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Can a foreign key be NULL and/or duplicate?
...at field must exist first in a different table (the parent table). That is all an FK is by definition. Null by definition is not a value. Null means that we do not yet know what the value is.
Let me give you a real life example. Suppose you have a database that stores sales proposals. Suppose furth...
Rounded UIView using CALayers - only some corners - How?
... CGRect rect = CGRectMake(10, 10, 200, 100);
MyView *myView = [[MyView alloc] initWithFrame:rect];
[self.view addSubview:myView];
[super viewDidLoad];
}
MyView is just a UIImageView subclass:
@interface MyView : UIImageView
{
}
I'd never used graphics contexts before, but I managed ...
JPA EntityManager: Why use persist() over merge()?
...ed (any changes you make will not be part of the transaction - unless you call merge again). You can through use returned instance (managed one).
Maybe a code example will help.
MyEntity e = new MyEntity();
// scenario 1
// tran starts
em.persist(e);
e.setSomeField(someValue);
// tran ends, an...
Failed to allocate memory: 8
...e, with 6GB sitting free while the Android emulator tells me that it can't allocate 1GB?
– Strozykowski
Nov 11 '11 at 1:38
5
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What is so bad about singletons? [closed]
...haned by the developer world. I still use quite a lot of singletons, especially for factory classes , and while you have to be a bit careful about multithreading issues (like any class actually), I fail to see why they are so awful.
...
Is it correct to use DIV inside FORM?
...
It is totally fine .
The form will submit only its input type controls ( *also Textarea , Select , etc...).
You have nothing to worry about a div within a form.
...
Difference between core and processor
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A core is usually the basic computation unit of the CPU - it can run a single program context (or multiple ones if it supports hardware threads such as hyperthreading on Intel CPUs), maintaining the correct program state, registers, and c...
How to loop through all but the last item of a list?
... would indicate I answered the question he asked, if not the question he really wanted to ask. Asker is welcome to demote this answer.
– freespace
May 27 '09 at 11:35
7
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How to duplicate virtualenv
...is to use pip to generate a requirements file. A requirements file is basically a file that contains a list of all the python packages you want to install (or have already installed in case of file generated by pip), and what versions they're at.
To generate a requirements file, go into your origin...
When to use Common Table Expression (CTE)
...
I use them to break up complex queries, especially complex joins and sub-queries. I find I'm using them more and more as 'pseudo-views' to help me get my head around the intent of the query.
My only complaint about them is they cannot be re-used. For example, I may have...