improved sqlserver support, fix #4068

src/main/java/com/alibaba/druid/sql/parser/SQLStatementParser.java

@@ -1784,6 +1784,10 @@ public class SQLStatementParser extends SQLParser {
                         SQLPrimaryKey primaryKey = this.exprParser.parsePrimaryKey();
                         SQLAlterTableAddConstraint item = new SQLAlterTableAddConstraint(primaryKey);
                         stmt.addItem(item);
+                    } else if (lexer.token == UNIQUE) {
+                        SQLUnique unique = this.exprParser.parseUnique();
+                        SQLAlterTableAddConstraint item = new SQLAlterTableAddConstraint(unique);
+                        stmt.addItem(item);
                     } else if (lexer.token == Token.IDENTIFIER) {
                         SQLAlterTableAddColumn item = parseAlterTableAddColumn();
                         stmt.addItem(item);

src/test/java/com/alibaba/druid/bvt/sql/sqlserver/SqlServerResourceTest.java

@@ -38,6 +38,10 @@ public class SqlServerResourceTest extends TestCase {
         exec_test("bvt/parser/sqlserver-0.txt");
     }
 
+    public void test_1() throws Exception {
+        exec_test("bvt/parser/sqlserver-1.txt");
+    }
+
     public void exec_test(String resource) throws Exception {
 //        System.out.println(resource);
         InputStream is = null;

src/test/resources/bvt/parser/sqlserver-1.txt

@@ -0,0 +1,4 @@
+ALTER TABLE [dbo].[sys_config] ADD UNIQUE ([param_key] ASC)
+---------------------------
+ALTER TABLE [dbo].[sys_config]
+	ADD UNIQUE ([param_key] ASC)