feat(core): add fetch_optional and fetch_one

sqlx-core/src/blocking/executor.rs

@@ -12,4 +12,20 @@ where
     where
         'e: 'x,
         'q: 'x;
+
+    fn fetch_all<'x, 'e, 'q>(
+        &'e mut self,
+        sql: &'q str,
+    ) -> crate::Result<Vec<<Self::Database as Database<Rt>>::Row>>
+    where
+        'e: 'x,
+        'q: 'x;
+
+    fn fetch_optional<'x, 'e, 'q>(
+        &'e mut self,
+        sql: &'q str,
+    ) -> crate::Result<Option<<Self::Database as Database<Rt>>::Row>>
+    where
+        'e: 'x,
+        'q: 'x;
 }

sqlx-core/src/error.rs

@@ -12,11 +12,20 @@ pub type Result<T> = std::result::Result<T, Error>;
 #[derive(Debug)]
 #[non_exhaustive]
 pub enum Error {
-    Configuration { message: Cow<'static, str>, source: Option<Box<dyn StdError + Send + Sync>> },
+    Configuration {
+        message: Cow<'static, str>,
+        source: Option<Box<dyn StdError + Send + Sync>>,
+    },
 
     Connect(Box<dyn DatabaseError>),
 
     Network(std::io::Error),
+
+    /// Returned by `fetch_one` when no row was returned from the query.
+    ///
+    /// Use `fetch_optional` to return `None` instead of signaling an error.
+    ///
+    RowNotFound,
 }
 
 impl Error {
@@ -56,6 +65,10 @@ impl Display for Error {
             Self::Configuration { message, source: Some(source) } => {
                 write!(f, "{}: {}", message, source)
             }
+
+            Self::RowNotFound => {
+                f.write_str("no row returned by a query required to return at least one row")
+            }
         }
     }
 }

sqlx-core/src/executor.rs

@@ -1,7 +1,7 @@
 #[cfg(feature = "async")]
-use futures_core::future::BoxFuture;
+use futures_util::future::{self, BoxFuture, FutureExt, TryFutureExt};
 
-use crate::{Database, Result, Runtime};
+use crate::{Database, Error, Result, Runtime};
 
 /// Describes a type that can execute SQL queries on a self-provided database connection.
 ///
@@ -55,5 +55,13 @@ pub trait Executor<Rt: Runtime> {
     where
         Rt: crate::Async,
         'e: 'x,
-        'q: 'x;
+        'q: 'x,
+    {
+        self.fetch_optional(sql)
+            .and_then(|maybe_row| match maybe_row {
+                Some(row) => future::ok(row),
+                None => future::err(Error::RowNotFound),
+            })
+            .boxed()
+    }
 }

sqlx-core/src/row.rs

@@ -1,6 +1,6 @@
 use crate::{Column, Database, Runtime};
 
-pub trait Row {
+pub trait Row: 'static + Send + Sync {
     type Column: Column;
 
     /// Returns `true` if the row contains only `NULL` values.