#include <string.h>
#include <stdio.h>

int main(){

    char username[25];
    char passwd[45];

    printf("Input your name ->");
    fgets(username, 25, stdin);

    printf("Input your password->");
    fgets(passwd, 45, stdin);

    char query[128]="select username, password from accounts where username='";
    char and[18]="' and password='";

    strncat(query, username, 25);
    strncat(query, and, 16);
    strncat(query, passwd, 45);
    strncat(query, "';n", 4);

    printf("%s", query);

    return 0;

    //OBS: Código de exemplo
}

gcc main.c
./a.out
Input your name ->linus
Input your password->123
select username, password from accounts where username='linus
' and password='123
';