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Minggu 6, Pertemuan 11 Programmatic SQL

Matakuliah : T0206-Sistem Basisdata

Tahun : 2005

Versi : 1.0/0.0

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa dapat mendemonstrasikan bagaimana SQL dapat diintegrasikan di dalam high level programming language (C3)

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Outline Materi

• How SQL statements can be embedded in high-level programming languages.

• Difference between static and dynamic embedded SQL.

• How to write programs that use static embedded SQL.

• How to write programs that use dynamic embedded SQL.

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Embedded SQL

• SQL can be embedded in high-level procedural language.

• In many cases, language is identical although SELECT statement differs.

• Two types of programmatic SQL:– Embedded SQL statements.

• SQL supports Ada, C, COBOL, FORTRAN, MUMPS, Pascal, and PL/1.

– Application program interface (API).

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Example 21.1 - CREATE TABLE

EXEC SQL CREATE TABLE Viewing (

propertyNo VARCHAR2(5) NOT NULL,

clientNo VARCHAR2(5) NOT NULL,

viewDate DATE NOT NULL,

comment VARCHAR2(40));

if (sqlca.sqlcode >= 0)

printf(“Creation successful\n”);

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Embedded SQL

• Embedded SQL starts with identifier, usually EXEC SQL [ ‘@SQL(’ in MUMPS].

• Ends with terminator dependent on host language:– Ada, ‘C’, and PL/1: terminator is semicolon

(;)– COBOL: terminator is END-EXEC– Fortran: ends when no more continuation

lines.• Embedded SQL can appear anywhere an

executable host language statement can appear.

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SQL Communications Area (SQLCA)

• Used to report runtime errors to the application program.

• Most important part is SQLCODE variable: 0 - statement executed successfully;

< 0 - an error occurred;

> 0 - statement executed successfully, but an exception occurred, such as no more rows returned by SELECT.

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SQLCA for Oracle

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WHENEVER Statement

• Every embedded SQL statement can potentially generate an error.

• WHENEVER is directive to precompiler to generate code to handle errors after every SQL statement:

EXEC SQL WHENEVER

<condition> <action>

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WHENEVER Statement

• condition can be:SQLERROR - generate code to handle errors

(SQLCODE < 0).

SQLWARNING - generate code to handle warnings.

NOT FOUND - generate code to handle specific warning that a retrieval operation has found no more records.

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WHENEVER Statement

• action can be:CONTINUE - ignore condition and proceed to next

statement.DO - transfer control to an error handling function. DO BREAK - place an actual “break” statement in the

program. DO CONTINUE - place an actual “continue” statement in

the program. GOTO label or GO TO label - transfer control to specified label.

STOP - rollback all uncommitted work and terminate the program.

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WHENEVER Statement

EXEC SQL WHENEVER SQLERROR GOTO error1;

EXEC SQL INSERT INTO Viewing VALUES (‘CR76’, ‘PA14’, ‘12-May-2001’, ‘Not enough space’);

• would be converted to:

EXEC SQL INSERT INTO Viewing VALUES (‘CR76’, ‘PA14’, ‘12-May-2001’, ‘Not enough space’);

if (sqlca.sqlcode < 0) goto error1;

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Host Language Variables

• Program variable declared in host language.

• Used in embedded SQL to transfer data from database into program and vice versa.

• Can be used anywhere a constant can appear.

• Cannot be used to represent database objects, such as table names or column names.

• To use host variable, prefix it by a colon (:).

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Host Language Variables

EXEC SQL UPDATE Staff SET salary = salary + :increment WHERE staffNo = ‘SL21’;

• Need to declare host language variables to SQL, as well as to host language:

EXEC SQL BEGIN DECLARE SECTION;float increment;

EXEC SQL END DECLARE SECTION;

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Indicator Variables

• Indicates presence of null: 0 associated host variable contains valid

value.

<0 associated host variable should be assumed to contain a null; actual contents of host variable irrelevant.

>0 associated host variable contains valid value.

• Used immediately following associated host variable with a colon (:) separating two variables.

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Indicator Variables - Example

EXEC SQL BEGIN DECLARE SECTION;

char address[51];

short addressInd;

EXEC SQL END DECLARE SECTION;

addressInd = -1;

EXEC SQL UPDATE PrivateOwner

SET address = :address :addressInd

WHERE ownerNo = ‘CO21’;

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Singleton SELECT - Retrieves Single Row

EXEC SQL SELECT fName, lName, address

INTO :firstName, :lastName, :address :addressIndFROM PrivateOwnerWHERE ownerNo = ‘CO21’;

• Must be 1:1 correspondence between expressions in SELECT list and host variables in INTO clause.

• If successful, SQLCODE set to 0; if there are no rows that satisfies WHERE, SQLCODE set to NOT FOUND.

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Cursors

• If query can return arbitrary number of rows, need to use cursors.

• Cursor allows host language to access rows of query one at a time.

• Cursor acts as a pointer to a row of query result. Cursor can be advanced by one to access next row.

• Cursor must be declared and opened before it can be used and it must be closed to deactivate it after it is no longer required.

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Cursors - DECLARE CURSOR

• Once opened, rows of query result can be retrieved one at a time using FETCH:

EXEC SQL DECLARE

propertyCursor CURSOR FOR

SELECT propertyNo, street, city

FROM PropertyForRent

WHERE staffNo = ‘SL41’;

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Cursors - OPEN

• OPEN statement opens specified cursor and positions it before first row of query result:

EXEC SQL OPEN propertyCursor;

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Cursors - FETCH and CLOSE

• FETCH retrieves next row of query result table:

EXEC SQL FETCH propertyCursor

INTO :propertyNo, :street, :city

• FETCH is usually placed in a loop. When there are no more rows to be returned, SQLCODE is set to NOT FOUND.

EXEC SQL CLOSE propertyCursor;

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ISO Standard for Embedded SQL

• Standard does not recognize SQLWARNING of WHENEVER statement.

• Standard does not mention an SQLCA. It does, however, recognize the integer variable SQLCODE, although this is a deprecated feature that is supported only for compatibility with earlier versions of the standard.

• Instead, defines a character string SQLSTATE parameter, comprising a two-character class code followed by a three-character subclass code.

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ISO Standard for Embedded SQL

• Standard specifies definition and processing of cursors slightly differently from that presented above. ISO DECLARE CURSOR is:

EXEC SQL DECLARE cursorName [INSENSITIVE] [SCROLL]

CURSOR FOR selectStatement

[FOR {READ ONLY |

UPDATE [OF columnNameList]}]

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ISO Standard for Embedded SQL

• FETCH statement:

EXEC SQL FETCH [[fetchOrientation] FROM] cursorName INTO hostVariable [, . . . ]

• fetchOrientation can be one of: – NEXT– PRIOR– FIRST– LAST– ABSOLUTE– RELATIVE

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Dynamic Embedded SQL

• With static embedded SQL, cannot use host variables where database object names required.

• Dynamic SQL allows this.• Idea is to place complete SQL statement in a

host variable, which is passed to DBMS to be executed.

• If SQL statements do not involve multi-row queries, use EXECUTE IMMEDIATE statement:

EXEC SQL EXECUTE IMMEDIATE [hostVariable | stringLiteral]

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Dynamic Embedded SQL

For example:

sprintf(buffer, “UPDATE Staff SET salary = salary + %f

WHERE staffNo = ‘SL21’ ”, increment);

EXEC SQL EXECUTE IMMEDIATE :buffer;

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PREPARE and EXECUTE

• DBMS must parse, validate, and optimize each EXECUTE IMMEDIATE statement, build execution plan, and execute plan.

• OK if SQL statement is only executed once in program; otherwise inefficient.

• Dynamic SQL provides alternative: PREPARE and EXECUTE.

• PREPARE tells DBMS to ready dynamically built statement for later execution.

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PREPARE and EXECUTE

• Prepared statement assigned name. When statement is subsequently executed, program need only specify this name:

EXEC SQL PREPARE statementName FROM [hostVariable | stringLiteral]

EXEC SQL EXECUTE statementName [ USING hostVariable [indicatorVariable] [, ...] |

USING DESCRIPTOR descriptorName ]

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Parameter Markers

• USING allows portions of prepared statement to be unspecified, replaced by placeholders (parameter markers).

• Placeholder can appear anywhere in hostVariable or stringLiteral of PREPARE that constant can appear.

• Tells DBMS value will be supplied later, in EXECUTE statement.

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Placeholders

sprintf(buffer, “UPDATE Staff SET salary = :sal

WHERE staffNo = :sn”);

EXEC SQL PREPARE stmt FROM :buffer;

EXEC SQL EXECUTE stmt

USING :newSalary, :staffNo;

• sal and sn are placeholders.

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Static versus Dynamic SQL

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SQL Descriptor Area (SQLDA)

• Alternative way to pass parameters to EXECUTE statement is through SQLDA.

• Used when number of parameters and their data types unknown when statement formulated.

• SQLDA can also be used to dynamically retrieve data when do not know number of columns to be retrieved or the types of the columns.

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SQLDA for Oracle

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Retrieving Data Using Dynamic SQL

• Again, use cursors to retrieve data from a query result table that has an arbitrary number of rows.

EXEC SQL DECLARE cursorName CURSOR FOR selectStatement

EXEC SQL OPEN cursorName [FOR READONLY] {USING hostVariable [indicatorVariable] [,...] |

USING DESCRIPTOR descriptorName } EXEC SQL FETCH cursorName

{INTO hostVariable [indicatorVariable] [,...] |USING DESCRIPTOR descriptorName }

EXEC SQL CLOSE cursorName

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Retrieving Data Using Dynamic SQL• OPEN allows values for placeholders

to be substituted using one or more hostVariables in:– USING clause or– passing values via descriptorName

(SQLDA) in a USING DESCRIPTOR clause.

• Main difference is with FETCH, which now uses descriptorName to receive rows of query result table (or one or more hostVariables/ indicatorVariables).

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Retrieving Data Using Dynamic SQL

• Before FETCH, program must provide data areas to receive retrieved data and indicator variables.

• Basic steps for dynamic SQL statement are:(1) Declare host string in DECLARE SECTION to hold

text of the query.(2) Declare a select SQLDA and, if required, a bind

SQLDA.(3) Allocate storage space for the SQLDA(s). (4) Set maximum number of columns in select SQLDA

and, if query can have placeholders, maximum number of placeholders in bind SQLDA.

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Retrieving Data Using Dynamic SQL

(5) Put the query text into the host string.

(6) PREPARE the query from the host string.

(7) DECLARE a cursor for the query.

(8) If the query can have placeholders: (a) DESCRIBE bind variables into bind SQLDA.

(b) Reset number of placeholders to number actually found by DESCRIBE.

(c) Get values and allocate storage space for bind variables found by DESCRIBE.

(9) OPEN cursor USING the bind SQLDA, or if no bind SQLDA has been used, USING the select SQLDA.

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Retrieving Data Using Dynamic SQL

(10) DESCRIBE the column list INTO the select SQLDA.

(11) Reset number of column list items to number actually

found by DESCRIBE.

(12) Reset length and data type of each column list item.

(13) FETCH each row from database into allocated data

buffers pointed to by the select SQLDA and process it,

as appropriate.

(14) Deallocate the storage space used for the column list

items, placeholders, indicator variables, and SQLDAs.

(15) CLOSE the cursor.