Performing Queries

Data that is persisted is practically useless if queries cannot be performed against it. PerformQueries.java (available here) demonstrates several ways to query SimpleDB.

The performQueriesUsingSelect() method contains the following code:

SelectRequest selectRequest = new SelectRequest();
        selectRequest.setSelectExpression("select * from StateInformation where Nickname = 'Green Mountain State'");
SelectResponse selectResponse = client.select(selectRequest);

Notice the String that is passed to the setSelectExpression() method. It looks exactly like any SQL query you have seen. The query is sent using a SelectRequest object and the reply comes back in a SelectResponse object. The following code shows how you can retrieve a ResultSet from the SelectResponse object, retrieve items for the ResultSet, retrieve attributes from each item and names and values from each attribute:

if (selectResponse.isSetSelectResult()) {
            SelectResult selectResult = selectResponse.getSelectResult();
            if (selectResult.isSetItem()) {
                List<Item> itemList = selectResult.getItem();                
                for (Item item : itemList) {
                    if (item.isSetName()) {
                        System.out.println("Name: " + item.getName());
                    }
                    if (item.isSetAttribute()) {
                        List<Attribute> attributeList = item.getAttribute();
                        for (Attribute a : attributeList) {
                            if (a.isSetName()) {
                            System.out.println("Name: " + a.getName());
                            }
                            if (a.isSetValue()) {
                                System.out.println("Value: " + a.getValue());
                            }
                        }
                    }
                }
                System.out.println("done");
            }
        }

If you examine the performSimpleQuery() and performQueryWithAttributes() methods, you will see that the query string used by both is ['Abbreviation' = 'CT']. This rather terse syntax was used early in the life of SimpleDB and my understanding is that it is being phased out. When you run the program, you get the following output:

performQueryUsingSelect
Item Name: Vermont
Attribute Name: Abbreviation
Value: VT
Attribute Name: Nickname
Value: Green Mountain State
done

performSimpleQuery
item name = Connecticut
done

performQueryWithAttributes
Item Name: Connecticut
Attribute Name: Area
Value: 5544
Attribute Name: Abbreviation
Value: CT
Attribute Name: Nickname
Value: Constitution State
Attribute Name: Nickname
Value: Nutmeg State
done