fixes #929

Author: wumpz

README.md

@@ -68,6 +68,7 @@ Also I would like to know about needed examples or documentation stuff.
 
 * allow **ON** as a value in a set statement (`SET myvalue = ON`)
 * support for **ALTER TABLE ONLY mytable ... **
+* allow foreign key definition in alter statements without referenced columns specification
 
 ## Extensions of JSqlParser releases
 

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

@@ -4224,8 +4224,8 @@ AlterExpression AlterExpression():
             [<K_USING> sk4=RelObjectName() { alterExp.addParameters("USING", sk4); }])
         |
 //following two choices regarding foreign keys should be merged
-        ( <K_FOREIGN> <K_KEY> columnNames=ColumnsNamesList() { alterExp.setFkColumns(columnNames); }
-             <K_REFERENCES> tk=<S_IDENTIFIER> columnNames=ColumnsNamesList()
+        ( <K_FOREIGN> <K_KEY> columnNames=ColumnsNamesList() { alterExp.setFkColumns(columnNames); columnNames = null; }
+             <K_REFERENCES> tk=<S_IDENTIFIER> [ columnNames=ColumnsNamesList() ]
                     { alterExp.setFkSourceTable(tk.image); alterExp.setFkSourceColumns(columnNames); }
             [<K_ON> <K_DELETE>
                             (<K_CASCADE> { alterExp.setOnDeleteCascade(true); }
@@ -4243,8 +4243,9 @@ AlterExpression AlterExpression():
                     fkIndex.setName(sk3);
                     fkIndex.setType(tk.image + " " + tk2.image);
                     fkIndex.setColumnsNames(columnNames);
+					columnNames = null;
                 }
-                <K_REFERENCES> fkTable=Table() columnNames=ColumnsNamesList()
+                <K_REFERENCES> fkTable=Table() [ columnNames=ColumnsNamesList() ]
                 {
                     fkIndex.setTable(fkTable);
                     fkIndex.setReferencedColumnNames(columnNames);

src/test/java/net/sf/jsqlparser/statement/alter/AlterTest.java

@@ -393,4 +393,9 @@ public void testIssue633_2() throws JSQLParserException {
     public void testAlterOnlyIssue928() throws JSQLParserException {
         assertSqlCanBeParsedAndDeparsed("ALTER TABLE ONLY categories ADD CONSTRAINT pk_categories PRIMARY KEY (category_id)");
     }
+    
+    @Test
+    public void testAlterConstraintWithoutFKSourceColumnsIssue929() throws JSQLParserException {
+        assertSqlCanBeParsedAndDeparsed("ALTER TABLE orders ADD CONSTRAINT fk_orders_customers FOREIGN KEY (customer_id) REFERENCES customers");
+    }
 }