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Figure 17-8.
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I Compile-time errors. Misplaced commas, misspelled SQL keywords, and similar errors in embedded SQL statements are detected by the SQL precompiler and reported to the programmer. The programmer can fix the errors and recompile the application program. I Runtime errors. An attempt to insert an invalid data value or lack of permission to update a table can be detected only at runtime. Errors such as these must be detected and handled by the application program. In embedded SQL programs, the DBMS reports runtime errors to the application program through a returned error code. If an error is detected, a further description of the error and other information about the statement just executed is available through additional diagnostic information. The earliest IBM-embedded SQL implementations defined an error-reporting mechanism that was adopted, with variations, by most of the major DBMS vendors. The central part of this scheme an error status variable named SQLCODE was also defined in the original ANSI/ISO SQL standard. The SQL2 standard, published in 1992, defined an entirely new, parallel error-reporting mechanism, built around an error status variable named SQLSTATE. These mechanisms are described in the next two sections.
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Under this scheme, pioneered by the earliest IBM products, the DBMS communicates status information to the embedded SQL program through an area of program storage called the SQL Communications Area, or SQLCA. The SQLCA is a data structure that contains error variables and status indicators. By examining the SQLCA, the application program can determine the success or failure of its embedded SQL statements and act accordingly.
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Notice in Figures 17-4, 17-5, 17-6, and 17-7 that the first embedded SQL statement in the program is INCLUDE SQLCA. This statement tells the SQL precompiler to include a SQL Communications Area in this program. The specific contents of the SQLCA vary slightly from one brand of DBMS to another, but the SQLCA always provides the same type of information. Figure 17-9 shows the definition of the SQLCA used by the IBM databases. The most important part of the SQLCA, the SQLCODE variable, is supported by all of the major embedded SQL products and was specified by the ANSI/ISO SQL1 standard. As the DBMS executes each embedded SQL statement, it sets the value of the variable SQLCODE in the SQLCA to indicate the completion status of the statement:
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struct sqlca { unsigned char sqlcaid[8]; long long short unsigned unsigned long unsigned unsigned } #define SQLCODE sqlca.sqlcode sqlcabc; sqlcode; sqlerrml; sqlerrmc[70]; sqlerrp[8]; sqlerrd[6]; sqlwarn[8]; sqlext[8];
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/* the string "SQLCA /* /* /* /* /* /* /* /*
" */
char char char char
length of SQLCA, in bytes */ SQL status code */ length of sqlerrmc array data */ name(s) of object(s) causing error */ diagnostic information */ various counts and error code */ warning flag array */ extension to sqlwarn array */
/* SQL status code */
/* A 'W' in any of the SQLWARN fields signals a warning condition; otherwise these fields each contain a blank */ #define #define #define #define #define #define #define #define SQLWARN0 SQLWARN1 SQLWARN2 SQLWARN3 SQLWARN4 SQLWARN5 SQLWARN6 SQLWARN7 sqlca.sqlwarn[0] sqlca.sqlwarn[1] sqlca.sqlwarn[2] sqlca.sqlwarn[3] sqlca.sqlwarn[4] sqlca.sqlwarn[5] sqlca.sqlwarn[6] sqlca.sqlwarn[7] /* /* /* /* /* /* /* /* master warning flag */ string truncated */ NULLs eliminated from column function */ too few/too many host variables */ prepared UPDATE/DELETE without WHERE */ SQL/DS vs DB2 incompatibility */ invalid date in arithmetic expr */ reserved */
` Figure 17-9.
The SQL Communications Area (SQLCA) for IBM databases
17:
Embedded SQL
I A SQLCODE of zero indicates successful completion of the statement, without any errors or warnings. I A negative SQLCODE value indicates a serious error that prevented the statement from executing correctly. For example, an attempt to update a read-only view would produce a negative SQLCODE value. A separate negative value is assigned to each runtime error that can occur. I A positive SQLCODE value indicates a warning condition. For example, truncation or rounding of a data item retrieved by the program would produce a warning. A separate positive value is assigned to each runtime warning that can occur. The most common warning, with a value of +100 in most implementations and in the SQL1 standard, is the out-of-data warning returned when a program tries to retrieve the next row of query results and no more rows are left to retrieve. Because every executable embedded SQL statement can potentially generate an error, a well-written program will check the SQLCODE value after every executable embedded SQL statement. Figure 17-10 shows a C program excerpt that checks the SQLCODE value. Figure 17-11 shows a similar excerpt from a COBOL program.
. . . exec sql delete from salesreps where quota < 150000; if (sqlca.sqlcode < 0) goto error_routine; . . . error_routine: printf("SQL error: %ld\n, sqlca.sqlcode); exit(); . . .
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