How to do it…

1. Initialize a MYSQL object:

mysql_init(NULL);

2. Establish a connection to the MySQL server running at the specified host. Also, generate a connection handler. The program will terminate if some error occurs in establishing the connection to the MySQL server engine or to the ecommerce database:

 if (!mysql_real_connect(conn, server, user, password, database, 0, NULL, 0)) 
 {
      fprintf(stderr, "%s\n", mysql_error(conn));
      exit(1);
 }

3. Enter the email address of the user whose information has to be updated:

printf("Enter email address of the user to update: ");
scanf("%s", emailaddress);

4. Create an SQL SELECT statement that will search the row in the users table that matches the email address that was entered by the user:

 strcpy(sqlquery,"SELECT * FROM users where email_address like \'");
 strcat(sqlquery,emailaddress);
 strcat(sqlquery,"\'");

5. Execute the SQL SELECT statement. The program will terminate if the SQL query does not execute successfully or some other error occurs:

if (mysql_query(conn, sqlquery) != 0) 
{ 
     fprintf(stderr, "No row found in the users table with this          email address\n"); 
     exit(1); 
 }  

6. If the SQL query executes successfully, then the row(s) that match the supplied email address will be retrieved and assigned to the resultset:

 resultset = mysql_store_result(conn);

7. Check if there is at least one row in the resultset:

if(mysql_num_rows(resultset) >0)

8. If there is no row in the resultset, then display the message that no row was found in the users table with the specified email address and exit from the program:

printf("No user found with this email address\n");

9. If there is any row in the resultset, then access it and assign it to the array row:

row = mysql_fetch_row(resultset)

10. Information about the user (that is, the email address, password, and address of delivery, which are assigned to the subscripts row[0], row[1], and row[2], respectively) are displayed on the screen:

printf("Email Address: %s \n", row[0]);
printf("Password: %s \n", row[1]);
printf("Address of delivery: %s \n", row[2]);

11. The memory allocated to the resultset is freed:

mysql_free_result(resultset);

12. Enter the new updated information of the user; that is, the new password and the new address of delivery:

printf("Enter new password: ");
scanf("%s", upassword);
printf("Enter new address of delivery: ");
getchar();
gets(deliveryaddress);

13. An SQL UPDATE statement is prepared that contains the information of the newly entered password and address of delivery:

strcpy(sqlquery,"UPDATE users set password=\'");
strcat(sqlquery,upassword);
strcat(sqlquery,"\', address_of_delivery=\'");
strcat(sqlquery,deliveryaddress);
strcat(sqlquery,"\' where email_address like \'");
strcat(sqlquery,emailaddress);
strcat(sqlquery,"\'");

14. Execute the SQL UPDATE statement. If any error occurs in executing the SQL UPDATE query, the program will terminate:

if (mysql_query(conn, sqlquery) != 0)                 
{                                                                                                                                                  fprintf(stderr, "The desired row in users table could not be 
    updated\n");  
    exit(1);
 }  

15. If the SQL UPDATE statement executes successfully, display a message on the screen informing that the user's information has been updated successfully:

printf("The information of user is updated successfully in users table\n");

16. Close the opened connection handler:

mysql_close(conn);

The updateuser.c program for updating a specific row of a MySQL database table with new content is shown in the following code:

#include <mysql/mysql.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void main() {
    MYSQL *conn;
    MYSQL_RES *resultset;
    MYSQL_ROW row;
    char *server = "127.0.0.1";
    char *user = "root";
    char *password = "Bintu2018$";
    char *database = "ecommerce";
    char emailaddress[30], sqlquery[255],             
    upassword[30],deliveryaddress[255];
    conn = mysql_init(NULL);
    if (!mysql_real_connect(conn, server, user, password, database, 0,     NULL, 0)) {
        fprintf(stderr, "%s\n", mysql_error(conn));
        exit(1);
    }
    printf("Enter email address of the user to update: ");
    scanf("%s", emailaddress);
    strcpy(sqlquery,"SELECT * FROM users where email_address like \'");
    strcat(sqlquery,emailaddress);
    strcat(sqlquery,"\'");
    if (mysql_query(conn, sqlquery) != 0)                 
    {                                                                                 
        fprintf(stderr, "No row found in the users table with this 
        email address\n");                                                                                                     
        exit(1);                                                                     
    }  
    resultset = mysql_store_result(conn);
    if(mysql_num_rows(resultset) >0)
    {
        printf("The details of the user with this email address are as 
        follows:\n");
        while ((row = mysql_fetch_row(resultset)) != NULL)
        {
            printf("Email Address: %s \n", row[0]);
            printf("Password: %s \n", row[1]);
            printf("Address of delivery: %s \n", row[2]);
        }
        mysql_free_result(resultset);
        printf("Enter new password: ");
        scanf("%s", upassword);
        printf("Enter new address of delivery: ");
        getchar();
        gets(deliveryaddress);
        strcpy(sqlquery,"UPDATE users set password=\'");
        strcat(sqlquery,upassword);
        strcat(sqlquery,"\', address_of_delivery=\'");
        strcat(sqlquery,deliveryaddress);
        strcat(sqlquery,"\' where email_address like \'");
        strcat(sqlquery,emailaddress);
        strcat(sqlquery,"\'");
        if (mysql_query(conn, sqlquery) != 0)                 
        {                                                                                                                                                         
            fprintf(stderr, "The desired row in users table could not 
            be updated\n");                                                             
            exit(1);                                                                     
        }  
        printf("The information of user is updated successfully in 
        users table\n");
    }
    else
        printf("No user found with this email address\n");
    mysql_close(conn);
}

Now, let's go behind the scenes to understand the code better.