diesel/expression/
exists.rs

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1//! This module contains the query dsl node definition
2//! for `EXISTS` expressions
3
4use crate::backend::{sql_dialect, Backend, SqlDialect};
5use crate::expression::subselect::Subselect;
6use crate::expression::{AppearsOnTable, Expression, SelectableExpression, ValidGrouping};
7use crate::helper_types;
8use crate::query_builder::*;
9use crate::result::QueryResult;
10use crate::sql_types::Bool;
11
12/// Creates a SQL `EXISTS` expression.
13///
14/// The argument must be a complete SQL query. The query may reference columns
15/// from the outer table.
16///
17/// # Example
18///
19/// ```rust
20/// # include!("../doctest_setup.rs");
21/// #
22/// # fn main() {
23/// #     use schema::users::dsl::*;
24/// #     use diesel::select;
25/// #     use diesel::dsl::exists;
26/// #     let connection = &mut establish_connection();
27/// let sean_exists = select(exists(users.filter(name.eq("Sean"))))
28///     .get_result(connection);
29/// let jim_exists = select(exists(users.filter(name.eq("Jim"))))
30///     .get_result(connection);
31/// assert_eq!(Ok(true), sean_exists);
32/// assert_eq!(Ok(false), jim_exists);
33/// # }
34/// ```
35pub fn exists<T>(query: T) -> helper_types::exists<T> {
36    Exists {
37        subselect: Subselect::new(query),
38    }
39}
40
41/// The query dsl node that represents a SQL `EXISTS (subselect)` expression.
42///
43/// Third party backend can customize the [`QueryFragment`]
44/// implementation of this query dsl node via
45/// [`SqlDialect::ExistsSyntax`]. A customized implementation
46/// is expected to provide the same semantics as an ANSI SQL
47/// `EXIST (subselect)` expression.
48#[derive(Clone, Copy, QueryId, Debug)]
49#[non_exhaustive]
50pub struct Exists<T> {
51    /// The inner subselect
52    pub subselect: Subselect<T, Bool>,
53}
54
55impl<T> Expression for Exists<T>
56where
57    Subselect<T, Bool>: Expression,
58{
59    type SqlType = Bool;
60}
61
62impl<T, GB> ValidGrouping<GB> for Exists<T>
63where
64    Subselect<T, Bool>: ValidGrouping<GB>,
65{
66    type IsAggregate = <Subselect<T, Bool> as ValidGrouping<GB>>::IsAggregate;
67}
68
69impl<T, DB> QueryFragment<DB> for Exists<T>
70where
71    DB: Backend,
72    Self: QueryFragment<DB, DB::ExistsSyntax>,
73{
74    fn walk_ast<'b>(&'b self, pass: AstPass<'_, 'b, DB>) -> QueryResult<()> {
75        <Self as QueryFragment<DB, DB::ExistsSyntax>>::walk_ast(self, pass)
76    }
77}
78
79impl<T, DB> QueryFragment<DB, sql_dialect::exists_syntax::AnsiSqlExistsSyntax> for Exists<T>
80where
81    DB: Backend + SqlDialect<ExistsSyntax = sql_dialect::exists_syntax::AnsiSqlExistsSyntax>,
82    T: QueryFragment<DB>,
83{
84    fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, DB>) -> QueryResult<()> {
85        out.push_sql("EXISTS (");
86        self.subselect.walk_ast(out.reborrow())?;
87        out.push_sql(")");
88        Ok(())
89    }
90}
91
92impl<T, QS> SelectableExpression<QS> for Exists<T>
93where
94    Self: AppearsOnTable<QS>,
95    Subselect<T, Bool>: SelectableExpression<QS>,
96{
97}
98
99impl<T, QS> AppearsOnTable<QS> for Exists<T>
100where
101    Self: Expression,
102    Subselect<T, Bool>: AppearsOnTable<QS>,
103{
104}
diesel/expression/exists.rs

diesel/expression/
exists.rs
