Function sqlx::query_scalar

pub fn query_scalar<'q, DB, O>(
    sql: &'q str,
) -> QueryScalar<'q, DB, O, <DB as Database>::Arguments<'q>>
where
    DB: Database,
    (O,): for<'r> FromRow<'r, <DB as Database>::Row>,

Execute a single SQL query as a prepared statement (transparently cached) and extract the first column of each row.

Extracts the first column of each row. Additional columns are ignored. Any type that implements Type<DB> + Decode<DB> may be used.

For details about prepared statements and allowed SQL syntax, see query().

Example: Simple Lookup

If you just want to look up a single value with little fanfare, this API is perfect for you:

use uuid::Uuid;

// MySQL and MariaDB: use `?`
let user_id: Option<Uuid> = sqlx::query_scalar("SELECT user_id FROM users WHERE username = $1")
    .bind("alice")
    // Use `&mut` where `conn` is a connection or a transaction, or use `&` for a `Pool`.
    .fetch_optional(&mut conn)
    .await?;

let user_id = user_id.ok_or("unknown user")?;

Note how we’re using .fetch_optional() because the lookup may return no results, in which case we need to be able to handle an empty result set. Any rows after the first are ignored.

Example: COUNT

This API is the easiest way to invoke an aggregate query like SELECT COUNT(*), because you can conveniently extract the result:

// Note that `usize` is not used here because unsigned integers are generally not supported,
// and `usize` doesn't even make sense as a mapping because the database server may have
// a completely different architecture.
//
// `i64` is generally a safe choice for `COUNT`.
let count: i64 = sqlx::query_scalar("SELECT COUNT(*) FROM users WHERE accepted_tos IS TRUE")
    // Use `&mut` where `conn` is a connection or a transaction, or use `&` for a `Pool`.
    .fetch_one(&mut conn)
    .await?;

// The above is functionally equivalent to the following:
// Note the trailing comma, required for the compiler to recognize a 1-element tuple.
let (count,): (i64,) = sqlx::query_as("SELECT COUNT(*) FROM users WHERE accepted_tos IS TRUE")
    .fetch_one(&mut conn)
    .await?;

Example: EXISTS

To test if a row exists or not, use SELECT EXISTS(<query>):

// MySQL and MariaDB: use `?`
let username_taken: bool = sqlx::query_scalar(
    "SELECT EXISTS(SELECT 1 FROM users WHERE username = $1)"
)
    .bind("alice")
    // Use `&mut` where `conn` is a connection or a transaction, or use `&` for a `Pool`.
    .fetch_one(&mut conn)
    .await?;

Example: Other Aggregates

Be aware that most other aggregate functions return NULL if the query yields an empty set:

let max_upvotes: Option<i64> = sqlx::query_scalar("SELECT MAX(upvotes) FROM posts")
    // Use `&mut` where `conn` is a connection or a transaction, or use `&` for a `Pool`.
    .fetch_one(&mut conn)
    .await?;

Note how we’re using Option<i64> with .fetch_one(), because we’re always expecting one row but the column value may be NULL. If no rows are returned, this will error.

This is in contrast to using .fetch_optional() with Option<i64>, which implies that we’re expecting either a row with a i64 (BIGINT), or no rows at all.

Either way, any rows after the first are ignored.

Example: Vec of Scalars

If you want to collect a single column from a query into a vector, try .fetch_all():

let top_users: Vec<String> = sqlx::query_scalar(
    // Note the `LIMIT` to ensure that this doesn't return *all* users:
    "SELECT username
     FROM (
         SELECT SUM(upvotes) total, user_id
         FROM posts
         GROUP BY user_id
     ) top_users
     INNER JOIN users USING (user_id)
     ORDER BY total DESC
     LIMIT 10"
)
    // Use `&mut` where `conn` is a connection or a transaction, or use `&` for a `Pool`.
    .fetch_all(&mut conn)
    .await?;

// `top_users` could be empty, too.
assert!(top_users.len() <= 10);