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In that Picture i have colored one part. i have attribute called "deviceModel". It contains more than one value.. i want to take using query from my domain which ItemName() contains deviceModel attribute values more than one value.
Thanks,
Senthil Raja
There is no direct approach to get what you are asking.. You need to manipulate by writing your own piece of code. By running SELECT query you will get the item Attribute-value pair. So here you need to traverse each each itemName() and count values of your desire attribute.
I think what you are refering to is called MultiValued Attributes. When you put a value in the attribute - if you don't replace the existing attribute value the values will multiply, giving you an array of items connected to the value of that attribute name.
How you create them will depend on the sdk/language you are using for your REST calls, however look for the Replace=true/false when you set the attribute's value.
Here is the documentation page on retrieving them: http://docs.amazonwebservices.com/AmazonSimpleDB/latest/DeveloperGuide/ (look under Using Amazon SimpleDB -> Using Select to Create Amazon SimpleDB Queries -> Queries on Attributes with Multiple Values)
Related
Is it possible to apply a filter based on values inside a dynamodb database?
Let's say the database contains an object info within a table:
info: {
toDo: x,
done: y,
}
Using the ExpressionAttributeValues, is it possible to check whether the info.toDo = info.done and apply a filter on it without knowing the current values of info.toDo and info.done ?
At the moment I tried using ExpressionAttributeNames so it contains:
'#toDo': info.toDo, '#done': info.done'
and the filter FilterExpression is
#toDo = #done
but I'm retrieving no items doing a query with this filter.
Thanks a lot!
DynamoDB is not designed to perform arbitrary queries as you might be used to in a relational database. It is designed for fast lookups based on keys.
Therefore, if you can add an index allowing you to access the records you look for, you can use it for this new access pattern. For example, if you add an index that uses info.toDo as the partition key and info.done as the sort key. You can then use the index to scan the records with the conditional expression of PK=x and SK=x, assuming that the list of possible values is limited and known.
I'm trying to query DynamoDB and get a result similar to select distinct(address) from ... in SQL.
I know DynamoDB is a document-oriented DB and maybe I need to change the data structure.
I'm trying to avoid getting all the data first and filtering later.
My data looks like this:
Attribute
Datatype
ID
String
Var1
Map
VarN
Map
Address
String
So I want to get the distinct addresses in the entire table.
How it's the best way to do it?
Unfortunately, no. You'll need to Scan the entire table (you can use the ProjectionExpression or AttributesToGet options to ask just for the "Address" attribute, but anyway you'll pay for scanning the entire contents of the table).
If you need to do this scan often, you can add a secondary-index which projects only the keys and the "Address" attribute, to make it cheaper to scan. But unfortunately, using a GSI whose partition key is the "Address" does not give you an ability to eliminate duplicates: Each partition will still contain a list of duplicate items, and unfortunately there is no way to just listing the different partition keys in an index - Scaning the index will give you the same partition key multiple times, as many items there are in this partition.
From the dynamoDBMapper docs
Querypage:
Queries a table or secondary index and returns a single page of
matching results. As with the query method, you must specify a
partition key value and a query filter that is applied on the sort key
attribute. However, queryPage will only return the first "page" of
data - that is, the amount of data that will fit within 1 MB.
I've applied query filter on a attribute which is not a sort key. But everything works. How is this possible?
I think that particular statement is misleading in the documentation. You can apply query-filter/filter-expression on any non-key attributes, but you cannot apply filter expression on key attributes :
http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/QueryAndScan.html#FilteringResults
You can send your feedback ("Feedback" button at the bottom of the doc page) to Amazon. I think they're pretty responsive for the feedbacks they receive.
I've been going through AWS DynamoDB docs and cannot figure out what's the difference between batchGetItem() and Query().
My use case: I have a table which has Id as primary hash key, and attribute values are Name and Marks.
I would like to perform batch query which returns list of names and marks by providing list of Id's which are primary keys.
Should I use batchGetItem() or Query()?
BatchGetItem: Allows to you parallelize "GetItem" requests for languages that don't support parallelism (i.e. javascript). This includes retrieving items from different tables (doesn't support indexes though).
Query: Allows you to page through tables with a Hash-Range schema (where you'll have multiple results associated with a Hash key) and allows you to retrieve items from the indexes on your table. Note you can also add an additional condition on range key in your KeyConditions and add conditions on any non primary key attribute in your QueryFilter.
It seems like that your use case calls for a BatchGetItem request, as you are trying to retrieve items from your base table by way of a Hash key.
Hope that helps!
I need to execute an in query on the key attribute. Since, query doesn't provide in condition, I am planning to use scan. Will scan on key attribute scan the entire table?
Will SCAN on key attribute scan the entire table?
Yes, see Query and Scan in Amazon DynamoDB:
Scan
A scan operation scans the entire table. You can specify filters to
apply to the results to refine the values returned to you, after the
complete scan. Amazon DynamoDB puts a 1MB limit on the scan (the limit
applies before the results are filtered). A scan can result in no
table data meeting the filter criteria.
Specifically, there is no difference between key and non key attributes as far as the Scan API is concerned, i.e. you simply provide the desired attributes by name, regardless of them being used as an attribute constituting the Primary Key as well or not:
AttributesToGet
Array of Attribute names. If attribute names are not specified then
all attributes will be returned. If some attributes are not found,
they will not appear in the result.
wouldn't batchGetItem work for you?