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Showing content from https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Attributes.html below:

Referring to item attributes when using expressions in DynamoDB

This section describes how to refer to item attributes in an expression in Amazon DynamoDB. You can work with any attribute, even if it is deeply nested within multiple lists and maps.

The examples on this page use the following sample item in the ProductCatalog table. (This table is described in Example tables and data for use in DynamoDB.)

{
    "Id": 123,
    "Title": "Bicycle 123",
    "Description": "123 description",
    "BicycleType": "Hybrid",
    "Brand": "Brand-Company C",
    "Price": 500,
    "Color": ["Red", "Black"],
    "ProductCategory": "Bicycle",
    "InStock": true,
    "QuantityOnHand": null,
    "RelatedItems": [
        341,
        472,
        649
    ],
    "Pictures": {
        "FrontView": "http://example.com/products/123_front.jpg",
        "RearView": "http://example.com/products/123_rear.jpg",
        "SideView": "http://example.com/products/123_left_side.jpg"
    },
    "ProductReviews": {
	    "FiveStar": [
	    		"Excellent! Can't recommend it highly enough! Buy it!",
	    		"Do yourself a favor and buy this."
	    ],
	    "OneStar": [
	    		"Terrible product! Do not buy this."
	    ]
    },
    "Comment": "This product sells out quickly during the summer",
    "Safety.Warning": "Always wear a helmet"
 }

Note the following:

• The partition key value (Id) is 123. There is no sort key.

• Most of the attributes have scalar data types, such as String, Number, Boolean, and Null.

• One attribute (Color) is a String Set.

• The following attributes are document data types:

  • A list of RelatedItems. Each element is an Id for a related product.

  • A map of Pictures. Each element is a short description of a picture, along with a URL for the corresponding image file.

  • A map of ProductReviews. Each element represents a rating and a list of reviews corresponding to that rating. Initially, this map is populated with five-star and one-star reviews.

An attribute is said to be top level if it is not embedded within another attribute. For the ProductCatalog item, the top-level attributes are as follows:

• Id

• Title

• Description

• BicycleType

• Brand

• Price

• Color

• ProductCategory

• InStock

• QuantityOnHand

• RelatedItems

• Pictures

• ProductReviews

• Comment

• Safety.Warning

All of these top-level attributes are scalars, except for Color (list), RelatedItems (list), Pictures (map), and ProductReviews (map).

An attribute is said to be nested if it is embedded within another attribute. To access a nested attribute, you use dereference operators:

• [n] — for list elements

• . (dot) — for map elements

The dereference operator for a list element is [N], where n is the element number. List elements are zero-based, so [0] represents the first element in the list, [1] represents the second, and so on. Here are some examples:

• MyList[0]

• AnotherList[12]

• ThisList[5][11]

The element ThisList[5] is itself a nested list. Therefore, ThisList[5][11] refers to the 12th element in that list.

The number within the square brackets must be a non-negative integer. Therefore, the following expressions are not valid:

The dereference operator for a map element is . (a dot). Use a dot as a separator between elements in a map:

• MyMap.nestedField

• MyMap.nestedField.deeplyNestedField

In an expression, you use a document path to tell DynamoDB where to find an attribute. For a top-level attribute, the document path is simply the attribute name. For a nested attribute, you construct the document path using dereference operators.

The following are some examples of document paths. (Refer to the item shown in Referring to item attributes when using expressions in DynamoDB.)

• A top-level scalar attribute.

  Description

• A top-level list attribute. (This returns the entire list, not just some of the elements.)

  RelatedItems

• The third element from the RelatedItems list. (Remember that list elements are zero-based.)

  RelatedItems[2]

• The front-view picture of the product.

  Pictures.FrontView

• All of the five-star reviews.

  ProductReviews.FiveStar

• The first of the five-star reviews.

  ProductReviews.FiveStar[0]

The maximum depth for a document path is 32. Therefore, the number of dereferences operators in a path cannot exceed this limit.

You can use any attribute name in a document path as long as they meet these requirements:

• The first character is a-z or A-Z and or 0-9

• The second character (if present) is a-z, A-Z

If an attribute name does not meet this requirement, you must define an expression attribute name as a placeholder.

For more information, see Expression attribute names (aliases) in DynamoDB.

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