C Prog Newb: retrieve MYSQL query true/false value - mysql

Question

Will try to make this simple - I can elaborate if needed:
Goal: trying to query MYSQL DB from C if a row or record exists using this command
SELECT EXISTS(SELECT * FROM users WHERE id = '123456789'
these are some of the test codes I've tried to get a proper return value from mysql.
sprintf(exist, "SELECT EXISTS(SELECT * FROM users WHERE id = '%s')", new_person.ID);
printf("\n\nExists result: %s\n\n", (mysql_query(conn, exist))); // debug code - return NULL
printf("\n\nExists = %s\n\n", exist); // debug code - output seems exactly what I want sent to database, I've tested with and without singles quotes ' '
printf("\n\nMYSQL_QUERY: %d\n\n", mysql_query(conn, exist)); // debug code - testing return value - seems all successfull query return 1 - always return 1 regardless if id exists or not.
printf("\n\nSQLSTATE: %d\n\n", mysql_stmt_sqlstate(exist)); // this does not return what's intended - looking for a value of 1 or 0 - returns 12935280
test = mysql_stmt_sqlstate(exist); // debug code testing if correct value is assigned
The goal is to get a return of 1 or 0 from my db. When I query mysql directly, i do get a true or false value as you can see below
mysql> select * FROM users;
+-----------+------------+-----------+------------+--------+--------+------+
| id | first_name | last_name | dob | height | weight | BMI |
+-----------+------------+-----------+------------+--------+--------+------+
| 123456789 | xxxx | xxxxx | xxxxxxxxxx | xx | xxx | xxxx |
| 254565 | Jessica | Perez | 05/03/2001 | 60 | 120 | 23.4 |
| 456132 | Tomas | Aquina | 02/01/1254 | 58 | 140 | 29.3 |
| 5245 | Herlinda | Jones | 12/01/1982 | 69 | 180 | 26.6 |
+-----------+------------+-----------+------------+--------+--------+------+
4 rows in set (0.00 sec)
mysql> SELECT EXISTS(SELECT * FROM users WHERE id = 123456789);
+--------------------------------------------------+
| EXISTS(SELECT * FROM users WHERE id = 123456789) |
+--------------------------------------------------+
| 1 |
+--------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT EXISTS(SELECT * FROM users WHERE id = 6548);
+---------------------------------------------+
| EXISTS(SELECT * FROM users WHERE id = 6548) |
+---------------------------------------------+
| 0 |
+---------------------------------------------+
1 row in set (0.00 sec)
I need the true/false value as a conditional for other parts of the code.
Sorry if this has been addressed or answered before. I couldn't find it anywhere and I looked in MySQL reference at: https://dev.mysql.com/doc/c-api/8.0/en/c-api-function-reference.html
Thanks again!
For those who want to see the full code:
// 911Prep - software to track of household emergency needs
// Hans Yunge (C)2022
#define _CRT_SECURE_NO_WARNINGS
// header inclusions
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <mysql.h>
#include "911prep_header.h"
// function declaration
void create_person(Person* new_person);
int main(int argc, char** argv) {
char *query;
char* exist;
// calling creating Person function - printing out data
Person new_person;
Person_p ptr_person = &new_person;
create_person(ptr_person);
// BMI calculation
new_person.BMI = new_person.weight / (new_person.height * new_person.height) * 703;
// connect to mysql
MYSQL* conn;
MYSQL_RES* res;
MYSQL_ROW row;
char* server = "localhost";
char* user = "root";
char* password = "password"; // set me first
char* database = "911prep";
conn = mysql_init(NULL);
if (conn == NULL) {
fprintf(stderr, "%s\n", mysql_error(conn));
exit(1);
}
// Connect to database
if (!mysql_real_connect(conn, server,
user, password, NULL, 0, NULL, 0)) {
fprintf(stderr, "%s\n", mysql_error(conn));
exit(1);
}
if (mysql_query(conn, "CREATE DATABASE IF NOT EXISTS testdb"))
{
fprintf(stderr, "%s\n", mysql_error(conn));
mysql_close(conn);
exit(1);
}
printf("\nMySQL version is: %s\n", mysql_get_client_info());
// select database
if (mysql_query(conn, "USE 911prep"))
{
fprintf(stderr, "%s\n", mysql_error(conn));
}
// check to see if row exists and update data to existing row
query = (char*)malloc(200);
exist = (char*)malloc(50);
sprintf(exist, "SELECT EXISTS(SELECT * FROM users WHERE id = '%s')", new_person.ID);
printf("\n\nExists result: %s\n\n", (mysql_query(conn, exist)));
printf("\n\nExists = %s\n\n", exist);
printf("\n\nMYSQL_QUERY: %d\n\n", mysql_query(conn, exist)); // testing return value - seems all successfull query return 1
int test = mysql_stmt_sqlstate(exist); // this does not return what's intended - looking for a value of 1 or 0
if (test == 1)
{
sprintf(query, "UPDATE users SET first_name ='%s', last_name = '%s', dob = '%s', height = %2.0f, weight = %3.0f, BMI = %2.1f WHERE id=%s", new_person.firstName, new_person.lastName, new_person.birthDate,
new_person.height, new_person.weight, new_person.BMI, new_person.ID);
if (mysql_query(conn, query))
{
fprintf(stderr, "%s\n", mysql_error(conn));
mysql_close(conn);
exit(1);
}
free(query);
}
// insert new data into database
if (test == 0)
{
sprintf(query, "INSERT INTO users (id, first_name, last_name, dob, height, weight, BMI) VALUES ('%s', '%s', '%s', '%s', %2.0f, %3.0f, %2.1f)", new_person.ID, new_person.firstName, new_person.lastName,
new_person.birthDate, new_person.height, new_person.weight, new_person.BMI);
if (mysql_query(conn, query))
{
fprintf(stderr, "%s\n", mysql_error(conn));
mysql_close(conn);
exit(1);
}
free(query);
}
mysql_close(conn);
//show data
// printf("ID: %s\nFirst Name: %s\nLast Name: %s\nBirth Date: %s\nHeight: %2.0f\nWeight: %3.0f\nBMI: %2.1f\n\n", new_person.ID, new_person.firstName, new_person.lastName, new_person.birthDate, new_person.height, new_person.weight, new_person.BMI);
return 0;
}

Answer 1

Thanks to my friend in the above comments leading me to pursue other unknown avenues and thanks to ZetCode # https://zetcode.com/db/mysqlc/
I'm on the right path... though have another issue. I'm not understanding how to use the resulting value - when I use row[0] in a conditional I get an error.
if (mysql_query(conn, exist) != 0)
{
finish_with_error(conn);
}
MYSQL_RES* result = mysql_store_result(conn);
if (result == NULL) {
finish_with_error(conn);
}
int num_fields = mysql_num_fields(result);
MYSQL_ROW row;
while ((row = mysql_fetch_row(result)))
{
for (int i = 0; i < num_fields; i++)
{
printf("%s ", row[i] ? row[i] : "NULL");
}
printf("\n");
}
mysql_free_result(result);
mysql_close(conn);
The above is the basic format of what I'm trying to do which provided correct feedback, true and false from the query.