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We will, however, cover a few more database concepts essential in understanding EJB 3 Persistence next, namely database constraints such as primary and foreign keys.
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Constraints are the concept that is closest to business rules in a basic relational database schema. In effect, constraints maintain data integrity by enforcing rules on how data may be modified. Since most database vendors try to differentiate their products by offering unique constraint features, coming up with a list of constraints to discuss is not easy. We have chosen to cover the bare minimum set necessary to understand EJB 3 persistence features, namely primary/foreign keys, uniqueness constraints, NULL constraints, and sequence columns.
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B.2.1 Primary keys and unique columns
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Just as a set of fields or properties uniquely identifies an entity, a set of columns uniquely identifies a given database record. The column or set of columns identifying a distinct record is called a primary key. For example, the CATEGORY_ID column is the primary key for the CATEGORIES table. When you identify a column or set of columns as the primary key, you essentially ask the database to enforce uniqueness. If the primary key consists of more than one column, it is called a compound or composite key. For example, instead of CATEGORY_ID, the combination of CATEGORY_NAME and CREATION_DATE could be the primary key for the CATEGORIES table. Primary keys that consist of business data are called natural keys. A classic example is using some business data such as a Social Security number (represented by an SSN column) as the primary key for an EMPLOYEES table. CATEGORY_ID or EMPLOYEE_ID, on the other hand, are examples of surrogate keys. Essentially, surrogate keys are columns created explicitly to function as primary keys. Surrogate keys are popular and we highly recommend using them, especially as opposed to compound keys. Other than naming, primary key and uniqueness constraints do exactly the same thing, and the constraint is usually applied to columns that can function as alternate natural keys.
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B.2.2 Foreign key
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The interaction of primary and foreign keys is what makes relational databases shine. Foreign keys are essentially primary key values of one table stored in another table. Foreign keys are the database equivalents of object references, and
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Reviewing relational databases
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Figure B.2 The CATEGORY_ID foreign key in the ITEMS table points to the primary key of the CATEGORIES table.
signify relationships between tables. As shown in figure B.2 (from our ActionBazaar example), a column named CATEGORY_ID in the ITEMS table pointing to the CATEGORY_ID column in the CATEGORIES table signifies the fact that an item belongs in a category. A database foreign key constraint means that the database will ensure every value that is put into the foreign key column exists in the primary key column it points to.
The NOT NULL constraint is essentially a data integrity mechanism that ensures some table columns always have valid, nonempty values. For example, if the business rules dictate that a Category record must always have a name, we can specify a NOT NULL constraint on the CATEGORY_NAME column, and the database will only allow rows to be inserted where a CATEGORY_NAME is specified. If no CATEGORY_NAME is provided, the database will not allow the row to be inserted.
Structured Query Language (SQL)
B.2.4 Sequences
An easy way to ensure uniqueness for surrogate primary keys is to set the key for a new record to a number greater than the last created record. Although you could manage this kind of column yourself, databases provide various mechanisms for managing key sequences. The easiest and most transparent of these mechanisms is an identity column constraint (such as the identity column constraints supported by DB2, Sybase, and SQL Server). When you designate a column as an identity, the database automatically generates a value for you when you create a new record. For example, if the ITEM_ID primary key for the ITEMS table is an identity, when we create a new record we do not specify a primary key value ourselves. Instead, during record creation the database looks at the last row inserted, generates a new value by incrementing the last key, and sets the ITEM_ID value on our behalf. Some other databases like Oracle don t support incrementing keys as an internal function of the column, but help you generate keys using an external mechanism called sequences (DB2 supports sequences in addition to identities). Each time you insert a new record, you can ask the sequence to generate a key that you can use in the INSERT statement. A few databases don t support sequence generation at all, in which case you must implement similar functionality yourself. Fortunately, EJB 3 transparently handles all these situations on your behalf, using the table generator.
B.3 Structured Query Language (SQL)
If relational theory is the bedrock of the relational database, SQL is the crown jewels. Java developers with strong OO roots may find SQL s verbose syntax and unmistakably relational feel less than ideal. The truth is that even O/R solutions such as the EJB 3 Persistence API generate SQL under the hood. The fact that you use O/R is no excuse not to have a solid understanding of SQL, particularly during debugging and fine-tuning. SQL (which stands for Structured Query Language) arose as a result of the initial relational research conducted at IBM. The American National Standards Institute (ANSI) has since standardized SQL. Almost all major databases comply with the SQL -92 standard for the most part. Even then, writing portable SQL is a tricky business at best. Luckily, O/R relieves us from this meticulous work to some degree by automatically generating SQL suited to a particular database. SQL statements include the familiar CREATE, INSERT, DELETE, UPDATE, and, of course, everyone s favorite, SELECT. As a testament to the power of the SELECT statement, some elements of it have been ported over into the O/R world through EJB-QL (which we cover in chapter 10).
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