Web hosting faq - The paradigm mismatch this problem: two in the
The paradigm mismatch this problem: two in the Java world and one in our SQL database. As expected, they work together only with some help. Java objects define two different notions of sameness: Object identity (roughly equivalent to memory location, checked with a==b) Equality as determined by the implementation of the equals() method (also called equality by value) On the other hand, the identity of a database row is expressed as the primary key value. As you ll see in chapter 9, section 9.2, Object identity and equality, neither equals() nor == is naturally equivalent to the primary key value. It s common for several nonidentical objects to simultaneously represent the same row of the database, for example, in concurrently running application threads. Furthermore, some subtle difficulties are involved in implementing equals() correctly for a persistent class. Let s discuss another problem related to database identity with an example. In our table definition for USERS, we used USERNAME as a primary key. Unfortunately, this decision makes it difficult to change a username; we need to update not only the USERNAME column in USERS, but also the foreign key column in BILLING_ DETAILS. To solve this problem, later in the book we ll recommend that you use surrogate keys whenever you can t find a good natural key (we ll also discuss what makes a key good). A surrogate key column is a primary key column with no meaning to the user; in other words, a key that isn t presented to the user and is only used for identification of data inside the software system. For example, we may change our table definitions to look like this: create table USERS ( USER_ID bigint not null primary key, USERNAME varchar(15) not null unique, NAME varchar(50) not null, … ) create table BILLING_DETAILS ( BILLING_DETAILS_ID bigint not null primary key, ACCOUNT_NUMBER VARCHAR(10) not null unique, ACCOUNT_NAME VARCHAR(50) not null, ACCOUNT_TYPE VARCHAR(2) not null, USER_ID bigint foreign key references USER ) The USER_ID and BILLING_DETAILS_ID columns contain system-generated values. These columns were introduced purely for the benefit of the data model, so how
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