constraints

问题 Using Oracle, if a column value can be 'YES' or 'NO' is it possible to constrain a table so that only one row can have a 'YES' value? I would rather redesign the table structure but this is not possible. [UDPATE] Sadly, null values are not allowed in this table. 回答1: Use a function-based index: create unique index only_one_yes on mytable (case when col='YES' then 'YES' end); Oracle only indexes keys that are not completely null, and the CASE expression here ensures that all the 'NO' values

问题 I am using auto layout for my ViewController I know how to define the gap between two views using constraints programmatically. What I want to do now is when the screen size increase, increase the gap also. Because my UIViews are positioned properly in iphone 4s and 5s but in 6 and 6 plus they are positioned in a small area of the screen. I know adding multiplier we can set the aspect ratio of a view but, how to increase the gap between 2 views when the screen hight increase. UPDATE Let say

问题 Consider this situation: a Car is bought from a Salesperson . A Salesperson works at a Showroom (and at only one Showroom). A Showroom is affiliated to a Manufacturer , and only sells cars made by that Manufacturer. At the same time, a Car is of a particular Model , and a Model is made by a Manufacturer. Restriction R: A Car's Model's Manufacturer must be the same Manufacturer as the Car's Salesperson's Showroom's affiliated Manufacturer. The diagram shows the obvious foreign key

问题 Consider this situation: a Car is bought from a Salesperson . A Salesperson works at a Showroom (and at only one Showroom). A Showroom is affiliated to a Manufacturer , and only sells cars made by that Manufacturer. At the same time, a Car is of a particular Model , and a Model is made by a Manufacturer. Restriction R: A Car's Model's Manufacturer must be the same Manufacturer as the Car's Salesperson's Showroom's affiliated Manufacturer. The diagram shows the obvious foreign key

问题 I have a generic class: public class ListObject<T> { // fields protected T _Value = null; // .. } Now I want to do something like the following: ListObject<MyClass> foo = new ListObject<MyClass>(); ListObject<MyClass> foo2 = new ListObject<MyClass>(); foo.compareTo(foo2); Question: How can I define the compareTo() method with resprect to the generic T ? I guess I have to somehow implement a constraint on the generic T , to tell that T implements a specific interface (maybe Comparable , if

问题 I have tried to ask this question before, but have never been able to word it correctly. I hope I have it right this time: I have a list of unique elements. I want to shuffle this list to produce a new list. However, I would like to constrain the shuffle, such that each element's new position is at most d away from its original position in the list. So for example: L = [1,2,3,4] d = 2 answer = magicFunction(L, d) Now, one possible outcome could be: >>> print(answer) [3,1,2,4] Notice that 3

问题 When adding a unique constraint to a Postgres table does that imply that an index has also been added to that table? Meaning, if I add a UNIQUE constraint on a text column, does that text column now have an index or does an index have to be added separately? 回答1: Yes. UNIQUE constraints are implemented using a btree index in Postgres. Details: How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use? 来源： https://stackoverflow.com/questions/29655439/does-a-postgres

问题 I know the syntax: ALTER TABLE [TheTable] DROP CONSTRAINT [TheDefaultConstraint] but how to I drop the default constraint when I don't know its name? (That is, it was autogenerated at CREATE TABLE time.) 回答1: If you want to do this manually, you can use Management Studio to find it (under the Constraints node inside the table). To do it using SQL: If the constraints are default constraints, you can use sys.default_constraints to find it: SELECT OBJECT_NAME(parent_object_id) AS TableName, name

问题 I understand the old Struts and Springs method of aligning, sizing and distributing views in Interface Builder. However, I cannot seem to figure out how to evenly distribute views using auto layout with Xcode 5. There was a way to do it using Xcode 4, but that option is gone. I have 7 buttons arranged in a vertical stack. On a 3.5" layout, it looks great. When I preview the screen in the 4" layout, all of the buttons remain tightly packed and there is a large amount of space below the last

问题 I am new to OCL, I am using papyrus on eclipse luna 4.4.2 I have a class diagram named CLIENT with the constraint {all attributes are out of scope} a second class named Customer with the constraint {all attributes are mandatory} and another class call Address having an attribute provenance with the constraint {provenance shall be mandatory} How should I write the OCL constraint that suit the formal language? will also like to know how to verify the correctness of the syntax? 回答1: In case of