diesel/expression/
exists.rs

1use backend::Backend;
2use expression::subselect::Subselect;
3use expression::{AppearsOnTable, Expression, NonAggregate, SelectableExpression};
4use query_builder::*;
5use result::QueryResult;
6use sql_types::Bool;
7
8/// Creates a SQL `EXISTS` expression.
9///
10/// The argument must be a complete SQL query. The query may reference columns
11/// from the outer table.
12///
13/// # Example
14///
15/// ```rust
16/// # #[macro_use] extern crate diesel;
17/// # include!("../doctest_setup.rs");
18/// #
19/// # fn main() {
20/// #     use schema::users::dsl::*;
21/// #     use diesel::select;
22/// #     use diesel::dsl::exists;
23/// #     let connection = establish_connection();
24/// let sean_exists = select(exists(users.filter(name.eq("Sean"))))
25///     .get_result(&connection);
26/// let jim_exists = select(exists(users.filter(name.eq("Jim"))))
27///     .get_result(&connection);
28/// assert_eq!(Ok(true), sean_exists);
29/// assert_eq!(Ok(false), jim_exists);
30/// # }
31/// ```
32pub fn exists<T>(query: T) -> Exists<T> {
33    Exists(Subselect::new(query))
34}
35
36#[derive(Debug, Clone, Copy, QueryId)]
37pub struct Exists<T>(Subselect<T, ()>);
38
39impl<T> Expression for Exists<T>
40where
41    Subselect<T, ()>: Expression,
42{
43    type SqlType = Bool;
44}
45
46impl<T> NonAggregate for Exists<T> {}
47
48impl<T, DB> QueryFragment<DB> for Exists<T>
49where
50    DB: Backend,
51    T: QueryFragment<DB>,
52{
53    fn walk_ast(&self, mut out: AstPass<DB>) -> QueryResult<()> {
54        out.push_sql("EXISTS (");
55        self.0.walk_ast(out.reborrow())?;
56        out.push_sql(")");
57        Ok(())
58    }
59}
60
61impl<T, QS> SelectableExpression<QS> for Exists<T>
62where
63    Self: AppearsOnTable<QS>,
64    Subselect<T, ()>: SelectableExpression<QS>,
65{
66}
67
68impl<T, QS> AppearsOnTable<QS> for Exists<T>
69where
70    Self: Expression,
71    Subselect<T, ()>: AppearsOnTable<QS>,
72{
73}
diesel/expression/exists.rs

diesel/expression/
exists.rs
