feat(functions/subquery): add exists function

Add postgres EXISTS(...subquery...) function.
This is required by typeorm exists method.
See: https://github.com/typeorm/typeorm/blob/e7649d2746f907ff36b1efb600402dedd5f5a499/src/query-builder/QueryBuilder.ts#L1273

src/functions/index.ts

@@ -3,6 +3,7 @@ import { dateFunctions } from './date';
 import { systemFunctions } from './system';
 import { sequenceFunctions } from './sequence-fns';
 import { numberFunctions } from './numbers';
+import { subqueryFunctions } from './subquery';
 
 
 export const allFunctions = [
@@ -11,4 +12,5 @@ export const allFunctions = [
     , ... systemFunctions
     , ... sequenceFunctions
     , ... numberFunctions
-]
+    , ... subqueryFunctions
+]

src/functions/subquery.ts

@@ -0,0 +1,14 @@
+import { FunctionDefinition } from '../interfaces';
+import { DataType } from '../interfaces-private';
+
+export const subqueryFunctions: FunctionDefinition[] = [
+    {
+        name: 'exists',
+        args: [DataType.integer],
+        argsVariadic: DataType.integer,
+        returns: DataType.bool,
+        allowNullArguments: true,
+        impure: true,
+        implementation: (...items: number[]) => items?.some?.(Boolean) ?? false,
+    },
+];

src/tests/subqueries.spec.ts

@@ -10,11 +10,13 @@ describe('Subqueries', () => {
         let db: _IDb;
         let many: (str: string) => any[];
         let none: (str: string) => void;
+        let one: (str: string) => any;
 
         beforeEach(() => {
                 db = newDb() as _IDb;
                 many = db.public.many.bind(db.public);
                 none = db.public.none.bind(db.public);
+                one = db.public.one.bind(db.public);
         });
 
         function setupBooks() {
@@ -90,4 +92,21 @@ describe('Subqueries', () => {
 
         //     expect(cnt).toBe(1, 'Was expecting subquery to be simplified');
         // })
+
+        it('can check if a subquery exists', () => {
+                setupBooks();
+
+                // returns single row, regardless of matches
+                expect(many(`SELECT EXISTS(SELECT 1 FROM books WHERE created_at < 5) AS x;`))
+                    .toHaveLength(1);
+
+                expect(one(`SELECT EXISTS(SELECT 1 FROM books WHERE created_at > 5) AS x;`))
+                    .toEqual({ x: false });
+
+                expect(one(`SELECT EXISTS(SELECT 1 FROM books WHERE name = 'one') AS x;`))
+                    .toEqual({ x: true });
+
+                expect(one(`SELECT EXISTS(SELECT 1 FROM books WHERE name = 'one') AS x;`))
+                    .toEqual({ x: true });
+        });
 });