大约有 18,400 项符合查询结果（耗时：0.0199秒） [XML]
CHECK constraint in MySQL is not working
... answered Jan 22 '10 at 7:11
David KerinsDavid Kerins
1,41911 gold badge88 silver badges33 bronze badges
...
Numbering rows within groups in a data frame
...l, df$cat, FUN = seq_along)
or:
library(plyr)
ddply(df, .(cat), mutate, id = seq_along(val))
or:
library(dplyr)
df %>% group_by(cat) %>% mutate(id = row_number())
or (the most memory efficient, as it assigns by reference within DT):
library(data.table)
DT <- data.table(df)
DT[, id...
NULL vs nil in Objective-C
...
nil should only be used in place of an id, what we Java and C++ programmers would think of as a pointer to an object. Use NULL for non-object pointers.
Look at the declaration of that method:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object...
How do you specify that a class property is an integer?
...rimenting with TypeScript, and in the process of creating a class with an "ID" field that should be an integer, I have gotten a little confused.
...
jQuery UI Dialog with ASP.NET button postback
...true,
show: 'Transfer',
hide: 'Transfer',
width: 320,
autoOpen: false,
minHeight: 10,
minwidth: 10
});
dlg.parent().appendTo(...
What are best practices for REST nested resources?
As far as I can tell each individual resource should have only one canonical path. So in the following example what would good URL patterns be?
...
Tying in to Django Admin's Model History
...min.models import LogEntry, ADDITION
LogEntry.objects.log_action(
user_id = request.user.pk,
content_type_id = ContentType.objects.get_for_model(object).pk,
object_id = object.pk,
object_repr = force_unicode(object),
action_flag = ADDITION
)
where object...
How to choose the id generation strategy when using JPA and Hibernate
I was going through Id generation section of the Hibernate reference guide and "java persistence with Hibernate"
4 Answers
...
@UniqueConstraint and @Column(unique = true) in hibernate annotation
...
As said before, @Column(unique = true) is a shortcut to UniqueConstraint when it is only a single field.
From the example you gave, there is a huge difference between both.
@Column(unique = true)
@ManyToOne(optional = false, fet...
If threads share the same PID, how can they be identified?
...
The four threads will have the same PID but only when viewed from above. What you (as a user) call a PID is not what the kernel (looking from below) calls a PID.
In the kernel, each thread has it's own ID, called a PID (although it would possibly make more sens...