Modify records while iterating over them in SQLite - sql

Question

I'm trying to update all the columns of an SQLite database if it meets certain conditions.
I can't just use an UPDATE statement, the actual production database has diverse values, think of it as a game with many "bases" and I need to simulate them producing and consuming resources, and I'm not sure how to do that in SQL.
Here's a Python script to create the sample database where I'm testing this.
Now, the callback's code to modify the table column returns code 5, and the values are not being updated. I'm not sure how to fix this.
import sqlite3
import uuid
db = sqlite3.connect("main.db")
cur = db.cursor()
cur.execute("DROP TABLE IF EXISTS things")
cur.execute("CREATE TABLE IF NOT EXISTS things (numid INTEGER PRIMARY KEY, uuid TEXT, value INTEGER)")
for i in range(1, 2000):
cur.execute(
"INSERT INTO things (uuid, value) VALUES (?, ?)", (str(uuid.uuid4()), 1)
)
db.commit()
And this is the C code that I'm using to go through the entire database. Make sure to link with the SQLite library.
#include <sqlite3.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct things_t
{
int numid;
int value;
} things_t;
static int callback(void *voidthing, int argc, char **argv, char **azColName)
{
int i;
things_t *thing = (things_t *)voidthing;
for (i = 0; i < argc; i++)
{
if (strcmp(azColName[i], "value") == 0)
{
thing->value = atoi(argv[i]);
thing->value++;
}
if (strcmp(azColName[i], "numid") == 0)
{
thing->numid = atoi(argv[i]);
}
}
sqlite3 *db;
int rc = sqlite3_open("main.db", &db);
printf("open file rc: %d (thing numid: %d, thing value: %d)\n", rc,
thing->numid, thing->value);
sqlite3_stmt *stmt;
sqlite3_prepare(db, "UPDATE things SET value = ? WHERE numid = ?", -1,
&stmt, NULL);
sqlite3_bind_int(stmt, 1, thing->value);
sqlite3_bind_int(stmt, 2, thing->numid);
rc = sqlite3_step(stmt);
printf("step rc: %d (thing numid: %d, thing value: %d)\n", rc,
thing->numid, thing->value);
sqlite3_finalize(stmt);
sqlite3_close(db);
return 0;
}
int main()
{
char *err = "";
sqlite3 *db;
sqlite3_open("main.db", &db);
things_t thing;
sqlite3_exec(db, "SELECT * FROM things LIMIT 200", callback, &thing,
&err);
sqlite3_close(db);
}

Answer 1

I've decided to just use SQL subqueries to do this.
For example, this query would show how much resources each faction would lose or gain per turn (once again, continuing the game example).
SELECT (SELECT faction_name FROM factions WHERE owner = numid) as owner, SUM(production - consumption) as resdiff FROM buildings GROUP BY owner;
Here's a dump of the sample database where these queries (including the one above) are being tested:
BEGIN TRANSACTION;
CREATE TABLE factions (numid INTEGER PRIMARY KEY, faction_name TEXT, resources INTEGER);
INSERT INTO factions VALUES(1,'Arch users',900);
INSERT INTO factions VALUES(2,'Debian users',912);
INSERT INTO factions VALUES(3,'Gentoo users',100);
CREATE TABLE buildings (numid INTEGER PRIMARY KEY, owner INTEGER, production INTEGER, consumption INTEGER, FOREIGN KEY(owner) REFERENCES factions(numid));
INSERT INTO buildings VALUES(1,1,339,291);
INSERT INTO buildings VALUES(2,1,211,440);
INSERT INTO buildings VALUES(3,1,773,445);
INSERT INTO buildings VALUES(4,2,143,498);
INSERT INTO buildings VALUES(5,2,339,113);
INSERT INTO buildings VALUES(6,3,10,9);
INSERT INTO buildings VALUES(7,2,144,14);
COMMIT;
I'll just update the data with SQL, then extract the results with other queries (and as part of the game example, presumably show them to the player).