replace field string list with SelectableExpression list in SelectExpression

src/main/java/jef/expressions/selectable/DatabaseFunctionExpression.java

@@ -0,0 +1,25 @@
+package jef.expressions.selectable;
+
+import jef.expressions.Expression;
+
+public class DatabaseFunctionExpression implements Expression, SelectableExpression {
+    private final String function;
+    public DatabaseFunctionExpression(String function) {
+        this.function = function;
+    }
+
+    @Override
+    public Type getType() {
+        return Type.CONSTANT;
+    }
+
+    @Override
+    public Priority getPriority() {
+        return Priority.UNDEFINED;
+    }
+
+    @Override
+    public String toString() {
+        return function;
+    }
+}