primary-key

I have a simple JPA entity that uses a generated long "ID" as its primary key: @Entity public class Player { private long id; protected Player() { // Do nothing; id defaults to 0L } @GeneratedValue @Id public long getId() { return id; } protected void setId(final long id) { this.id = id; } // Other code } At some point in the life-cycle of an object of this type the JPA must call setId() to record the generated ID value. My question is, when does this happen, and where is the documentation that states this . I've looked through the JPA Specification and can not find a clear statement. The JPA

I have a table Messages with columns ID (primary key, autoincrement) and Content (text). I have a table Users with columns username (primary key, text) and Hash. A message is sent by one Sender (user) to many recipients (user) and a recipient (user) can have many messages. I created a table Messages_Recipients with two columns: MessageID (referring to the ID column of the Messages table and Recipient (referring to the username column in the Users table). This table represents the many to many relation between recipients and messages. So, the question I have is this. The ID of a new message

When should I use KEY , PRIMARY KEY , UNIQUE KEY and INDEX ? KEY and INDEX are synonyms in MySQL. They mean the same thing. In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN , WHERE , and ORDER BY clauses. Imagine you have a table called users and you want to search for all the users which have the last name 'Smith'. Without an index, the database would have to go through all the records of the table: this is slow, because the more records you have in your database, the more work it has to do to find the result. On