primary-key

问题 i have declared an entity the following way: public class MyEntity { @PrimaryKey @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) private Long id; @Persistent private String text; //getters and setters } Now I want to retrieve the objects using the id. I tried to manage it from the Google Appengine Data Viewer with "SELECT * FROM MyEntity Where id = 382005" or via a query in a servlet. I get no results returned. But i know for sure that the object with the id exists (i made a jsp

问题 i have declared an entity the following way: public class MyEntity { @PrimaryKey @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) private Long id; @Persistent private String text; //getters and setters } Now I want to retrieve the objects using the id. I tried to manage it from the Google Appengine Data Viewer with "SELECT * FROM MyEntity Where id = 382005" or via a query in a servlet. I get no results returned. But i know for sure that the object with the id exists (i made a jsp

问题 I am new to core data and iphone programming so dont mind my silly qn. I couldn't figure out how to mark a attribute as a primary key in the xcdatamodel file or may be outside. I looked around and found some people saying that it is not possible. I am getting identifier for every object that i am getting, i would need to store that object through core data and i might need to use that identifier in my web service to retrieve additional data. How do i keep the primary key of an entity equal to

问题 I am very new to SQL and if my question is silly please forgive my ignorance. What is the difference between Primary Key only and Primary Key constraint? Difference between This CREATE TABLE CUSTOMERS( ID INT NOT NULL, PRIMARY KEY (ID, NAME) and This CREATE TABLE CUSTOMERS( ID INT NOT NULL, CONSTRAINT [Pk_ID_Name] PRIMARY KEY (ID, NAME) Thank you, Dash 回答1: In the second version, you can give a name to your constraint, allowing you to drop it using the name instead of having to name every

问题 How to provide primary key for multiple column in a single table using PostgreSQL? Example: Create table "Test" ( "SlNo" int not null primary key, "EmpID" int not null, /* Want to become primary key */ "Empname" varchar(50) null, "EmpAddress" varchar(50) null ); Note: I want to make "EmpID" also a primary key. 回答1: There can only be one primary key per table. That's what the word "primary" hints at. You can have additional UNIQUE columns. CREATE TABLE test( sl_no int PRIMARY KEY, -- NOT NULL

问题 How to provide primary key for multiple column in a single table using PostgreSQL? Example: Create table "Test" ( "SlNo" int not null primary key, "EmpID" int not null, /* Want to become primary key */ "Empname" varchar(50) null, "EmpAddress" varchar(50) null ); Note: I want to make "EmpID" also a primary key. 回答1: There can only be one primary key per table. That's what the word "primary" hints at. You can have additional UNIQUE columns. CREATE TABLE test( sl_no int PRIMARY KEY, -- NOT NULL

问题 I'm having problem with SubSonic 3 and multiple Primary Key columns. It seems like it isn't supported via the ActiveRecord T4 script. In line 173 of Settings.ttinclude return this.Columns.SingleOrDefault(x=>x.IsPK) ?? this.Columns[0]; It tries to get a Single Primary Key Column and failed. Any solutions? 回答1: Many ORM products do not support composite keys due to the overwhelming complexity of doing so. As far as I know, NHibernate is the only well-known .Net ORM product that does. Mindscape

问题 A basic simple question for all of you DBA. When I do a select, is it always guaranteed that my result will be ordered by the primary key, or should I specify it with an 'order by'? I'm using Oracle as my DB. 回答1: No, if you do not use "order by" you are not guaranteed any ordering whatsoever. In fact, you are not guaranteed that the ordering from one query to the next will be the same. Remember that SQL is dealing with data in a set based fashion. Now, one database implementation or another

问题 Forewarning - Something could be wrong from the start. More than one thing. Any additional help outside of the question I ask would be much appreciated :) Also, admin_tbl can be ignored. It is simply usernames/passwords for administrators for the site. Aim (and question): To create two relationship tables. Using my HTML Form I want to be able to insert information such as: albumName - Exploration name - Dead Sea timeDate - 2015-12-21 comment - A beautiful landscape of the Dead Sea in Israel.

问题 I wonder if anyone can help me with this. I ran the whole thing in sql but its gives me this error: Error report: SQL Error: ORA-00001: unique constraint (GAMES.ATHLETE_PK) violated 00001. 00000 - "unique constraint (%s.%s) violated" *Cause: An UPDATE or INSERT statement attempted to insert a duplicate key. For Trusted Oracle configured in DBMS MAC mode, you may see this message if a duplicate entry exists at a different level. *Action: Either remove the unique restriction or do not insert