unique

问题 Java: I have a problem using System.currentTimeMillis() function i am using System.currentTimeMillis() to generate unique values in foor loop problem is loop executes too fast and System.currentTimeMillis() gives me duplicate values. How can i generate for sure unique values. for(int a=0;a<=10;a++){ System.out.println(System.currentTimeMillis()) } I also tried following but it is also not generaet to generate unique number System.currentTimeMillis()+Math.random() 回答1: why don't you use System

问题 I understand how str_shuffle() or shuffle works but I don't know it in this case. $word="tea"; I want to echo out all unique shuffling possibilities (tea, tae, eta, eat, ate, aet) 回答1: You need to produce all of the permutations of the string, either by iterating through the possibilities, or using a recursive method like this one below. Note that for a moderately sized array this will grow very large very quickly. For a word with unique characters, the number of possible permutations is n!

问题 I want make value of TEXT field unique in my MySQL table. After small research I discovered that everybody are discouraging using UNIQUE INDEX on TEXT fields, due to performance issues. What I want to use now is: 1) create another field to contain hash of TEXT value (md5(text_value)) 2) make this hash field UNIQUE index 3) use INSERT IGNORE in queries Is this solution complete, secure and optimal? (found it on SO) Is there a better way of achiving this goal? 回答1: As I was asked in the

问题 Is it possible to add a constraint like ALTER TABLE `t1` ADD UNIQUE(`col1`, `col2`); without creating an index? The index wouldn't be used for any queries so it would be a waste of space. It wouldn't be a problem if inserts and updates would be way slower, because the table doesn't get updated very often. 回答1: No, this is not possible. A UNIQUE constraint contains an index definition and I barely imagine how it might be implemented without creating an index (in DBMS terms). You should realize

问题 We have a script that should function like so: Script inserts n rows into database inserting the same unique number into the batch column. Script puts a job on the AWS queue with the batch number. AWS worker does some processing and fires off another script on our server. Script on our server inserts the response from the AWS worker into all rows in the batch. This is all easy except - creating the batch number. We can't just get the max batch number and add 1 due to multiple users being able

问题 I want to save a simple model with Django REST Framework. The only requirement is that UserVote.created_by is set automatically within the perform_create() method. This fails with this exception: { "created_by": [ "This field is required." ] } I guess it is because of the unique_together index. models.py: class UserVote(models.Model): created_by = models.ForeignKey(User, related_name='uservotes') rating = models.ForeignKey(Rating) class Meta: unique_together = ('created_by', 'rating')

问题 I have a list of sets : L = [set([1, 4]), set([1, 4]), set([1, 2]), set([1, 2]), set([2, 4]), set([2, 4]), set([5, 6]), set([5, 6]), set([3, 6]), set([3, 6]), set([3, 5]), set([3, 5])] (actually in my case a conversion of a list of reciprocal tuples) and I want to remove duplicates to get : L = [set([1, 4]), set([1, 2]), set([2, 4]), set([5, 6]), set([3, 6]), set([3, 5])] But if I try : >>> list(set(L)) TypeError: unhashable type: 'set' Or >>> list(np.unique(L)) TypeError: cannot compare sets

问题 I have a list of sets : L = [set([1, 4]), set([1, 4]), set([1, 2]), set([1, 2]), set([2, 4]), set([2, 4]), set([5, 6]), set([5, 6]), set([3, 6]), set([3, 6]), set([3, 5]), set([3, 5])] (actually in my case a conversion of a list of reciprocal tuples) and I want to remove duplicates to get : L = [set([1, 4]), set([1, 2]), set([2, 4]), set([5, 6]), set([3, 6]), set([3, 5])] But if I try : >>> list(set(L)) TypeError: unhashable type: 'set' Or >>> list(np.unique(L)) TypeError: cannot compare sets

问题 I have a basic users table I want to create in MySQL. I do not want duplicate emails or duplicate usernames appearing in the database. What is the best way of preventing this upon table creation? And what is the difference between the following: 1. UNIQUE (username), UNIQUE (email), 2. UNIQUE KEY (username), UNIQUE KEY (email), 3. CONSTRAINT ucons_login UNIQUE (username, email), I assume some of these are synonymous, yet I've been reading conflicting information online and was seeking

问题 With help of Zohar Answer, I got SQL function to generate random string but I am facing the problem with duplicate. Query Create FUNCTION [dbo].[MaskGenerator] ( @Prefix nvarchar(4000), -- use null or an empty string for no prefix @suffix nvarchar(4000), -- use null or an empty string for no suffix @MinLength int, -- the minimum length of the random part @MaxLength int, -- the maximum length of the random part @Count int, -- the maximum number of rows to return. Note: up to 1,000,000 rows