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Resources
- DatabaseSystems: The Complete Book by Hector Garcia, Jeff Ullman, and JenniferWidom.
- AFirst Course in Database Systems by Jeff Ullman and Jennifer Widom.
- Gradiance SQLTutorial.
Overview
Embedded SQL is a method of combining the computing power of a high-levellanguage like C/C++ and the database manipulation capabilities of SQL.It allows you to execute any SQL statement from an application program.Oracle's embedded SQL environment is called Pro*C.A Pro*C program is compiled in two steps. First, the Pro*C precompilerrecognizes the SQL statements embedded in the program, and replaces themwith appropriate calls to the functions in the SQL runtime library. Theoutput is pure C/C++ code with all the pure C/C++ portions intact. Then,a regular C/C++ compiler is used to compile the code and produces the executable. For details, see the section on Demo Programs.
Pro*C Syntax
SQL
All SQL statements need to start with EXEC SQL and end with asemicolon ';'. You can place the SQL statements anywhere withina C/C++ block, with the restriction that the declarative statements donot come after the executable statements. As an example:PreprocessorDirectives
The C/C++ preprocessor directives that work with Pro*C are #includeand #if. Pro*C does not recognize #define. For example,the following code is invalid:Statement Labels
You can connect C/C++ labels with SQL as in:We will come to what WHENEVER means later in the section on ErrorHandling.Host Variables
Basics
Host variables are the key to the communication between the host programand the database. A host variable expression must resolve to an lvalue(i.e., it can be assigned). You can declare host variables accordingto C syntax, as you declare regular C variables. The host variable declarationscan be placed wherever C variable declarations can be placed. (C++ usersneed to use a declare section; see the section on C++Users.) The C datatypes that can be used with Oracle include:- char
- char[n]
- int
- short
- long
- float
- double
- VARCHAR[n] - This is a psuedo-type recognizedby the Pro*C precompiler. It is used to represent blank-padded, variable-lengthstrings. Pro*C precompiler will convert it into a structure with a 2-bytelength field and a n-byte character array.
A host variable reference must be prefixed with a colon ':'in SQL statements, but should not be prefixed with a colon in C statements.When specifying a string literal via a host variable, the single quotesmust be omitted; Pro*C understands that you are specifying a string basedon the declared type of the host variable. C function calls and most ofthe pointer arithmetic expressions cannot be used as host variablereferences even though they may indeed resolve to lvalues. The followingcode illustrates both legal and illegal host variable references:
Pointers
You can define pointers using the regular C syntax, and use them in embeddedSQL statements. As usual, prefix them with a colon:The result of this SELECT statement will be written into *x,not x.Structures
Structures can be used as host variables, as illustrated in the followingexample:Arrays
Host arrays can be used in the following way:which will insert all the 50 tuples in one go.Arrays can only be single dimensional. The example char name[50][11]would seem to contradict that rule. However, Pro*C actually considers namea one-dimensional array of strings rather than a two-dimensional arrayof characters. You can also have arrays of structures.
When using arrays to store the results of a query, if the size of thehost array (say n) is smaller than the actual number of tuples returnedby the query, then only the first n result tuples will be enteredinto the host array.
IndicatorVariables
Indicator variables are essentially 'NULL flags' attached to hostvariables. You can associate every host variable with an optional indicatorvariable. An indicator variable must be defined as a 2-byte integer (usingthe type short) and, in SQL statements, must be prefixed by acolon and immediately follow its host variable. Or, you may use the keywordINDICATORin between the host variable and indicator variable. Here is an example:You can use indicator variables in the INTO clause of a SELECTstatement to detect NULL's or truncated values in the output hostvariables. The values Oracle can assign to an indicator variable have thefollowing meanings:-1 | The column value is NULL, so the value of the host variableis indeterminate. |
0 | Oracle assigned an intact column value to the host variable. |
>0 | Oracle assigned a truncated column value to the host variable. Theinteger returned by the indicator variable is the original length of thecolumn value. |
-2 | Oracle assigned a truncated column variable to the host variable, butthe original column value could not be determined. |
-1 | Oracle will assign a NULL to the column, ignoring the valueof the host variable. |
>=0 | Oracle will assign the value of the host variable to the column. |
DatatypeEquivalencing
Oracle recognizes two kinds of datatypes: internal and external. Internaldatatypes specify how Oracle stores column values in database tables.Externaldatatypes specify the formats used to store values in input and outputhost variables. At precompile time, a default Oracle external datatypeis assigned to each host variable. Datatype equivalencing allows you tooverride this default equivalencing and lets you control the way Oracleinterprets the input data and formats the output data.The equivalencing can be done on a variable-by-variable basis usingthe VAR statement. The syntax is:For example, suppose you want to select employee names from the emptable, and then pass them to a routine that expects C-style '0'-terminatedstrings. You need not explicitly '0'-terminate the names yourself.Simply equivalence a host variable to the STRING external datatype,as follows:The length of the ename column in the emp table is 20characters, so you allot emp_name 21 characters to accommodatethe '0'-terminator. STRING is an Oracle external datatypespecifically designed to interface with C-style strings. When you selecta value from the ename column into emp_name, Oracle willautomatically '0'-terminate the value for you.
You can also equivalence user-defined datatypes to Oracle external datatypesusing the TYPE statement. The syntax is:You can declare a user-defined type to be a pointer, either explicitly,as a pointer to a scalar or structure, or implicitly as an array, and thenuse this type in a TYPE statement. In these cases, you need touse the REFERENCE clause at the end of the statement, as shownbelow:Here we allocated more memory than the type length (4000) because the precompileralso returns the length, and may add padding after the length in orderto meet the alignment requirement on your system.
Dynamic SQL
While embedded SQL is fine for fixed applications, sometimes it is importantfor a program to dynamically create entire SQL statements. With dynamicSQL, a statement stored in a string variable can be issued. PREPAREturns a character string into a SQL statement, andEXECUTE executesthat statement. Consider the following example.Alternatively, PREPARE and EXECUTE may be combined intoone statement:TransactionsOraclePRO*C supports transactions as defined by the SQL standard. A transactionis a sequence of SQL statements that Oracle treats as a single unit ofwork. A transaction begins at your first SQL statement. A transaction endswhen you issue 'EXEC SQL COMMIT' (to make permanent any databasechanges during the current transaction) or 'EXEC SQL ROLLBACK'(to undo any changes since the current transaction began). After the currenttransaction ends with yourCOMMIT or ROLLBACK statement,the next executable SQL statement will automatically begin a new transaction. If your program exits without calling EXEC SQL COMMIT,all database changes will be discarded.
Error HandlingAftereach executable SQL statement, your program can find the status of executioneither by explicit checking of SQLCA, or by implicit checking using theWHENEVERstatement. These two ways are covered in details below.SQLCA
SQLCA (SQL Communications Area) is used to detect errors and status changesin your program. This structure contains components that are filled inby Oracle at runtime after every executable SQL statement.To use SQLCA you need to include the header file sqlca.h usingthe #include directive. In case you need to include sqlca.hat many places, you need to first undefine the macro SQLCA with#undefSQLCA. The relevant chunk of sqlca.h follows:The fields in sqlca have the following meaning:
sqlcaid | This string component is initialized to 'SQLCA' to identify the SQLCommunications Area. | ||||||||||||||||
sqlcabc | This integer component holds the length, in bytes, of the SQLCA structure. | ||||||||||||||||
sqlcode | This integer component holds the status code of the most recently executedSQL statement:
| ||||||||||||||||
sqlerrm | This embedded structure contains the following two components:
| ||||||||||||||||
sqlerrp | Reserved for future use. | ||||||||||||||||
sqlerrd | This array of binary integers has six elements:
| ||||||||||||||||
sqlwarn | This array of single characters has eight elements used as warningflags. Oracle sets a flag by assigning to it the character 'W'.
| ||||||||||||||||
sqlext | Reserved for future use. |
Proc Pdf
SQLCA can only accommodate error messages up to 70 characters long inits sqlerrm component. To get the full text of longer (or nested)error messages, you need the sqlglm() function:where msg_buf is the character buffer in which you want Oracleto store the error message; buf_size specifies the size of msg_bufin bytes; Oracle stores the actual length of the error message in *msg_length.The maximum length of an Oracle error message is 512 bytes.
WHENEVER Statement
This statement allows you to do automatic error checking and handling.The syntax is:Oracle automatically checks SQLCA for <condition>, and if suchcondition is detected, your program will automatically perform <action>.<condition> can be any of the following:
- SQLWARNING - sqlwarn[0] is set because Oracle returneda warning
- SQLERROR - sqlcode is negative because Oracle returnedan error
- NOT FOUND - sqlcode is positive because Oracle couldnot find a row that meets your WHERE condition, or a SELECTINTO or FETCH returned no rows
- CONTINUE - Program will try to continue to run with the next statementif possible
- DO - Program transfers control to an error handling function
- GOTO <label>- Program branches to a labeled statement
- STOP - Program exits with an exit() call, and uncommittedwork is rolled back
EXEC SQL WHENEVER NOT FOUND DO break;
All SQL statements that occur after this line in the file would be affected. Make sure you use the following line to cancel the effect of WHENEVER whenit is no longer needed (such as after your loop):
EXEC SQL WHENEVER NOT FOUND CONTINUE;
Demo Programs
Note: The demo programs will create and use four tables named DEPT,EMP,PAY1,and PAY2. Be careful if any table in your database happens tohave the same name!Several demo programs are available in /afs/ir/class/cs145/code/procon the leland system. They are named sample*.pc (for C users)and cppdemo*.pc (for C++ users). '.pc' is the extensionfor Pro*C code. Do not copy these files manually, since there area couple of customizations to do. To download and customize the demo programs,follow the instructions below:
- Make sure that you have run source /afs/ir/class/cs145/all.env
- In your home directory, run load_samples <db_username> <db_passwd><sample_dir>, where <sample_dir> is the name of thedirectory where you wish to put demo programs (e.g., load_samplessally etaoinshrdlu cs145_samples)
- cd <sample_dir>
- Run make samples (or make cppsamples for C++) to compileall demo programs
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sample1![Pro C 7 Pdf Pro C 7 Pdf](https://sharesoftwarepremium.com/wp-content/uploads/2019/05/maxresdefault-43.jpg)
If you happen to make any mistake when entering username or passwordin Step (2), just run clean_samples <db_username> <db_passwd><sample_dir> Starcraft legacy of the void. in your home directory, and then repeat Steps (2)to (4).
For Step (4), you can also compile each sample program separately. Forexample, make sample1 compiles sample1.pc alone. Thecompilation process actually has two phases: Open pdf plus.
- proc iname=sample1.pc
- cc <a_number_of_flags_here> sample1.c
converts the embedded SQL code to corresponding library calls and outputssample1.c
generates the executable sample1
The demo programs operate on the following tables:These tables are created automatically when you run load_samplesin Step (2). A few tuples are also inserted. You may like to browse thetables before running the samples on them. You can also play with themas you like (e.g., inserting, deleting, or updating tuples). Thesetables will be dropped automatically when you run clean_samples.Note:clean_samplesalso wipes out the entire <sample_dir>; make sure you moveyour own files to some other place before running this command!
You should take a look at the sample source code before running it.The comments at the top describe what the program does. For example, sample1takes an employee's EMPNO and retrieve the name, salary, and commissionfor that employee from the table EMP.
You are supposed to study the sample source code and learn the following: 3ds emulator animal crossing new leaf.
- How to connect to Oracle from the host program
- How to embed SQL in C/C++
- How to use cursors
- How to use host variables to communicate with the database
- How to use WHENEVER to take different actions on error messages.
- How to use indicator variables to detect NULL's in the output
C++ Users
To get the precompiler to generate appropriate C++ code, you need to beaware of the following issues:- Code emission by precompiler. To get C++ code, you need to set the optionCODE=CPPwhile executing proc. C users need not worry about this option;the default caters to their needs.
- Parsing capability. The PARSE option of proc may takethe following values:
- PARSE=NONE. C preprocessor directives are understood only insidea declare section, and all host variables need to be declared inside adeclare section.
- PARSE=PARTIAL. C preprocessor directives are understood; however,all host variables need to be declared inside a declare section.
- PARSE=FULL. C preprocessor directives are understood and hostvariables can be declared anywhere. This is the default when CODEis anything other than CPP; it is an error to specify PARSE=FULLwith CODE=CPP.
- So, C++ users must specify PARSE=NONE or PARSE=PARTIAL.They therefore lose the freedom to declare host variables anywhere in thecode. Rather, the host variables must be encapsulated in declare sectionsas follows:You need to follow this routine for declaring the host and indicator variablesat all the places you do so.
- File extension. You need to specify the option CPP_SUFFIX=cc orCPP_SUFFIX=C.
- Location of header files. By default, proc searches for headerfiles like stdio.h in standard locations. However, C++ has itsown header files, such as iostream.h, located elsewhere. So youneed to use the SYS_INCLUDE option to specify the paths that procshould search for header files.
Listof Embedded SQL Statements Supported by Pro*C
Declarative Statements | |
EXEC SQL ARRAYLEN | To use host arrays with PL/SQL |
EXEC SQL BEGIN DECLARE SECTION EXEC SQL END DECLARE SECTION | To declare host variables |
EXEC SQL DECLARE | To name Oracle objects |
EXEC SQL INCLUDE | To copy in files |
EXEC SQL TYPE | To equivalence datatypes |
EXEC SQL VAR | To equivalence variables |
EXEC SQL WHENEVER | To handle runtime errors |
Executable Statements | |
EXEC SQL ALLOCATE | To define and control Oracle data |
EXEC SQL ALTER | |
EXEC SQL ANALYZE | |
EXEC SQL AUDIT | |
EXEC SQL COMMENT | |
EXEC SQL CONNECT | |
EXEC SQL CREATE | |
EXEC SQL DROP | |
EXEC SQL GRANT | |
EXEC SQL NOAUDIT | |
EXEC SQL RENAME | |
EXEC SQL REVOKE | |
EXEC SQL TRUNCATE | |
EXEC SQL CLOSE | |
EXEC SQL DELETE | To query and manipulate Oracle data |
EXEC SQL EXPLAIN PLAN | |
EXEC SQL FETCH | |
EXEC SQL INSERT | |
EXEC SQL LOCK TABLE | |
EXEC SQL OPEN | |
EXEC SQL SELECT | |
EXEC SQL UPDATE | |
EXEC SQL COMMIT | To process transactions |
EXEC SQL ROLLBACK | |
EXEC SQL SAVEPOINT | |
EXEC SQL SET TRANSACTION | |
EXEC SQL DESCRIBE | To use dynamic SQL |
EXEC SQL EXECUTE | |
EXEC SQL PREPARE | |
EXEC SQL ALTER SESSION | To control sessions |
EXEC SQL SET ROLE | |
EXEC SQL EXECUTE END-EXEC | To embed PL/SQL blocks |