From 4427868fc2a9eacf1910859a78124be4ef0c3230 Mon Sep 17 00:00:00 2001
From: emirmrkaljevic <93012621+emirmrkaljevic@users.noreply.github.com>
Date: Thu, 15 Oct 2026 10:54:25 +0200
Subject: [PATCH] fix(mysql): apply keywordCase to INTERVAL duration
 expressions

INTERVAL is listed as both a function and a keyword in MySQL-like
dialects. Without a following '(', it was demoted to IDENTIFIER, so
keywordCase left `interval 30 day` unchanged while MariaDB uppercased
it. Keep the non-call form as RESERVED_KEYWORD in the shared MariaDB
post-process path.
---
 src/languages/mariadb/likeMariaDb.ts |  9 +++++++++
 test/mysql.test.ts                   | 21 +++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/src/languages/mariadb/likeMariaDb.ts b/src/languages/mariadb/likeMariaDb.ts
index 63c7cba1f7..1c43361417 100644
--- a/src/languages/mariadb/likeMariaDb.ts
+++ b/src/languages/mariadb/likeMariaDb.ts
@@ -14,6 +14,15 @@ export function postProcess(tokens: Token[]) {
       // This is VALUES() function, not VALUES clause
       return { ...token, type: TokenType.RESERVED_FUNCTION_NAME };
     }
+    // INTERVAL is both a comparison function (INTERVAL(N, ...)) and a
+    // duration keyword (INTERVAL n DAY). Without '(', keep it as a keyword
+    // so keywordCase applies; otherwise disambiguateTokens would demote it
+    // to IDENTIFIER (see #985).
+    if (token.type === TokenType.RESERVED_FUNCTION_NAME && token.text === 'INTERVAL') {
+      if (nextToken.text !== '(') {
+        return { ...token, type: TokenType.RESERVED_KEYWORD };
+      }
+    }
     return token;
   });
 }
diff --git a/test/mysql.test.ts b/test/mysql.test.ts
index e6bcd37af9..0b654d4d6e 100644
--- a/test/mysql.test.ts
+++ b/test/mysql.test.ts
@@ -98,6 +98,27 @@ describe('MySqlFormatter', () => {
     `);
   });
 
+  it('applies keywordCase to INTERVAL duration expressions (#985)', () => {
+    expect(
+      format(`select now() - interval 30 day; select date_sub(now(), interval 30 day);`, {
+        keywordCase: 'upper',
+      })
+    ).toBe(dedent`
+      SELECT
+        now() - INTERVAL 30 day;
+
+      SELECT
+        date_sub(now(), INTERVAL 30 day);
+    `);
+  });
+
+  it('still treats INTERVAL(...) as a function name', () => {
+    expect(format(`select interval(55, 0, 50, 100);`, { functionCase: 'upper' })).toBe(dedent`
+      select
+        INTERVAL(55, 0, 50, 100);
+    `);
+  });
+
   it('formats ALTER TABLE ... ALTER COLUMN', () => {
     expect(
       format(