2 Precompiler Concepts

Embedded SQL Statements

The term embedded SQL refers to SQL statements placed within an application program. Because it houses the SQL statements, the application program is called a host program, and the language in which it is written is called the host language. For example, Pro*C/C++ provides the ability to embed certain SQL statements in a C or C++ host program.

To manipulate and query Oracle data, you use the INSERT, UPDATE, DELETE, and SELECT statements. INSERT adds rows of data to database tables, UPDATE modifies rows, DELETE removes unwanted rows, and SELECT retrieves rows that meet your search condition.

The powerful SET ROLE statement lets you dynamically manage database privileges. A role is a named group of related system and object privileges, or a named group of related system or object privileges granted to users or other roles. Role definitions are stored in the Oracle data dictionary. Your applications can use the SET ROLE statement to enable and disable roles as needed.

Only SQL statements—not SQL*Plus statements—are valid in an application program. (SQL*Plus has additional statements for setting environment parameters, editing, and report formatting.)

Embedded SQL Syntax

In your application program, you can freely mix complete SQL statements with complete C statements and use C variables or structures in SQL statements. The only special requirement for building SQL statements into your host program is that you begin them with the keywords EXEC SQL and end them with a semicolon. Pro*C/C++ translates all EXEC SQL statements into calls to the runtime library SQLLIB.

Many embedded SQL statements differ from their interactive counterparts only through the addition of a new clause or the use of program variables. The following example compares interactive and embedded ROLLBACK statements:

ROLLBACK WORK:           -- interactive
EXEC SQL ROLLBACK WORK;  -- embedded

These statements have the same effect, but you would use the first in an interactive SQL environment (such as when running SQL*Plus), and the second in a Pro*C/C++ program.

Static Versus Dynamic SQL Statements

Most application programs are designed to process static SQL statements and fixed transactions. In this case, you know the makeup of each SQL statement and transaction before runtime; that is, you know which SQL commands will be issued, which database tables might be changed, which columns will be updated, and so on.

However, some applications might be required to accept and process any valid SQL statement at runtime. So, you might not know until runtime all the SQL commands, database tables, and columns involved.

Dynamic SQL is an advanced programming technique that lets your program accept or build SQL statements at run time and take explicit control over datatype conversion.

Embedded PL/SQL Blocks

Pro*C/C++ treats a PL/SQL block like a single embedded SQL statement. You can place a PL/SQL block anywhere in an application program that you can place a SQL statement. To embed PL/SQL in your host program, you simply declare the variables to be shared with PL/SQL and bracket the PL/SQL block with the keywords EXEC SQL EXECUTE and END-EXEC.

From embedded PL/SQL blocks, you can manipulate Oracle data flexibly and safely because PL/SQL supports all SQL data manipulation and transaction processing commands.

Host and Indicator Variables

Host variables are the key to communication between Oracle and your program. A host variable is a scalar or aggregate variable declared in C and shared with Oracle, meaning that both your program and Oracle can reference its value.

Your program uses input host variables to pass data to Oracle. Oracle uses output host variables to pass data and status information to your program. The program assigns values to input host variables; Oracle assigns values to output host variables.

Host variables can be used anywhere a SQL expression can be used. In SQL statements, host variables must be prefixed with a colon (:) to set them apart from the SQL keywords.

You can also use a C struct to contain a number of host variables. When you name the structure in an embedded SQL statement, prefixed with a colon, Oracle uses each of the components of the struct as a host variable.

You can associate any host variable with an optional indicator variable. An indicator variable is a short integer variable that "indicates" the value or condition of its host variable. You use indicator variables to assign NULLs to input host variables and to detect NULLs or truncated values in output host variables. A NULL is a missing, unknown, or inapplicable value.

In SQL statements, an indicator variable must be prefixed with a colon and immediately follow its associated host variable. The keyword INDICATOR can be placed between the host variable and its indicator for additional clarity.

If the host variables are packaged in a struct, and you want to use indicator variables, you simply create a struct that has an indicator variable for each host variable in the host structure, and name the indicator struct in the SQL statement, immediately following the host variable struct, and prefixed with a colon. You can also use the INDICATOR keyword to separate a host structure and its associated indicator structure.

Oracle Datatypes

Typically, a host program inputs data to Oracle, and Oracle outputs data to the program. Oracle stores input data in database tables and stores output data in program host variables. To store a data item, Oracle must know its datatype, which specifies a storage format and valid range of values.

Oracle recognizes two kinds of datatypes: internal and external. Internal datatypes specify how Oracle stores data in database columns. Oracle also uses internal datatypes to represent database pseudocolumns, which return specific data items but are not actual columns in a table.

External datatypes specify how data is stored in host variables. When your host program inputs data to Oracle, if necessary, Oracle converts between the external datatype of the input host variable and the internal datatype of the target database column. When Oracle outputs data to your host program, if necessary, Oracle converts between the internal datatype of the source database column and the external datatype of the output host variable.

Arrays

Pro*C/C++ lets you define array host variables (called host arrays) and arrays of structures and operate on them with a single SQL statement. Using the array SELECT, FETCH, DELETE, INSERT, and UPDATE statements, you can query and manipulate large volumes of data with ease. You can also use host arrays inside a host variable struct.

Datatype Equivalencing

Pro*C/C++ adds flexibility to your applications by letting you equivalence datatypes. That means you can customize the way Oracle interprets input data and formats output data.

On a variable-by-variable basis, you can equivalence supported C datatypes to the Oracle external datatypes. You can also equivalence user-defined datatypes to Oracle external datatypes.

Private SQL Areas, Cursors, and Active Sets

To process a SQL statement, Oracle opens a work area called a private SQL area. The private SQL area stores information needed to execute the SQL statement. An identifier called a cursor lets you name a SQL statement, access the information in its private SQL area, and, to some extent, control its processing.

For static SQL statements, there are two types of cursors: implicit and explicit. Oracle implicitly declares a cursor for all data definition and data manipulation statements, including SELECT statements (queries) that return only one row. However, for queries that return more than one row, to process beyond the first row, you must explicitly declare a cursor (or use host arrays).

The set of rows returned is called the active set; its size depends on how many rows meet the query search condition. You use an explicit cursor to identify the row currently being processed, called the current row.

Imagine the set of rows being returned to a terminal screen. A screen cursor can point to the first row to be processed, then the next row, and so on. In the same way, an explicit cursor "points" to the current row in the active set. This allows your program to process the rows one at a time.

Transactions

A transaction is a series of logically related SQL statements (two UPDATEs that credit one bank account and debit another, for example) that Oracle treats as a unit, so that all changes brought about by the statements are made permanent or undone at the same time.

All the data manipulation statements executed since the last data definition, COMMIT, or ROLLBACK statement was executed make up the current transaction.

To help ensure the consistency of your database, Pro*C/C++ lets you define transactions using the COMMIT, ROLLBACK, and SAVEPOINT statements.

COMMIT makes permanent any changes made during the current transaction. ROLLBACK ends the current transaction and undoes any changes made since the transaction began. SAVEPOINT marks the current point in the processing of a transaction; used with ROLLBACK, it undoes part of a transaction.

Errors and Warnings

When you execute an embedded SQL statement, it either succeeds or fails, and might result in an error or warning. You need a way to handle these results. Pro*C/C++ provides two error handling mechanisms: the SQL Communications Area (SQLCA) and the WHENEVER statement.

The SQLCA is a data structure that you include (or hard-code) in your host program. It defines program variables used by Oracle to pass runtime status information to the program. With the SQLCA, you can take different actions based on feedback from Oracle about work just attempted. For example, you can check to see if a DELETE statement succeeded and, if so, how many rows were deleted.

With the WHENEVER statement, you can specify actions to be taken automatically when Oracle detects an error or warning condition. These actions are: continuing with the next statement, calling a function, branching to a labeled statement, or stopping.

SQL99 Syntax Support

The SQL standard enables the portability of SQL applications across all conforming software products. Oracle features are compliant with the ANSI/ISO SQL99 standard, including ANSI compliant joins. Pro*C/C++ supports all SQL99 features that are supported by Oracle database, which means that the SQL99 syntax for the SELECT, INSERT, DELETE, and UPDATE statements and the body of the cursor in a DECLARE CURSOR statement are supported.

Comments

You can place C-style Comments (/* ... */) in a SQL statement wherever blanks can be placed (except between the keywords EXEC SQL). Also, you can place ANSI-style Comments (-- ...) within SQL statements at the end of a line, as the following example shows:

EXEC SQL SELECT ENAME, SAL 
    INTO :emp_name, :salary  -- output host variables 
    FROM EMP 
    WHERE DEPTNO = :dept_number; 

You can use C++ style Comments (//) in your Pro*C/C++ source if you precompile using the CODE=CPP precompiler option.

Constants

An L or l suffix specifies a long integer constant, a U or u suffix specifies an unsigned integer constant, a 0X or 0x prefix specifies a hexadecimal integer constant, and an F or f suffix specifies a float floating-point constant. These forms are not allowed in SQL statements.

Declare Section

A Declare Section contains the host variable declarations and is of the form:

EXEC SQL BEGIN DECLARE SECTION;
/* Declare all host variables inside this section:  */
    char *uid = "username/password";
    ...
EXEC SQL END DECLARE SECTION;

A Declare Section begins with the statement:

EXEC SQL BEGIN DECLARE SECTION;

and ends with the statement:

EXEC SQL END DECLARE SECTION;

Between these two statements only the following are allowed:

• Host-variable and indicator-variable declarations

• Non-host C/C++ variables

• EXEC SQL DECLARE statements

• EXEC SQL INCLUDE statements

• EXEC SQL VAR statements

• EXEC SQL TYPE statements

• EXEC ORACLE statements

• C/C++ comments

A Declare Section is required when MODE=ANSI or CODE=CPP (in a C++ application) or PARSE=NONE or PARTIAL. For details of the PARSE option, see also "Parsing Code".

More than one Declare Section is allowed. They can be in different code modules.

Delimiters

While C uses single quotes to delimit single characters, as in

ch = getchar(); 
switch (ch)
{ 
case 'U': update();  break; 
case 'I': insert();  break; 
... 

SQL uses single quotes to delimit character strings, as in

EXEC SQL SELECT ENAME, SAL FROM EMP WHERE JOB = 'MANAGER'; 

While C uses double quotes to delimit character strings, as in

printf("\nG'Day, mate!"); 

SQL uses double quotes to delimit identifiers containing special or lowercase characters, as in

EXEC SQL CREATE TABLE "Emp2" (empno  number(4), ...); 

File Length

Pro*C/C++ cannot process arbitrarily long source files. There is a limit to the number of lines allowed. The following aspects of the source file are contributing factors to the file-size constraint:

• Complexity of the embedded SQL statements (for example, the number of bind and define variables).

• Whether a database name is used (for example, connecting to a database with an AT clause).

• Number of embedded SQL statements.

To prevent problems related to this limitation, use multiple program units to sufficiently reduce the size of the source files.

Function Prototyping

The ANSI C standard (X3.159-1989) provides for function prototyping. A function prototype declares a function and the data types of its arguments, so that the C compiler can detect missing or mismatched arguments.

The CODE option, which you can enter on the command line or in a configuration file, determines the way that the precompiler generates C or C++ code.

extern void sqlora(long *, void *); 

extern void sqlora(/*_ long *, void *  _*/);

Hint Length

Maximum length of a sql hint in an embedded sql statement is limited to 256 characters. Any hint exceeding this limit will be truncated.

Host Variable Names

Host variable names can consist of upper or lowercase letters, digits, and underscores, but must begin with a letter. They can be any length, but only the first 31 characters are significant to Pro*C/C++. Your C compiler or linker might require a shorter maximum length, so check your C compiler user's guide.

For portability, you may wish to restrict the length of host variable names to 18 or fewer characters (the length mandated by the SQL standard).

Line Continuation

You can continue SQL statements from one line to the next. You must use a backslash (\) to continue a string literal from one line to the next, as the following example shows:

EXEC SQL INSERT INTO dept (deptno, dname) VALUES (50, 'PURCHAS\ 
ING'); 

In this context, the precompiler treats the backslash as a continuation character.

Line Length

The maximum line length is 1299 for lines consisting of only ASCII characters, or 324 for multibyte characters.

MAXLITERAL Default Value

The precompiler option MAXLITERAL lets you specify the maximum length of string literals generated by the precompiler. The MAXLITERAL default value is 1024. Specify a smaller value if required. For example, if your C compiler cannot handle string literals longer than 512 characters, you then specify MAXLITERAL=512. Check your C compiler user's guide.

Operators

The logical operators and the "equal to" relational operator are different in C and SQL, as the following list shows. These C operators are not allowed in SQL statements:

The following C operators also not allowed in SQL statements:

Statement Terminator

Embedded SQL statements are always terminated by a semicolon, as the following example shows:

EXEC SQL DELETE FROM emp WHERE deptno = :dept_number;

Symbol Definition

You can define a symbol in two ways. Either include the statement:

EXEC ORACLE DEFINE symbol;

in your host program or define the symbol on the command line using the syntax:

... DEFINE=symbol ...

where symbol is not case-sensitive.

Some port-specific symbols are predefined for you when Pro*C/C++ is installed on your system. For example, predefined operating symbols include CMS, MVS, MS-DOS, UNIX, and VMS.

Example SELECT Statement

In the following example, the SELECT statement is precompiled only when the symbol site2 is defined:

EXEC ORACLE IFDEF site2;
   EXEC SQL SELECT DNAME
       INTO :dept_name
       FROM DEPT
       WHERE DEPTNO= :dept_number;
EXEC ORACLE ENDIF;

You can "comment out" C, C++, or embedded SQL code by placing it between IFDEF and ENDIF and not defining the symbol.

Guidelines

The following guidelines will help you avoid some common problems.

#define SQLCA_STORAGE_CLASS extern 

Example Data

Respectively, the DEPT and EMP tables contain the following rows

of data:

DEPTNO  DNAME      LOC
------- ---------- ---------
10      ACCOUNTING NEW YORK
20      RESEARCH   DALLAS
30      SALES      CHICAGO
40      OPERATIONS BOSTON

EMPNO ENAME   JOB          MGR  HIREDATE    SAL   COMM  DEPTNO
----- ------- --------- ------ --------- ------ ------ -------
 7369 SMITH   CLERK       7902 17-DEC-80    800             20
 7499 ALLEN   SALESMAN    7698 20-FEB-81   1600    300      30
 7521 WARD    SALESMAN    7698 22-FEB-81   1250    500      30
 7566 JONES   MANAGER     7839 02-APR-81   2975             20
 7654 MARTIN  SALESMAN    7698 28-SEP-81   1250   1400      30
 7698 BLAKE   MANAGER     7839 01-MAY-81   2850             30
 7782 CLARK   MANAGER     7839 09-JUN-81   2450             10
 7788 SCOTT   ANALYST     7566 19-APR-87   3000             20
 7839 KING    PRESIDENT        17-NOV-81   5000             10
 7844 TURNER  SALESMAN    7698 08-SEP-81   1500             30
 7876 ADAMS   CLERK       7788 23-MAY-87   1100             20
 7900 JAMES   CLERK       7698 03-DEC-81    950             30
 7902 FORD    ANALYST     7566 03-DEC-81   3000             20
 7934 MILLER  CLERK       7782 23-JAN-82   1300             10