Database Languages

Database Languages

A database system provides a data definition language to specify the database schema and a data manipulation language to express database queries and updates. In practice, the data definition and data manipulation languages are not two separate languages; instead they simply form parts of a single database language, such as the widely used SQL language.

Data-Definition Language

We specify a database schema by a set of definitions expressed by a special language called a data-definition language (DDL).

For instance, the following statement in the SQL language defines the account table:

Execution of the above DDL statement creates the account table. In addition, it up- dates a special set of tables called the data dictionary or data directory.

A data dictionary contains metadata — that is, data about data. The schema of a table is an example of metadata. A database system consults the data dictionary before reading or modifying actual data.

We specify the storage structure and access methods used by the database system by a set of statements in a special type of DDL called a data storage and definition language. These statements define the implementation details of the database schemas, which are usually hidden from the users.

The data values stored in the database must satisfy certain consistency constraints. For example, suppose the balance on an account should not fall below $100. The DDL

provides facilities to specify such constraints. The database systems check these constraints every time the database is updated.

Data-Manipulation Language

Data manipulation is

• The retrieval of information stored in the database

• The insertion of new information into the database

• The deletion of information from the database

• The modification of information stored in the database

A data-manipulation language (DML) is a language that enables users to access or manipulate data as organized by the appropriate data model. There are basically two types:

• Procedural DMLs require a user to specify what data are needed and how to get those data.

• Declarative DMLs (also referred to as nonprocedural DMLs) require a user to specify what data are needed without specifying how to get those data.

Declarative DMLs are usually easier to learn and use than are procedural DMLs. However, since a user does not have to specify how to get the data, the database system has to figure out an efficient means of accessing data. The DML component of the SQL language is nonprocedural.

A query is a statement requesting the retrieval of information. The portion of a DML that involves information retrieval is called a query language. Although technically incorrect, it is common practice to use the terms query language and data manipulation language synonymously.

This query in the SQL language finds the name of the customer whose customer-id is 192-83-7465:

select customer.customer-name

from customer

where customer.customer-id = 192-83-7465

The query specifies that those rows from the table customer where the customer-id is 192-83-7465 must be retrieved, and the customer-name attribute of these rows must be displayed. If the query were run on the table in Figure 1.3, the name Johnson would be displayed.

Queries may involve information from more than one table. For instance, the following query finds the balance of all accounts owned by the customer with customer id = 192-83-7465.

select account.balance

from depositor, account

where depositor.customer-id = 192-83-7465 and

depositor.account-number = account.account-number

If the above query were run on the tables in Figure 1.3, the system would find that the two accounts numbered A-101 and A-201 are owned by customer 192-83-7465 and would print out the balances of the two accounts, namely 500 and 900.

There are a number of database query languages in use, either commercially or experimentally. We study the most widely used query language, SQL, in Chapter 4. We also study some other query languages in Chapter 5.

The levels of abstraction that we discussed in Section 1.3 apply not only to defining or structuring data, but also to manipulating data. At the physical level, we must define algorithms that allow efficient access to data. At higher levels of abstraction, we emphasize ease of use. The goal is to allow humans to interact efficiently with the system. The query processor component of the database system (which we study in Chapters 13 and 14) translates DML queries into sequences of actions at the physical level of the database system.

Database Access from Application Programs

Application programs are programs that are used to interact with the database. Ap- plication programs are usually written in a host language, such as Cobol, C, C++, or Java. Examples in a banking system are programs that generate payroll checks, debit accounts, credit accounts, or transfer funds between accounts.

To access the database, DML statements need to be executed from the host language. There are two ways to do this:

• By providing an application program interface (set of procedures) that can be used to send DML and DDL statements to the database, and retrieve the results.

The Open Database Connectivity (ODBC) standard defined by Microsoft for use with the C language is a commonly used application program inter- face standard. The Java Database Connectivity (JDBC) standard provides corresponding features to the Java language.

• By extending the host language syntax to embed DML calls within the host language program. Usually, a special character prefaces DML calls, and a pre- processor, called the DML precompiler, converts the DML statements to nor- mal procedure calls in the host language.