Rusqlite query with SUM panics with “InvalidColumn

问题:

I have this working code:

extern crate rusqlite;

use rusqlite::Connection;

fn main() {
    let conn = Connection::open("db.sqlite").unwrap();

    conn.execute("CREATE TABLE toto (size INTEGER NOT NULL DEFAULT 0);", &[]).unwrap();
    conn.execute("INSERT INTO toto(size) VALUES (42);", &[]).unwrap();

    let filter = 0;
    let i: i64 = conn.query_row("SELECT SUM(size) FROM toto", &[], |r| r.get(0)).unwrap();
    println!("Coucou");
    println!("Coucou: {}", i);
}

But if I switch

"SELECT SUM(size) FROM toto", &[]

to

"SELECT SUM(size) FROM toto WHERE size=?1", &[&filter]

and re-run, it panics:

rm db.sqlite
RUST_BACKTRACE=1 cargo run

        Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
         Running `target/debug/testsqlite`
    thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidColumnType(0, Null)', /checkout/src/libcore/result.rs:860

If I execute this query in sqliteman, it works, so why does rusqlite panic?

回答1:

If I run your query directly in the sqlite command line utility, there is no value, the result is NULL:

sqlite> SELECT SUM(size) FROM toto WHERE size=42;

sqlite> SELECT coalesce(SUM(size), 'NULL!') FROM toto WHERE size=42;
NULL!
sqlite>

Since there are no rows matching the where clause, the sum is NULL:

If there are no non-NULL input rows then sum() returns NULL but total() returns 0.0

Your NULL result cannot be converted to an i64, so you get an error:

InvalidColumnType(0, Null)

If you print that error out, it says the same thing:

let i: i64 = conn.query_row("SELECT SUM(size) FROM toto WHERE size = ?1", &[&filter], |r| {
    match r.get_checked(0) {
        Ok(v) => v,
        Err(e) => panic!("query_row error: {}", e),
    }
}).expect("select failed");

query_row error: Invalid column type Null at index: 0

Fixing it Rust-side

If you update your code to account for a NULL by using an Option, it will correctly execute:

let i: Option<i64> = conn.query_row(
    "SELECT SUM(size) FROM toto WHERE size = ?1",
    &[&filter],
    |r| r.get(0)
).expect("select failed");

You can then use i.unwrap_or(0).

Fixing it SQL-side with TOTAL

let i: f64 = conn.query_row(
    "SELECT TOTAL(size) FROM toto WHERE size = ?1",
    &[&filter],
    |r| r.get(0)
).expect("select failed");

Note that we switched to a f64.

Fixing it SQL-side with COALESCE

let i: i64 = conn.query_row(
    "SELECT COALESCE(SUM(size), 0) FROM toto WHERE size = ?1",
    &[&filter],
    |r| r.get(0)
).expect("select failed");