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PostgreSQL 8 for Windows
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PQclear() function is used to clear the memory used by the PGresult object It is always a good idea to clear this memory, especially if you reuse the PGresult object for another command After compiling the updatec program, execute it on your PostgreSQL system If the INSERT command succeeds, nothing should display (not too exciting) You can then look at the auctionbidders table and see the newly added record If you run the program a second time, you should get an error message, indicating that the Bidder table key value already exists:
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C:\Program Files\PostgreSQL\82\bin>update Problem with command: ERROR: duplicate key violates unique constraint "bidder_pkey" C:\Program Files\PostgreSQL\82\bin>
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For SQL commands that return data, you must use the PQgetvalue() function to retrieve the returned information The PQgetvalue() function retrieves the data values as a character string data type, no matter what the actual data type of the table column data For integer or floating-point values, you can convert the character string value into the appropriate data type using standard C functions, such as atoi() for integers or atof() for floating-point values The PQgetvalue() function allows you to retrieve data from the result set in any order You are not limited in walking forward through each record in the result set The format of the PQgetvalue() function is
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char *PQgetvalue(PGresult *result, int record, int column)
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In the function, you must specify the result set result, the desired record number record within the result set, and also the column number column within the record to retrieve a value from Records and column numbers both start at 0 For commands that produce only one record, that is always record 0 The getvalsc program demonstrates how to retrieve and convert data from a result set of a SQL function:
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#include <stdioh> #include <stdlibh> #include "libpq-feh" int main(int argc, char **argv) { const char *conninfo; PGconn *conn; PGresult *result; char *time, *pi; float fpi;
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conninfo = "hostaddr = 127001 dbname = test user = earl password = auction"; conn = PQconnectdb(conninfo); if (PQstatus(conn) != CONNECTION_OK) { printf("Unable to establish connection: %s", PQerrorMessage(conn)); return 1; } else { result = PQexec(conn, "SELECT timeofday()"); if (PQresultStatus(result) != PGRES_TUPLES_OK) { printf("Problem with command1: %s\n", PQerrorMessage(conn)); PQclear(result); return 1; } time = PQgetvalue(result, 0, 0); printf("Time of day: %s\n", time); PQclear(result); result = PQexec(conn, "Select pi()"); if (PQresultStatus(result) != PGRES_TUPLES_OK) { printf("Problem with command: %s\n", PQerrorMessage(conn)); PQclear(result); return 1; } pi = PQgetvalue(result, 0, 0); fpi = atof(pi); printf("The value of pi is: %lf\n", fpi); PQclear(result); } PQfinish(conn); return 0; }
After establishing the connection, the PQexec() function is used to send a standard SELECT SQL command to the server Notice that to check the status of the result set, you must use the PGRES_TUPLES_OK value, since the query returns a result set The PQgetvalue() function is used to retrieve the result set, using record 0 and column 0 since there is only one data value returned In the first instance, the timeofday() PostgreSQL function returns a string value, which can be directly placed in a character string variable After using the PQclear() function to reset the PGresult value, another query is made with the PQexec() function Again, the PQgetvalue() function is used to retrieve the text result set from the query This time, since the desired data type is a floating-point value, the atof() C function is used to convert the string value to a floating point variable
PostgreSQL 8 for Windows
Running the program produces the following results:
C:\Program Files\PostgreSQL\82\bin>getvals Time of day: Sun Oct 01 16:37:29092000 2006 EDT The value of pi is: 3141593 C:\Program Files\PostgreSQL\82\bin>
Handling Column Data
The getvalsc program was trivial in that we knew there was only one column of data in the result set For more complicated result sets, you must determine the number of records returned and the order of the data columns in the result set If there are multiple records in the result set, you must loop through the result set, reading all of the data records until you have read the last one To determine the number of records and columns in a result set, you can use the PQntuples() and PQnfields() functions, respectively:
int recs; int cols; result = PQexec(conn, "SELECT * from auctionitems"); recs = PQntuples(result); cols = PQnfields(result);
Once you know the number of records and columns in a result set, it is a snap to loop through the result set, extracting individual data items:
for (i = 0; i < recs; i++) {
There is one thing you must be careful about, though, when extracting the column data Remember, the PQexec() function returns all data items in text format This means that if you intend to use the table data within your application program as another data type, you must use standard C functions to convert the string to the appropriate data type: atoi() For converting to an integer value atof() For converting to a floating-point value The getitemswonc program demonstrates how to extract individual column data elements from a result set:
#include <stdioh> #include <stdlibh> #include "libpq-feh"
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