I'd like to add the autoIncrement property for a numeric column in SORM. How do I do that?
There is no way to configure this in SORM and there's a reason: in practice auto-increment property is only needed for id-columns, and SORM manages ids for you.
If you're, however, absolutely sure that you need this, you can always let SORM generate the schema and then alter such details according to your needs with db-administration tool. But again, I'm sure that you're approaching the problem incorrectly. What do you need that for?
Related
I just answered this question: Can I get the ID of the data that I just inserted?. I explained that in order to know the ID of the last record inserted in a table, what I would do is inserting it manually, instead of using some sequence or serial field.
What I like to do is to run a Max(id) query before INSERT, add 1 to that result, and use that number as ID for the record I'm about to insert.
Now, what I would like to ask: is this a good idea? Can it give some trouble? What are the reasons to use automatically set field on IDs fields?
Note: this is not exactly a question, but looking help center it seems like a good question to ask. If you find it to be off-topic, please tell me and I'll remove it.
This is a bad idea and it will fail in a multi threaded (or multi users) environment.
Please note that the surrogate-key vs natural-key debate is still far from having a concrete definitive solution - but putting that aside for a minute - even if you do go with a surrogate key - you should never try to manually auto-increment columns. Let the database do that for you and avoid all kinds of problems that can occur if you try to do that manually - such as primary key constraint violations in the best case, or duplicate values in the worst case.
If an Entity uses an ID as the Primary-Key, it is in general a good idea to let the DB autocreate it, so you don't need to determine an unused one while creating this Entity in your code. Furthermore a DateAccessObject(DAO) does not need to operate on the ID.
Dependant on what DB u might use, you might even not be allowed to retrieve all IDs of that Table..
I guess there might be other good reasons to let the DB manage this part.
I am using OrientDB for the first time. I read that this database operates in a schema less mode.
Although there seems to be some confusion between Field and Property. What is the difference between these two?
The ALTER command does work on fields but fields are shown under the property name in OrientDB studio in query results. Field operations are done through UPDATE. Am I missing something. Please clarify.
The field and property are the same thing in OrientDB.
When you use UPDATE you should not specify the word property or field, the property is just used in ALTER queries, for example ALTER PROPETY, see:
https://orientdb.com/docs/last/SQL-Alter-Property.html
and
https://orientdb.com/docs/last/SQL-Update.html
From the SORM documentation, it's not really clear and the API does not help.
So, how do I specify a custom column name in an entity? I would also like to change the column name of the primary key for each entity. Is that possible?
Thanks.
No. Changing column names isn't possible because SORM is all about convention over configuration. That's what makes it simple.
The primary key isn't configurable either, but for different reasons.
I am totally new to SQL Server management, I tried to add a constraint in a table.
The situation is that I created a column with only 'Y'or 'N' allowed to be valued.
So I tried to create a constraint in Management Studio by right clicking "Constraints" in the table.
However, I really have no idea about the syntax of creating a constraint. Finally, I tried to input the code into "Check Constraint Expression" window by referring the template from the Internet. The SQL Server always tell me "Error Validating constaint".
Can you guys just help me to write the first constraint for me? Because I really dont know how to start.
My requirement is:
I have a table called "Customer"
I created a column called "AllowRefund"
The column "AllowRefund" is only allowed to 'Y' or 'N'
Thanks.
I'd advise against what you are trying to do. There is a datatype (Bit) that is designed to represent values with two states. If you use this type you won't even need the constraint at all. SQL Server will enforce the value to be either one or zero with no additonal work required. You just have to design your app to treat 1 as yes, and 0 as No.
The approach you are attempting is just not a good idea and no good will come of it.
You can do this as follows:
ALTER TABLE Customer
ADD CONSTRAINT CK_Customer_AllowRefund
CHECK (AllowRefund in ('Y','N'))
However #JohnFx is correct - you'd be better off making this column a bit field.
I partly agree with JohnFix, but as knowing the correct syntax to define a check constraint might be useful for you in the future (as you apparently don't read manuals), here is the SQL to create such a constraint:
alter table customer
add constraint check_yes_no check (AllowRefund in ('Y', 'N'));
You probably want to also define the column as NOT NULL in order to make sure you always have a value for that.
(As I don't use "Management Studio" I cannot tell you where and how you have to enter that SQL).
(Table)File has many (Table)Words
FK Words.file_id related to a single File.id
Default value of Words.frame is equal to File.frame for that PK/FK
Does this type of default relationship have a name? Examples on getting this setup? (MySQL)
Edit
The reason for this is that words may have the same frame as the file and if they do, we want to use that default, however some may not and need to be set manually. Is this really bad practice to handle it this way as described in one of the answers? Any improvement suggestions?
You may want to use a Trigger. You should be able to mimick the "default value" of Words.frame to be based on the value of another field from the File table.
It doesn't have a name, but feels like denormalization / data duplication to me.
#Daniel Vassallo suggests an insert trigger for this, and I think that would be the best approach as well if this is really what you need.