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15 Database Languages
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A database system provides a data de nition language to specify the database schema and a data manipulation language to express database queries and updates In practice, the data de nition 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
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151 Data-De nition Language
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We specify a database schema by a set of de nitions expressed by a special language called a data-de nition language (DDL) For instance, the following statement in the SQL language de nes the account table: create table account (account-number char(10), balance integer) Execution of the above DDL statement creates the account table In addition, it updates 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 de nition language These statements de ne 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
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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1 Introduction
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The McGraw Hill Companies, 2001
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Introduction
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152 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 modi cation 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 gure out an ef cient 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 datamanipulation language synonymously This query in the SQL language nds the name of the customer whose customer-id is 192-83-7465: select customercustomer-name from customer where customercustomer-id = 192-83-7465 The query speci es 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 13, the name Johnson would be displayed Queries may involve information from more than one table For instance, the following query nds the balance of all accounts owned by the customer with customerid 192-83-7465 select accountbalance from depositor, account where depositorcustomer-id = 192-83-7465 and depositoraccount-number = accountaccount-number
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
1 Introduction
Text
The McGraw Hill Companies, 2001
Database Users and Administrators
If the above query were run on the tables in Figure 13, the system would nd 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 4 We also study some other query languages in 5 The levels of abstraction that we discussed in Section 13 apply not only to de ning or structuring data, but also to manipulating data At the physical level, we must de ne algorithms that allow ef cient access to data At higher levels of abstraction, we emphasize ease of use The goal is to allow humans to interact ef ciently with the system The query processor component of the database system (which we study in s 13 and 14) translates DML queries into sequences of actions at the physical level of the database system
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