Specifying Queries at Runtime Using Dynamic SQL
13.2.3 Specifying Queries at Runtime Using Dynamic SQL
In the previous examples, the embedded SQL queries were written as part of the host program source code. Hence, any time we want to write a different query, we must modify the program code, and go through all the steps involved (compiling, debugging, testing, and so on). In some cases, it is convenient to write a program that can execute different SQL queries or updates (or other operations) dynamically at runtime. For example, we may want to write a program that accepts an SQL query typed from the monitor, executes it, and displays its result, such as the interactive interfaces available for most relational DBMSs. Another example is when a user- friendly interface generates SQL queries dynamically for the user based on point- and-click operations on a graphical schema (for example, a QBE-like interface; see Appendix C). In this section, we give a brief overview of dynamic SQL, which is one technique for writing this type of database program, by giving a simple example to illustrate how dynamic SQL can work. In Section 13.3, we will describe another approach for dealing with dynamic queries.
Program segment E3 in Figure 13.4 reads a string that is input by the user (that string should be an SQL update command) into the string program variable sqlupdatestring in line 3. It then prepares this as an SQL command in line 4 by associating it with the SQL variable sqlcommand . Line 5 then executes the command. Notice that in this case no syntax check or other types of checks on the command are possible at compile time, since the SQL command is not available until runtime. This contrasts with our previous examples of embedded SQL, where the query could be checked at compile time because its text was in the program source code.
Although including a dynamic update command is relatively straightforward in dynamic SQL, a dynamic query is much more complicated. This is because usually we do not know the types or the number of attributes to be retrieved by the SQL query when we are writing the program. A complex data structure is sometimes needed to allow for different numbers and types of attributes in the query result if no prior information is known about the dynamic query. Techniques similar to those that we discuss in Section 13.3 can be used to assign query results (and query parameters) to host program variables.
In E3, the reason for separating PREPARE and EXECUTE is that if the command is to
be executed multiple times in a program, it can be prepared only once. Preparing the command generally involves syntax and other types of checks by the system, as
//Program Segment E3:
Figure 13.4
0) EXEC SQL BEGIN DECLARE SECTION ; Program segment E3, a C program segment 1) varchar sqlupdatestring [256] ;
that uses dynamic SQL for updating a table. 2) EXEC SQL END DECLARE SECTION ; ... 3) prompt("Enter the Update Command: ", sqlupdatestring) ; 4) EXEC SQL PREPARE sqlcommand FROM :sqlupdatestring ; 5) EXEC SQL EXECUTE sqlcommand ;
13.2 Embedded SQL, Dynamic SQL, and SQLJ 459
well as generating the code for executing it. It is possible to combine the PREPARE and EXECUTE commands (lines 4 and 5 in E3) into a single statement by writing
EXEC SQL EXECUTE IMMEDIATE :sqlupdatestring ; This is useful if the command is to be executed only once. Alternatively, the pro-
grammer can separate the two statements to catch any errors after the PREPARE statement, if any.
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» Fundamentals_of_Database_Systems,_6th_Edition
» Characteristics of the Database Approach
» Advantages of Using the DBMS Approach
» A Brief History of Database Applications
» Schemas, Instances, and Database State
» The Three-Schema Architecture
» The Database System Environment
» Centralized and Client/Server Architectures for DBMSs
» Classification of Database Management Systems
» Domains, Attributes, Tuples, and Relations
» Key Constraints and Constraints on NULL Values
» Relational Databases and Relational Database Schemas
» Integrity, Referential Integrity, and Foreign Keys
» Update Operations, Transactions, and Dealing with Constraint Violations
» SQL Data Definition and Data Types
» Specifying Constraints in SQL
» The SELECT-FROM-WHERE Structure of Basic SQL Queries
» Ambiguous Attribute Names, Aliasing, Renaming, and Tuple Variables
» Substring Pattern Matching and Arithmetic Operators
» INSERT, DELETE, and UPDATE Statements in SQL
» Comparisons Involving NULL and Three-Valued Logic
» Nested Queries, Tuples, and Set/Multiset Comparisons
» The EXISTS and UNIQUE Functions in SQL
» Joined Tables in SQL and Outer Joins
» Grouping: The GROUP BY and HAVING Clauses
» Discussion and Summary of SQL Queries
» Specifying General Constraints as Assertions in SQL
» Introduction to Triggers in SQL
» Specification of Views in SQL
» View Implementation, View Update, and Inline Views
» Schema Change Statements in SQL
» Sequences of Operations and the RENAME Operation
» The UNION, INTERSECTION, and MINUS Operations
» The CARTESIAN PRODUCT (CROSS PRODUCT) Operation
» Variations of JOIN: The EQUIJOIN and NATURAL JOIN
» Additional Relational Operations
» Examples of Queries in Relational Algebra
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» The Role of Information Systems
» The Database Design and Implementation Process
» Use of UML Diagrams as an Aid to Database Design Specification 6
» Rational Rose: A UML-Based Design Tool
» Automated Database Design Tools
» Introduction to Object-Oriented Concepts and Features
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» Database Programming: Techniques
» Retrieving Single Tuples with Embedded SQL
» Retrieving Multiple Tuples with Embedded SQL Using Cursors
» Specifying Queries at Runtime Using Dynamic SQL
» SQLJ: Embedding SQL Commands in Java
» Retrieving Multiple Tuples in SQLJ Using Iterators
» Database Programming with SQL/CLI Using C
» JDBC: SQL Function Calls for Java Programming
» Database Stored Procedures and SQL/PSM
» PHP Variables, Data Types, and Programming Constructs
» Overview of PHP Database Programming
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» General Definitions of Second and Third Normal Forms
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» Inference Rules for Functional Dependencies
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» Dependency-Preserving Decomposition
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» Implementing the SELECT Operation
» Implementing the JOIN Operation
» Algorithms for PROJECT and Set
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» Examples of Cost Functions for JOIN
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» Factors That Influence Physical Database Design
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» Distributed Database Concepts 1
» Types of Distributed Database Systems
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» Distributed Databases in Oracle 13
» Generalized Model for Active Databases and Oracle Triggers
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» Examples of Statement-Level Active Rules
» Time Representation, Calendars, and Time Dimensions
» Incorporating Time in Relational Databases Using Tuple Versioning
» Incorporating Time in Object-Oriented Databases Using Attribute Versioning
» Temporal Querying Constructs and the TSQL2 Language
» Spatial Database Concepts 24
» Multimedia Database Concepts
» Clausal Form and Horn Clauses
» Datalog Programs and Their Safety
» Evaluation of Nonrecursive Datalog Queries
» Introduction to Information Retrieval
» Types of Queries in IR Systems
» Evaluation Measures of Search Relevance
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» Grouping, Aggregation, and Database Modification in QBE
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