improved sql parser, support pg udpate_from_clause. issue #1430

src/main/java/com/alibaba/druid/sql/ast/statement/SQLUpdateStatement.java

@@ -27,6 +27,7 @@ public class SQLUpdateStatement extends SQLStatementImpl {
 
     protected final List<SQLUpdateSetItem> items = new ArrayList<SQLUpdateSetItem>();
     protected SQLExpr                      where;
+    protected SQLTableSource               from;
 
     protected SQLTableSource               tableSource;
 
@@ -81,6 +82,17 @@ public class SQLUpdateStatement extends SQLStatementImpl {
         item.setParent(this);
     }
 
+    public SQLTableSource getFrom() {
+        return from;
+    }
+
+    public void setFrom(SQLTableSource from) {
+        if (from != null) {
+            from.setParent(this);
+        }
+        this.from = from;
+    }
+
     @Override
     public void output(StringBuffer buf) {
         buf.append("UPDATE ");