作者:手机用户2502896851 | 来源:互联网 | 2023-02-08 16:42
我有这个工作代码:
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);
}
但如果我改变
"SELECT SUM(size) FROM toto", &[]
至
"SELECT SUM(size) FROM toto WHERE size=?1", &[&filter]
它重新运行,恐慌:
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
如果我在sqliteman中执行此查询,它可以工作,那么为什么rusqlite会出现恐慌?
1> Shepmaster..:
如果我直接在sqlite
命令行实用程序中运行查询,则没有值,结果是NULL
:
sqlite> SELECT SUM(size) FROM toto WHERE size=42;
sqlite> SELECT coalesce(SUM(size), 'NULL!') FROM toto WHERE size=42;
NULL!
sqlite>
由于没有与where子句匹配的行,因此总和为NULL:
如果没有非NULL输入行,则sum()返回NULL但total()返回0.0
您的NULL
结果无法转换为i64
,因此您收到错误消息:
InvalidColumnType(0, Null)
如果你打印出那个错误,它会说同样的事情:
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错误:索引处的列类型无效:0
修复它生锈的一面
如果您NULL
使用a更新代码以考虑a Option
,它将正确执行:
let i: Option = conn.query_row(
"SELECT SUM(size) FROM toto WHERE size = ?1",
&[&filter],
|r| r.get(0)
).expect("select failed");
然后你可以使用i.unwrap_or(0)
.
用SQL修复它 TOTAL
let i: f64 = conn.query_row(
"SELECT TOTAL(size) FROM toto WHERE size = ?1",
&[&filter],
|r| r.get(0)
).expect("select failed");
请注意,我们切换到了f64
.
用SQL修复它 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");