC directly, I'm going to have to do the Person lookup by hand, insert if it doesn't exist, then create the Car entry.
Of course, in real life, it's more than just 2 tables and the pain scales exponentially. Surely there's a better way?

You can use ORM solutions for Java - there are various solutions available.
Links worth looking at:
Hibernate - http://www.hibernate.org/ - probably the leading Java ORM solution
SO Question - Hibernate, iBatis, Java EE or other Java ORM tool
Having said that, I've usually found that for complex applications ORM frequently causes more trouble than it is worth (and yes, this does include Ruby projects with Activerecord). Sometimes it really does make sense to just get at the data directly via SQL rather than attempt to force on object-oriented facade on top of it.

The better way is learn SQL! The ORM you like so much writes SQL for you behind the scenes.
So you can make a quick helper function that tries to select the record, and if it doesn't exist creates it for you.
In MySQL you can use INSERT IGNORE ..... which will insert the row only if it doesn't exist.
And here is a special bit of SQL you may like (MySQL only):
INSERT INTO table (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE id=LAST_INSERT_ID(id), c=3;
This tries to insert the record, if it doesn't exist it returns the auto_increment like usual that you retrieve in your program.
But: If it does exist then it updates it - (only c is set to update in that case), but the cool part is it sets the LAST_INSERT_ID() just like it would on an insert.
So either way you get the ID field. And all in a single bit of SQL.
SQL is a very nice language - you should learn it and not rely on the psudo-language of orm.

If you are using JDBC you need to lookup yourself and create person if it does not exist. There is no better way if you use JDBC.
But you can use Hibernate, it will help you reduce writing the O-R mapping yourself and reduce the boilerplate.
As you come from Ruby and If you find it painful to write all the SQL queries, JDBC boilerplate then the better way is to use ORM. I recommend one of the following,
Hibernate
JPA (If you want to change the ORM implementation then use JPA)

Sormula contains an active record package. The save method will update an existing record or insert if no record exists.
See the active record example on web site.
Also see org.sormula.tests.active.SaveTest.java within the project:
SormulaTestAR record = new SormulaTestAR();
record.attach(getActiveDatabase()); // record needs to know data source
record.setId(8002);
record.setType(8);
record.setDescription("Save one AR 2");
record.save();

Looks like I'm late here, however, ActiveJDBC will do what you want in Java:
Person p = Person.findOrCreateIt("name", "Joe Bob");
Car car = Car.createIt("make", "Toyota", "plate", "ABC 123", "owner", p);
There is a ton more it can do, check out at: http://javalite.io/

Can we write describe table query in JPQL?

I am taking a 'Keyword' and table name from user.
Now, I want to find all the columns of table whose data type is varchar(String).
Then I will create query which will compare the keyword with those column and matching rows will be returned as result set.
I tried desc table_name query, but it didn't work.
Can we write describe table query in JPQL?
If not then is there any other way to solve above situation?
Please help and thank you in advance.

No workaround is necessary, because it's not a drawback of the technology. It is not JPQL that needs to be changed, it's your choice of technology. In JPQL you cannot even select data from a table. You select from classes, and these can be mapped to multiple tables at once, resulting in SQL joins for simplest queries. Describing such a join would be meaningless. And even if you could describe a table, you do not use names of columns in JPQL, but properties of objects. Describing tables in JPQL makes no sense.
JPQL is meant for querying objects, not tables. Also, it is meant for static work (where classes are mapped to relations once and for good) and not for dynamic things like mapping tables to objects on-the-fly or live inspection of database (that is what ror's AR is for). Dynamic discovery of properties is not a part of that.
Depending on what you really want to achieve (we only know what you are trying to do, that's different) you have two basic choices:
if you are trying to write a piece of software in a dynamic way, so that it adjusts itself to changes in schema - drop JPQL (or any other ORM). Java classes are meant to be static, you can't really map them to dynamic tables (or grow new attributes). Use rowsets, they work fine and they will let you use SQL;
if you are building a clever library that can be shared by many projects and so has to work with many different static mappings, use reflection API to find properties of objects that you query for. Names of columns in the table will not help you anyway, since in JPQL queries you have to use names defined in mappings.

Map the database dictionary tables and read the required data from them. For Oracle database you will need to select from these three tables: user_tab_comments, user_tab_cols, user_col_comments; to achieve the full functionality of the describe statement.
There are some talks over the community about dynamic definition of the persistent unit in the future releases of JPA: http://www.oracle.com/goto/newsletters/javadev/0111/blogs_sun_devoxx.html?msgid=3-3156674507

According to me, we can not use describe query in jpql.