Rdbms Day6

March 9, 2018 | Author: api-3707589 | Category: Database Index, Sql, Databases, Table (Database), Compiler
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RDBMS- Day5 Views Concept of an index Embedded SQL

In today’s session we would be discussing about the concept of views, the concept of an index and how it is useful in database implementations and the concept embedded SQL programming using pro*c.

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Views

“Views “ is an important and useful feature in RDBMS. It helps achieve sharing with proper security. We will be looking at What is a view How to create a view How to update/delete a view Different types of views Advantages and disadvantages of views in the next few slides

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What is a view? A view is a kind of “virtual table” Contents are defined by a query like: Select Empno, Name, age from Employee Where designation=‘developer’; As shown in the figure

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Views are tables whose contents are taken or derived from other tables. To the user, the view appears like a table with columns and rows But in reality, the view doesn’t exists in the database as a stored set of values The rows and columns that we find inside a view are actually the results generated by a query that defines the view View is like a window through which we see a limited region of the actual table The table from where the actual data is obtained is called the source table

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What is a view to the DBMS We can use views in select statements like Select * from view_employees where age > 23; DBMS translates the request to an equivalent request to the source table

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For simple queries the results are generated row by row on the fly. For more complex views the DBMS must generate a temporary table representing the view and later discard it.

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Create a view CREATE VIEW ViewSupplier AS SELECT * FROM Supplier;

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You must have permission on the table refereed in the view to successfully create the view Can assign a new name to each column chosen in the view Only names can be different. The data type etc remain the same as the source table because, the values are after all going to be derived from that table If no names are specified for columns, the same name as used in the source table is used

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Assigning names to columns CREATE VIEW ViewSupplier AS SELECT S.SNO, S.SName, S.City FROM Supplier S WHERE City = ‘BHU’;

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We can assign names for the various columns in the view. This may be totally different from what has been used in the source table

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Types of views Horizontal views Vertical views Row/column subset views Grouped views Joined views

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Based on how the view is created. We will discuss them in detail in the following slides

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Horizontal views Create view Bgraders as select * from employee where grade =‘B’; Bgraders

Employee

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Here a horizontal subset of the source table is taken to create the view. Very useful when data belonging to different categories are present in the table. A private( virtual) table for each category can be created and given access to the person concerned

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Vertical views A view which selects only few columns of a table:

Genemp

For e.g: Create view Genemp as select Empno, Name, Age from Employee;

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Helps restricting access to sensitive information like salary, grade etc.

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Row/column subset views CREATE VIEW ViewSupplier AS SELECT S.SNO, S.SName, S.City FROM Supplier S WHERE City = ‘BHU’;

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SQL standard doesn’t define any notion of horizontal or vertical views . It is for our own understanding that we have given names for views which select only selected rows/columns from the source table as horizontal/vertical views. There could be a combination of these two concepts where a view selects a subset of rows and columns

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Grouped views The query contains a group by clause

CREATE VIEW Emp_Gradewise (grade ,total) AS SELECT grade, count(*) FROM employee GROUP BY grade;

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Grouped views are created based on a grouped query. They group rows of data and produce one row in the result corresponding to each group So, the rows in the view don’t have a one to one correspondence with the rows of the source table For this reason , grouped views cannot be updated.

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Joined views Created by specifying a two-table or three-table query in the view creation command

CREATE VIEW Ship_View (Supplier_id, Supplier_Name, Part_id, Partname, QTY_Shipped) AS SELECT SP.SNO,S.Name,SP.PNO,P.NAME,SP.Qty FROM Shipment sp, Suppliers S, Parts P WHERE SP.SNO=S.SNO AND SP.PNO=P.PNO

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(Refer to the supplier,parts, shipment tables we have used in our earlier slides on DDL,DML) The advantage with this type of view is, any subsequent info can be extracted from this view as a single-table query instead of writing a more complex two or three table query . For e.g: SELECT Supplier_id, Partname, SUM(Qty) from SHIP_VIEW (Group by Supplier_id, Partname; Would help extract data which is actually spread over more than one table

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Updating a VIEW

A view can be modified by the DML update command. CREATE VIEW ViewSupplier AS SELECT S.SNO, S.SName, S.City FROM Supplier S WHERE City = ‘BHU’;

UPDATE VIEW ViewSupplier set SName=‘mnb’ where SNO=1000;

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Depending on the commercial implementation being used, views may or may not be updateable. In some cases, all views are not updateable. In some others a view is updateable if it is from one table, else it is not. In still others, a view is updateable if it is the result of a simple query; if it is defined by some GROUP BY statements, for example, the view is not updateable. According to ANSI/ISO standards, A view is updatable if The DBMS is able to trace back the rows and columns of the view to the corresponding rows and columns of the source table. So a view is updatable if : DISTINCT is not specified in the query used to create the view The FROM clause specifies only one source table The select list doesn’t contain expressions/calculated columns The WHERE clause doesn’t include a subquery The query doesn’t include a GROUP BY or HAVING An example of a view which can’t be updated is given in the next slide

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An example C rea te view view _su p p lier As S elect S u p p n um , su p p na m e, city F rom S u p plier W h ere C ity = " D elh i";

Insert into view_supplier Values("sup04","Manekchand and co. ", "Bhubaneswar"); Subsequently if you execute SELECT * from View_Suppliers – What happens? Copyright © 2004, 14 Infosys Technologies Ltd

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The newly inserted row, gets added to the source table subject to non-violation of other integrity constraints. What is the result of a Select * from view_suppliers? It shows only the details of suppliers belonging to Delhi. Why? Reason is, the view is created to display only suppliers based at Delhi. To ensure that the user of the view, view_supplier should not be able to add data for any other town than "Delhi", check option can be used as follows: CREATE VIEW View_Supplier AS SELECT Suppnum, Suppname, City FROM Supplier WHERE City=‘Delhi’ WITH CHECK OPTION

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Dropping Views Views are dropped similar to the way in which the tables are dropped. However, you must own a view in order to drop it. DROP VIEW ; DROP VIEW ViewSupplier;

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if some other views depend on this view, then if you say DROP VIEW ViewSupplier CASCADE then this view plus all other views that are based on this view are deleted. If you specify DROP VIEW ViewSupplier RESTRICT then this view cannot be deleted if other views depend on it.

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Advantages of views Security Query simplicity Structural simplicity

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Security: only a limited set of rows/ columns are viewable by certain users Query simplicity: A view can derive data from many tables. So, subsequently we can use queries on the view as single table queries rather than writing queries against the source tables as multi-table queries Structural simplicity: views can show only a portion of the table which is relevant to the user there by keeping it simple.

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Disadvantages of views Performance Restrictions

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Performance: views based on joins are merely virtaul tables. Every time a query is placed against the view, the query representing creation of the view has to be executed . So, complex joins may have to be performed every time a query is placed against the view. Restrictions: Not all views are updateable

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Concept of an Index

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What is an index? Physical storage structure Provides rapid access to rows of a table Mostly created on the primary key column Can be created on any other column also Transparent to the user

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An index is like the index page we see at the end of a book or like the index card used in a library. Let us understand this … Say you wish to know where you can find some description on “linkage editors” in a system software book. Imagine how difficult it is if you have to go through all the pages of the book to locate this information. Instead, if you go through the index which is sorted based on alphabetical order, it is very easy to locate a term called linkage editor under the letter L. Now against this term you will find the page numbers where this information is present in the book. So, you can straightawy go to that page number(s) and find your information. Apply the same principle to searching for a row in a database table. If you want to locate those rows of the employee table where the department is finance, if you have to search through the entire hard disk to locate this information, it would be extremely time consuming. Instead, if we store an index in a specific block of the disk which contains something like the following Department

address

Finance

2001f, 4005f,1002f

HRD

2000f, 3045f

Then you know that you have look for rows corresponding to finance dept only in three locations in the hard disk. Here the index has been formed on the department column. This is the benefit of having indexes.

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Advantage Speeds the execution of search queries

Disadvantages Consumes additional disk space Must be updated for every row insert into the table and also for every update of the key column

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Creation of index-Syntax Creation: CREATE [unique] INDEX index-name ON Table-name (column-name) E.g : CREATE UNIQUE INDEX Emp_index ON Employee (Empid); Deletion: DROP INDEX index-name E.g: DROP INDEX Emp_index;

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Embedded SQL

SQL can be used in two ways: 1. As an interactive database language to issue queries against the database and get results online 2. As a programmatic language used along with some high level language for performing database access Based on this, commercial products support two flavors of using SQL with programming languages a) Embedded SQL: directly writing SQL commands inside HLL (High level language) program interspersed with the other HLL code. A precompiler would process this kind of a code, separate out the programming language code and sql, a series of tools act on them and the executable is produced b) API: Application programming interface. The program uses some API function calls and passes the sql as a parameter. This doesn’t require any precompiler The following discussions on embedded SQL are purely from a theoretical point of view. As far as this course is concerned, no practical is involved with respect to embedded SQL.

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How does the DBMS process a Query SQL Statement Parse Validate Optimize Generate application plan Execute plan

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a) The first step in processing an SQL is Parsing. The statement is split into words and checked against syntax. Syntax errors and spelling mistakes can be detected at this stage b)

The second step is Validation. Semantic errors are checked. It is verified as to whether the tables referred in the query are present, whether column names are valid etc. The system catalog is refereed for this purpose

c)

The next step is optimization. RDBMS explores possibility of using an index, the order of execution of join etc

d) The next step is to generate the ‘Application plan’. It is binary representation of the sequence of steps to be executed by the DBMS in executing the query e) The ‘Application plan’ is executed.

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What is Embedded SQL Interleaving SQL statements along with the other code in a host language program Variables declared in the host program can be used in sql to send /receive values from/to the DBMS

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Special variables are used to pass and receive “NULL” New SQL statements not found in interactive SQL are added to enable processing of query results.

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Processing of an Embedded SQL Program

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The Embedded SQL program is submitted to a precompiler. This is a language specific tool. Ir we have separate precompiler for c, for cobol etc.

2. The precompiler generates two files as output. One is the source file in which all SQL commands are removed and replaced with calls to some special DBMS routines. These routines provide the necessary run-time link with the DBMS. This is aut0-generated by the precompiler The other file is the DBRM (Data Base Request Module). This contains all the SQL commands removed from the program 3. Now, this source file is compiled by the compiler to generate the object file. 4. The linker links the object code with other library routines (which includes the DBMS library) and generates the executable 5. parallely, the DBRM is submitted to a “Bind” program. This program parses each SQL statement, validates, optimizes and generates the binary DBMS-executable form called the application plan. After all statements are thus converted , the combined application plan is stored under the DBMS with the same name as the application name from where the SQL statements were stripped off 6. The run-time communication between the executable form of source program and the Application plan is totally hidden .

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Using embedded SQL

EXEC SQL Keyword indicates to the pre compiler that the next line of code is for the SQL handling. The block of sql code is ended with a ;

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This depends on the host language . EXEC SQL is a common way to begin. But the way to end is language specific.

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Declaring host variables SQL DECLARE SECTION; Compatible data types; Preceded by a colon(:).

EXEC SQL INSERT INTO Supplier VALUE (:Sup_No, :Sup_Name, :Sup_City);

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An example is as shown below: exec sql begin declare section; int emp_no; varchar emp_name[20]; varchar emp_addr[20]; char emp_des[3]; char emp_dept[3]; int emp_sal; short emp_addri; short emp_desi; short emp_depti; short emp_sali; varchar username[20]; varchar password[20]; exec sql end declare section; The data type of the variable which is going to be used in a query should be compatible with the data type of the column of the table for which it is going to be substituted/ compared with For example in the query shown in the slide, we are passing values for supplier number , name and city through the variables declared in the c program named: Sup_No, Sup_Name etc . If the data type of the column SNO in the Supplier table is INTEGER then the datatype of the variable Sup_No should be long … The variables declared in the host language are preceded with a : when they are used in a query.

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Example of embedded SQL EXEC SQL SELECT name,addr,des,dept,sal INTO :emp_name, :emp_addr, :emp_des, :emp_dept, :emp_sal FROM employee WHERE eno = :emp_no; Where are the values of the rows returned by SQL stored?

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The below code snippet shows how a connection to a DBMS is established by supplying the userid and password. main() { exec sql whenever sqlerror goto sqlerrorlabel; strcpy(username.arr,"pramodv"); username.len = strlen(username.arr); strcpy(password.arr,"pramodv"); password.len=strlen(password.arr); exec sql connect :username identified by :password; After this step, the remaining sql statements are written. The answer to the question in the slide lies in the next few slides..

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Indicator variables Variable to indicate whether the returned value is NULL.

EXEC SQL SELECT name,addr,des,dept,sal INTO :emp_name, :emp_addr:emp_addi, :emp_des:emp_desi, :emp_dept, :emp_sal FROM employee WHERE eno = :emp_no ;

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The indicator variables are used to indicate the status of the value received into the host variable. Zero Host variable has been assigned the retrieved value. Host variable can be used in the application program. Negative

Retrieved value is NULL. Host variable value not reliable.

Positive Retrieved value is in host variable, but this indicates a warning, rounding off or truncation for example. The following highlights the declaration of indicator variables exec sql begin declare section; ……… short emp_addri; short emp_desi; short emp_depti; short emp_sali; varchar username[20]; varchar password[20]; exec sql end declare section; Indicator variables are required only for NULLABLE columns. They are also used with a prefixed colon

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A complete program example Please double click the icon below

Text Document

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This example shows use of indicator variables, host variables etc.

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CURSOR Needed for select queries which return multiple rows of output from DBMS A kind of variable associated with a query. The value of this variable will be each row, in turn, of that query’s output.

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A cursor is used along with an SQL statement in an embedded SQL program, when the SELECT statement returns multiple rows. A cursor is declared as follows: exec sql declare emp cursor for select * from employee; Another example follows in the next slide

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Declaring Cursor The query is not executed immediately; This is only the definition.

EXEC SQL DECLARE CURSOR SupplierCur SELECT * FROM Supplier Where City=‘BHU’;

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The Declaration of the cursor is like a directive to the precompiler. It doesn’t call for any immediate action

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Opening Cursor - OPEN

EXEC SQL OPEN CURSOR SupplierCur;

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The cursor thus declaraed can be opened by saying: exec sql openSupplierCur; This opens the table of query results for the application’s use. Once opened, the cursor remains open until it is explicitly closed or till the transaction ends. You may imagine a cursor to be like a buffer which receives query results and allows the application to leisurely go through each row of result and process it.

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Cursor’s FETCH command Used to extract the values of OPEN CURSOR

EXEC SQL FETCH SupplierCur INTO :Sup_No, :Sup_Name, :Sup_City, :Sup_Status;

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The open statement positions the cursor on top of the query results. At this point, the cursor is not pointing at any row When the first fetch statement is encountered, the cursor is automatically positioned at the first row and retrieves the same this goes on by automatically incrementing the cursor position for executing the fetch statement each time until the end of rows is reached. After the last row has been retrieved, when the fetch statement is executed again, an SQLWARNING is raised. for(;;) { exec sql fetch emp into :emp_no, :emp_name, :emp_addr:addri, :emp_des:desi, :emp_dept:depti, :emp_sal:sali; emp_name.arr[emp_name.len] = '\0'; if (addri == -1) emp_addr.arr[0] ='\0'; else emp_addr.arr[emp_addr.len] = '\0'; if (desi == -1) strcpy(emp_des," "); if (depti == -1) strcpy(emp_dept," "); if (sali == -1) emp_sal=0; printf("%d\t%20s\t%20s\t%3s\t%3s\t%d\n", emp_no, emp_name.arr, emp_addr.arr, emp_des, emp_dept, emp_sal);

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Cursor’s CLOSE command Closes an opened cursor

EXEC SQL CLOSE SupplierCur

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This closes the cursor. The query results represented by the cursor would not be available after this. To get back the values, the cursor has to be opened again

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A program using cursor Refer to Notes page for an example

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exec sql begin declare section; int

emp_no; varchar

emp_name[20];

varchar

emp_addr[20];

char

emp_des[3];

char

emp_dept[3];

int

emp_sal;

varchar username[20]; varchar password[20]; short

addri,desi,depti,sali;

exec sql declare emp cursor for select * from employee; exec sql end declare section; /* COMMENT BEGINS A cursor is declared for a SELECT statement. This SELECT statement is executed only when the cursor is opened (see below). COMMENT ENDS */

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A program using cursor Refer to Notes page ( contd.. From previous Notes page)

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exec sql include sqlca; main() { exec sql whenever sqlerror goto sqlerrorlabel; strcpy(username.arr,"pramodv"); username.len = strlen(username.arr); strcpy(password.arr,"pramodv"); password.len=strlen(password.arr); exec sql connect :username identified by :password; exec sql open emp; exec sql whenever not found goto end_of_fetch; /* COMMENT BEGINS The cursor must be first declared, then opened and then fetched before the records can be accessed. The error not found will be raised when the last record has been fetched and then we try to fetch another record. Recall that the scope of the whenever command is from the point where it is declared upto another whenever command. If the whenever condition is declared after the for loop, then when the cursor tries to fetch the next record after all records have been fetched, the not found error is raised - but since we have not handled this error, the program will come to a stop. In this program, since the whenever statement is defined before the for loop, when the not found error is raised, it will be handled in the manner that we have specified in the whenever command and control moves to the label specified in the whenever condition. COMMENT ENDS */

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A program using cursor Refer to Notes page ( contd.. From previous Notes page)

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system("cls"); printf("\t\t List of employees\n\n"); printf("No\t\t\tName\t\t\tAddress\t Desig dept Salary\n\n"); for(;;) { exec sql fetch emp into :emp_no, :emp_name, :emp_addr:addri, :emp_des:desi, :emp_dept:depti, :emp_sal:sali; emp_name.arr[emp_name.len] = '\0'; if (addri == -1) emp_addr.arr[0] ='\0'; else emp_addr.arr[emp_addr.len] = '\0'; if (desi == -1) strcpy(emp_des," "); if (depti == -1) strcpy(emp_dept," "); if (sali == -1) emp_sal=0; printf("%d\t%20s\t%20s\t%3s\t%3s\t%d\n", emp_no, emp_name.arr, emp_addr.arr, emp_des, emp_dept, emp_sal); } end_of_fetch: exec sql close emp; exit(0); sqlerrorlabel: printf("\n %s \n", sqlca.sqlerrm.sqlerrmc); exit(1); }

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Error handling EXEC SQL WHENEVER SQLERROR goto sqlerrorlabel;

Any SQL error the controls go to the label. WHENEVER is a positional command.

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In embedded SQL programs, run-time errors are reported to the program by the DBMS in the form of some error code. The DBMS communicates the error status information through an area of program storage called “SQLCA” (SQL communications area) This SQLCA is a data structure that contains error variables and status indicators. You might have observed a statement like: exec sql include sqlca; In the program. This includes the SQLCA datastructure into the program. By checking the fileds in this datastructure, the program can check the status of execution and act accordingly. AS and when the DBMS executes an embedded SQL code, it updates the value of the SQLCA According to the SQL2 standrad, programs can use an “SQLSTATE” error code to handle errors.This is one of the fileds inside the SQLCA datastructure. SQLSTATE consists of two parts: a) b)

An error-class that identifies the type of error An error subclass that identifies the specific type of error

A code snippet making use of this would look something like this: Exec sql delete * from emp where sal < 3000; If(strcmp(sqlca.sqlstate,”00000”) Goto error_lablel;

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Error handling ...

EXEC SQL WHENEVER NOT FOUND CONTINUE;

NOT FOUND is used when there are no records that the SQL can return It is not an error case

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It is laborious task to check the error code after every sql statement. To simplify this process, there is a WHENEVER statement. This is a directive that tells the precompiler what code code should be included following every executable SQL statement This statement can be used to handle three types of errors 1) SQLERROR 2) SQLWARNING 3) NOT FOUND (when we try to fetch rows from a cursor while there are no more) WHENEVER/GOTO: to generate a branch to the given label WHENEVER/CONTINUE to alolow the normal flow to continue This is summarised in the following slide

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Error handling - syntax

WHENEVER {NOT FOUND | SQLERROR | SQLWARNING} {CONTINUE | GO TO host-label}

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This whenever is a positional command. It is in effect until the next such whenever statement is encountered in the code.

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Summary Views create a window into the table thereby allowing restricted access Index helps speed up the search process Embedded SQL programs are written in some HLL in which SQL statements are intermixed A special processing separates the SQL , converts and generates executable code

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Thank You! Copyright © 2004, 43 Infosys Technologies Ltd

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