diesel/expression/
exists.rs

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")))).get_result(connection);
28/// let jim_exists = select(exists(users.filter(name.eq("Jim")))).get_result(connection);
29/// assert_eq!(Ok(true), sean_exists);
30/// assert_eq!(Ok(false), jim_exists);
31/// # }
32/// ```
33pub fn exists<T>(query: T) -> helper_types::exists<T> {
34    Exists {
35        subselect: Subselect::new(query),
36    }
37}
38
39/// The query dsl node that represents a SQL `EXISTS (subselect)` expression.
40///
41/// Third party backend can customize the [`QueryFragment`]
42/// implementation of this query dsl node via
43/// [`SqlDialect::ExistsSyntax`]. A customized implementation
44/// is expected to provide the same semantics as an ANSI SQL
45/// `EXIST (subselect)` expression.
46#[derive(Clone, Copy, QueryId, Debug)]
47#[non_exhaustive]
48pub struct Exists<T> {
49    /// The inner subselect
50    pub subselect: Subselect<T, Bool>,
51}
52
53impl<T> Expression for Exists<T>
54where
55    Subselect<T, Bool>: Expression,
56{
57    type SqlType = Bool;
58}
59
60impl<T, GB> ValidGrouping<GB> for Exists<T>
61where
62    Subselect<T, Bool>: ValidGrouping<GB>,
63{
64    type IsAggregate = <Subselect<T, Bool> as ValidGrouping<GB>>::IsAggregate;
65}
66
67impl<T, DB> QueryFragment<DB> for Exists<T>
68where
69    DB: Backend,
70    Self: QueryFragment<DB, DB::ExistsSyntax>,
71{
72    fn walk_ast<'b>(&'b self, pass: AstPass<'_, 'b, DB>) -> QueryResult<()> {
73        <Self as QueryFragment<DB, DB::ExistsSyntax>>::walk_ast(self, pass)
74    }
75}
76
77impl<T, DB> QueryFragment<DB, sql_dialect::exists_syntax::AnsiSqlExistsSyntax> for Exists<T>
78where
79    DB: Backend + SqlDialect<ExistsSyntax = sql_dialect::exists_syntax::AnsiSqlExistsSyntax>,
80    T: QueryFragment<DB>,
81{
82    fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, DB>) -> QueryResult<()> {
83        out.push_sql("EXISTS (");
84        self.subselect.walk_ast(out.reborrow())?;
85        out.push_sql(")");
86        Ok(())
87    }
88}
89
90impl<T, QS> SelectableExpression<QS> for Exists<T>
91where
92    Self: AppearsOnTable<QS>,
93    Subselect<T, Bool>: SelectableExpression<QS>,
94{
95}
96
97impl<T, QS> AppearsOnTable<QS> for Exists<T>
98where
99    Self: Expression,
100    Subselect<T, Bool>: AppearsOnTable<QS>,
101{
102}
diesel/expression/exists.rs

diesel/expression/
exists.rs
