大约有 40,000 项符合查询结果（耗时：0.0657秒） [XML]
Possible to do a MySQL foreign key to one of two possible tables?
...lways references one target table.
Polymorphic Associations are supported by frameworks such as Rails and Hibernate. But they explicitly say that you must disable SQL constraints to use this feature. Instead, the application or framework must do equivalent work to ensure that the reference is sat...
What is the error “Every derived table must have its own alias” in MySQL?
...
@ToBe I'm curious what you meant by that? The answer holds true in any query, that if you have a derived table in your from clause you need to give it an alias.
– AdamMc331
May 4 '15 at 13:11
...
Hibernate throws org.hibernate.AnnotationException: No identifier specified for entity: com..domain.
....
Using a wrapper type is a better choice when using Hibernate because, by checking if the id is null or not, Hibernate can better determine if an entity is transient (it does not have an associated table row) or detached (it has an associated table row, but it's not managed by the current Persis...
Getting the thread ID from a thread
...proach to identify threads as the ManagedThreadId property id's get reused by your app. So it is not a reliable identifier for threads in some scenarios and you will experience the exception : "An item with the same key has already been added." at line... Give the thread a unique name when you creat...
How do I select an element in jQuery by using a variable for the ID?
...
Doing $('body').find(); is not necessary when looking up by ID; there is no performance gain.
Please also note that having an ID that starts with a number is not valid HTML:
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, di...
Difference between id and name attributes in HTML
...t be processed at all according to the HTML spec)
– ebyrob
May 9 '19 at 17:47
...
ORA-00979 not a group by expression
...
You must put all columns of the SELECT in the GROUP BY or use functions on them which compress the results to a single value (like MIN, MAX or SUM).
A simple example to understand why this happens: Imagine you have a database like this:
FOO BAR
0 A
0 B
and you run SELE...
How to get the instance id from within an ec2 instance?
...me up with: At the end of the line ($), find one or more digits following by one or more lowercase letters. Substitute with the digits only. (Backslash + parentheses tell sed to remember a substring, which is recalled with \1.) I found this a little easier to read--the only backslashes are those r...
Rails migration for has_and_belongs_to_many join table
...ny table must match this format. I'm assuming the two models to be joined by has_and_belongs_to_many are already in the DB : apples and oranges:
create_table :apples_oranges, :id => false do |t|
t.references :apple, :null => false
t.references :orange, :null => false
end
# Adding the...
How do I find a default constraint using INFORMATION_SCHEMA?
...ed for—"I'm looking to get [whether 'a given default constraint exists'] by the name of the constraint." I have edited my answer to make its direct question-satisfying nature much more clear. Hope that helps.
– ErikE
Sep 24 '15 at 20:03
...