I have a SQL Server database that holds a table where a varchar column has a check constraints on it to make sure only a few different words can be entered as a value (names).
Like this
CONSTRAINT chk_Names CHECK (name IN ('John', 'Eva', 'Carl', 'Fred'))
What I want do do is to populate a combobox in java with these names, and I don't want to manually enter them since they might change in the database. I want to populate it from metadata.
But I haven't been able to find a way to get the information from the database either with the INFORMATION_SCHEMA or sys.objects (or from DatabaseMetaData in java for that matter)
I'm quite new to SQL Server but is it possible to get that information somehow?
Regards
/Fred
It sounds like you should move the list of names to a table. You're Java form could select the data from the table.
And, because the data can change, it will be better to update the table than to change the check constraint. You can change the check constraint to a foreign key constraint too.
You can also find the check-constraint definitions in INFORMATION_SCHEMA.CHECK_CONSTRAINTS. The expression is in the CHECK_CLAUSE column; and, you'll have to extract the values from the expression.
Related
I am trying to track changes made to a database (schema) using a java app. We are trying to track changes for each column/unique-constraint/index and table.
Functionally I know table.column is unique. So, if the datatype of a column changes, we know which column to find and record the change. But what if the name changes? If JDBC's result set is ordered (it asks for index), then I can rely on the order to give me the same column everytime, even if the name changes. Will there be any surprises here, since it is a result 'set'?
However, I learnt that we can change the order of the columns as well. Isn't there any unique ID associated with the columns so that they can be picked up on that basis?
I would mostly not want to use information_schema route, but even though i checked there for mysql, found nothing useful.
I am using JPA and Hibernate to connect to a table and insert data in the same. I have a table say : User which has three columns ID, Name and Address. I have an entity class for the same and to insert the data I simply use the EntityManager's object and persist the data in the db which works like a charm for me.
Now I have a scenario where I want to check whether the values that I am persisting already exist, if that is the case I have to log an error. Currently how I am doing that is manually loading the rows from the table and manually checking if the same values exist or not which is fairly simple for the example table (User) that has only three columns. But what if I have a table with 30 columns?
Do I manually load the data based on one condition and check for other columns or is there a better and a short way to do that ?
30 columns, is that you primary key as? If the data you are checking for duplication is the primary key, or unique constraint then you can use Hibnerate to fetch an object before save and report back if it exists. If the 30 columns are not part of the key then I would use equals method, and as such fetch all rows. However if there are many rows and this would be slow then I would probably write an dedicated SQL to check wherever an object exists, i.e.
UserDao public boolean rowExists(User user) { ... }
I'm dealing with a legacy database that uses a strange key/ID configuration for one of its tables. It's the table that defines user information. Here are the columns (I've simplified things a little):
ID
Secondary ID
First Name
Last Name
Change Type
All of these columns are part of the key in the database itself and are needed to uniquely identify a row, with one exception. When the Change Type column has a null value then the ID column uniquely identifies a row. This exception is heavily relied on to get a user's name based on their ID. However I need to specify all columns as #Id for hibernate to work correctly with this table ... or do I? Assuming I do, how would I go about also implementing the exception so that objects can be loaded from the database by just the ID? Ideally I'd like to be able to interact with this object as if ID was the only key since in practice that's how it's done in straight SQL by the DBAs.
Just a quick question about locking tables in a postgres database using JDBC. I have a table for which I want to add a new record to, however, To do this for the primary key, I use an increasing integer value.
I want to be able to retrieve the max value of this column in Java and store it as a variable to be used as a new primary key when adding a new row.
This gives me a small problem, as this is going to be modelled as a multi-user system, what happens when 2 locations request the same max value? This will of course create a problem when trying to add the same primary key.
I realise that I should be using an EXCLUSIVE lock on the table to prevent reading or writing while getting the key and adding a new row. However, I can't seem to find any way to deal with table locking in JDBC, just standard transactions.
psuedo code as such:
primaryKey = "SELECT MAX(id) FROM table1;";
primary key++;
//id retrieved again from 2nd source
"INSERT INTO table1 (primaryKey, value 1, value 2);"
You're absolutely right, if two locations request at around the same time, you'll run into a race condition.
The way to handle this is to create a sequence in postgres and select the nextval as the primary key.
I don't know exactly what direction you're heading and how your handle your data, but you could also set the column as a serial and not even include the column in your insert query. The column will automatically auto increment.
In my web application I have a form in that I am creating dynamic select boxes and text boxes as per need(It can be more than 1), default there is one pair of select box and text box and my table has two columns for storing values of that select box and text box and values are getting store in my table , but problem is if I add more than one pair of select And text box in my form how to store values in my table? Could it be possible to create columns in table at RUN TIME as per need.
If you can count the number of text boxes and select boxes you can use sql's alter table query at runtime to add columns dynamically.
You created a bad schema for your application.
To simply solve this try schema like this:
groupid, selectbox, textbox
and store the data in more rows.
For example if there is 2 pairs of boxes then you store two rows:
1, 'select', 'text'
1, 'select2', 'text2'
And so on.
You can easily create by using the good old JDBC; you can see an example here. As suggested by others, this is not a proper design approach.
I do not know if it is possible by using JPA since the table creation is a property that you need to specify in persistence.xml . JPA can automatically create tables, but you will lose all data each time you want persist data to an existing table (unless there is some override configuration parameter which is unknown to me).