primary-key

问题 I am using Spring MVC + Hibernate @Resource(name = "sessionFactory") private SessionFactory sessionFactory; // save public <T> int save(T entity) throws DataAccessException { Session session = sessionFactory.getCurrentSession(); session.save(entity); } As New Record Save , New Primary Key generated which in auto increment (db.MySQL). I want to get and return the new auto incremented value with respect to the above method. Update me ! 回答1: Save method should return generated ID: http://docs

问题 In a RoR migration, how do I auto increment a non-primary-key field? I'd like to do this in the db definition, and not in the model. 回答1: You need to execute an SQL statement. statement = "ALTER TABLE `users` CHANGE `id` `id` SMALLINT( 5 ) UNSIGNED NOT NULL AUTO_INCREMENT" ActiveRecord::Base.connection.execute(statement) you can entry manually in your migration Note this is just an example. The final SQL statement syntax depends on the database. 来源： https://stackoverflow.com/questions/3220473

问题 I need the field or fields (just the name of the field will do) that form the primary key of a Microsoft Access Table, given a connection and a tableName. 回答1: ok, I guess I found it. It should work for all oledb and is sth. like : public static List<string> getKeyNames(String tableName, DbConnection conn) { var returnList = new List<string>(); DataTable mySchema = (conn as OleDbConnection). GetOleDbSchemaTable(OleDbSchemaGuid.Primary_Keys, new Object[] {null, null, tableName}); // following

问题 I need the field or fields (just the name of the field will do) that form the primary key of a Microsoft Access Table, given a connection and a tableName. 回答1: ok, I guess I found it. It should work for all oledb and is sth. like : public static List<string> getKeyNames(String tableName, DbConnection conn) { var returnList = new List<string>(); DataTable mySchema = (conn as OleDbConnection). GetOleDbSchemaTable(OleDbSchemaGuid.Primary_Keys, new Object[] {null, null, tableName}); // following

问题 Struggling with ActiveRecord auto assigning the :id attribute as the primary key even though it is a separate column. Table - legacy-table id - int pk_id - int (primary key) name - varchar2 info - varchar2 Model class LegacyModel < ActiveRecord::Base self.table_name = 'legacy-table' self.primary_key = 'pk_id' default_scope {order(:name => :asc)} alias_attribute :other_id, :id end I don't care that ActiveRecord automatically assigns the primary key (pk_id) to the :id attribute however I lose

问题 Is it good practice to give every database table a primary key? It seems to me that if the primary key is not explicitly needed, then it would just be extra clutter in my database. 回答1: When you probably WOULD: In an OLTP database you'd almost always (in my case always) have a primary key of some sort. Sometimes Guid, sometimes autonumber/identity fields, sometimes set by the application or the customer. Sometimes even a combination of more than one field. This is because you'll typically

问题 As an example create table indexing_table ( id SERIAL PRIMARY KEY, created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(), ); Is there a difference between the following tables? Table 1: create table referencing_table ( indexing_table_id INTEGER references indexing_table ); Table 2: create table referencing_table ( indexing_table_id INTEGER references indexing_table NOT NULL ); Alternatively, in the case of Table 1, where there is no NOT NULL constraint, are we allowed to insert records containing

问题 I was wondering whether there is a possibility for me to discover the underlying primary (or unique) key columns for all tables involved in an Oracle view. Here's an example to show what I mean: CREATE TABLE t_a ( id number(7), primary key(id) ); CREATE VIEW v_a AS SELECT * FROM t_a; So by naming convention, I know that v_a.id is actually the primary key column of the underlying t_a table. Is there any way of formally discovering this information by using system views, such as SYS.ALL

问题 Why is it necessary to have a primary key on a column of one table to which a column of the other table having foreign key references. create table D(Did int) create table E(Eid int foreign key references D(Did)) The above query gives error: There are no primary or candidate keys in the referenced table 'D' that match the referencing column list in the foreign key 'FK__E__Eid__79C80F94'. 回答1: Easy. If you have 2 values the same in the parent table, how do you know which one to associate child

问题 Why is it necessary to have a primary key on a column of one table to which a column of the other table having foreign key references. create table D(Did int) create table E(Eid int foreign key references D(Did)) The above query gives error: There are no primary or candidate keys in the referenced table 'D' that match the referencing column list in the foreign key 'FK__E__Eid__79C80F94'. 回答1: Easy. If you have 2 values the same in the parent table, how do you know which one to associate child