fix: allow UNIQUE INDEX in CREATE TABLE

Accept an optional UNIQUE prefix on the INDEX branch of CreateTableConstraint, so MySQL-style CREATE TABLE ... UNIQUE INDEX name (cols) ... parses and round-trips. UNIQUE INDEX is disambiguated from UNIQUE KEY and the bare UNIQUE constraint by the LOOKAHEAD.

Fixes #1893

Signed-off-by: 付典 <fudianchn@gmail.com>

Author: fudianchn

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -10456,14 +10456,15 @@ Index CreateTableConstraint():
 }
 {
     (
-        LOOKAHEAD(3) (
-            { idxSpec.clear(); }
+        LOOKAHEAD(4) (
+            { idxSpec.clear(); tk3=null; }
+            [ tk3=<K_UNIQUE> ]
             tk=<K_INDEX>
             sk3=RelObjectName()
             colNames = IndexColumnsWithParamsList()
             ( parameter=CreateParameter() { idxSpec.addAll(parameter); } )*
             {
-                index = new Index().withType(tk.image).withName(sk3).withColumns(colNames).withIndexSpec(new ArrayList<String>(idxSpec));
+                index = new Index().withType((tk3!=null ? tk3.image + " " : "") + tk.image).withName(sk3).withColumns(colNames).withIndexSpec(new ArrayList<String>(idxSpec));
             }
         )
         |

src/test/java/net/sf/jsqlparser/statement/create/CreateTableTest.java

@@ -1206,4 +1206,15 @@ void testCreateTableQuotedColumnNamedVisible() throws JSQLParserException {
         assertSqlCanBeParsedAndDeparsed(
                 "CREATE TABLE t1 (`visible` int, `invisible` varchar (10))", true);
     }
+
+    @Test
+    void testUniqueIndexIssue1893() throws JSQLParserException {
+        // MySQL `UNIQUE INDEX name (cols) ...` was rejected (only INDEX / [UNIQUE] KEY were
+        // accepted).
+        assertSqlCanBeParsedAndDeparsed(
+                "CREATE TABLE t (a int, b int, UNIQUE INDEX idx (a, b) USING BTREE COMMENT 'unique index')",
+                true);
+        // A plain INDEX must still parse unchanged.
+        assertSqlCanBeParsedAndDeparsed("CREATE TABLE t (a int, INDEX idx (a))", true);
+    }
 }