Fix capitalization of IS NOT NULL

Fixes #872

Author: nene

src/languages/duckdb/duckdb.functions.ts

@@ -659,7 +659,7 @@ export const functions: string[] = [
   // Keywords that also need to be listed as functions
   'CAST',
   'COALESCE',
-  'NULL',
+  // 'NULL', we really prefer treating it as keyword
   'RANK',
   'ROW_NUMBER',
 ];

src/languages/duckdb/duckdb.keywords.ts

@@ -43,6 +43,7 @@ export const keywords: string[] = [
   'INITIALLY',
   'INTERSECT',
   'INTO',
+  'IS', // additional
   'LATERAL',
   'LEADING',
   'LIMIT',

test/duckdb.test.ts

@@ -202,4 +202,16 @@ describe('DuckDBFormatter', () => {
       })
     ).toBe('CREATE TABLE foo (bar JSON, baz JSON);');
   });
+
+  // Issue #872
+  it('capitalizes IS NOT NULL', () => {
+    expect(
+      format(`SELECT 1 is not null;`, {
+        keywordCase: 'upper',
+      })
+    ).toBe(dedent`
+      SELECT
+        1 IS NOT NULL;
+    `);
+  });
 });