Add support for unnamed CONSTRAINT CHECK syntax

Fixes parsing of `CONSTRAINT CHECK (expr)` without a constraint name,
which is valid in MySQL and nom-sql.

Previously, the parser would consume the CHECK keyword as the constraint
name, then fail when it encountered the opening parenthesis.

The fix checks if the next token after CONSTRAINT is a constraint keyword
(PRIMARY, UNIQUE, FOREIGN, or CHECK). If so, it treats the constraint as
unnamed rather than trying to parse the keyword as an identifier.

Examples that now parse correctly:
- CREATE TABLE t (x INT, CONSTRAINT CHECK (x > 1))
- ALTER TABLE t ADD CONSTRAINT PRIMARY KEY (id)
- ALTER TABLE t ADD CONSTRAINT UNIQUE (email)

These canonicalize to their forms without the CONSTRAINT keyword when
no name is provided.

Author: claude

src/parser/mod.rs

@@ -8825,7 +8825,15 @@ impl<'a> Parser<'a> {
         &mut self,
     ) -> Result<Option<TableConstraint>, ParserError> {
         let name = if self.parse_keyword(Keyword::CONSTRAINT) {
-            Some(self.parse_identifier()?)
+            if self.peek_keyword(Keyword::PRIMARY)
+                || self.peek_keyword(Keyword::UNIQUE)
+                || self.peek_keyword(Keyword::FOREIGN)
+                || self.peek_keyword(Keyword::CHECK)
+            {
+                None
+            } else {
+                Some(self.parse_identifier()?)
+            }
         } else {
             None
         };

tests/sqlparser_common.rs

@@ -5088,6 +5088,33 @@ fn parse_alter_table_constraints() {
     }
 }
 
+#[test]
+fn parse_alter_table_constraint_check_without_name() {
+    // CONSTRAINT CHECK without a name should parse and canonicalize to just CHECK
+    one_statement_parses_to(
+        "ALTER TABLE tab ADD CONSTRAINT CHECK (x > 1)",
+        "ALTER TABLE tab ADD CHECK (x > 1)",
+    );
+    one_statement_parses_to(
+        "CREATE TABLE t (x INT, CONSTRAINT CHECK (x > 1))",
+        "CREATE TABLE t (x INT, CHECK (x > 1))",
+    );
+
+    // Other constraint types should also work without names
+    one_statement_parses_to(
+        "ALTER TABLE tab ADD CONSTRAINT PRIMARY KEY (id)",
+        "ALTER TABLE tab ADD PRIMARY KEY (id)",
+    );
+    one_statement_parses_to(
+        "ALTER TABLE tab ADD CONSTRAINT UNIQUE (email)",
+        "ALTER TABLE tab ADD UNIQUE (email)",
+    );
+    one_statement_parses_to(
+        "ALTER TABLE tab ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES users(id)",
+        "ALTER TABLE tab ADD FOREIGN KEY (user_id) REFERENCES users(id)",
+    );
+}
+
 #[test]
 fn parse_alter_table_drop_column() {
     check_one("DROP COLUMN IF EXISTS is_active");