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In the program examples thus far in this chapter, the queries to be performed were known in advance. The columns of query results could be bound to program variables by explicit dbbind() calls hard-coded in the program. Most programs that use SQL Server can be written using this technique. (This static column binding corresponds to the fixed list of host variables used in the static SQL FETCH statement in standard embedded SQL, described in 17.) If the query to be carried out by a program is not known at the time the program is written, the program cannot include hard-coded dbbind() calls. Instead, the program must ask dblib for a description of each column of query results, using special API functions. The program can then bind the columns on the fly to data areas that it allocates at runtime. (This dynamic column binding corresponds to the use of the dynamic SQL DBNUMCOLS() statement and SQLDA, in dynamic embedded SQL, as described in 18.) Figure 19-10 shows an interactive query program that illustrates the dblib technique for handling dynamic queries. The program accepts a table name entered by the user and then prompts the user to choose which columns are to be retrieved from the table. As the user selects the columns, the program constructs a SELECT statement and then uses these steps to execute the SELECT statement and display the data from the selected columns: 1. The program passes the generated SELECT statement to SQL Server using the dbcmd() call, requests its execution with the dbsqlexec() call, and calls dbresults() to advance the API to the query results, as it does for all queries. 2. The program calls dbnumcols() to find out how many columns of query results were produced by the SELECT statement. 3. For each column, the program calls dbcolname() to find out the name of the column, calls dbcoltype() to find out its data type, and calls dbcollen() to find out its maximum length. 4. The program allocates a buffer to receive each column of query results and calls dbbind() to bind each column to its buffer. 5. When all columns have been bound, the program calls dbnextrow() repeatedly to retrieve each row of query results.
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main() { /* This is a simple general-purpose query program. It prompts the user for a table name and then asks the user which columns of the table are to be included in the query. After the user s selections are complete, the program runs the requested query and displays the results. */ LOGINREC DBPROCESS char char char int int int int char char char int char int *loginrec; *dbproc; stmbuf[2001]; querytbl[32]; querycol[32]; status; first_col = 0; colcount; i; inbuf[101]; *item_name[100]; *item_data[100]; item_type[100]; *address; length; /* /* /* /* /* /* /* /* /* /* /* /* /* /* /* data structure for login information */ data structure for connection */ SQL text to be executed */ user-specified table */ user-specified column */ dblib return status */ is this the first column chosen */ number of columns of query results */ index for columns */ input entered by user */ array to track column names */ array to track column buffers */ array to track column data types */ address of buffer for current column */ length of buffer for current column */
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/* Open a connection to SQL Server */ loginrec = dblogin(); DBSETLUSER(loginrec, scott ); DBSETLPWD (loginrec, tiger ); dbproc = dbopen(loginrec, ); /* Prompt the user for which table to query */ printf( *** Mini-Query Program ***\n ); printf( Enter name of table for query: ); gets(querytbl); /* Start the SELECT statement in the buffer */ strcpy(stmbuf, select );
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