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Expression attribute names (aliases) in DynamoDB

An expression attribute name is an alias (or placeholder) that you use in an Amazon DynamoDB expression as an alternative to an actual attribute name. An expression attribute name must begin with a pound sign (#) and be followed by one or more alphanumeric characters. The underscore (_) character is also allowed.

This section describes several situations in which you must use expression attribute names.

The examples in this section use the AWS Command Line Interface (AWS CLI).

Sometimes you might need to write an expression containing an attribute name that conflicts with a DynamoDB reserved word. (For a complete list of reserved words, see Reserved words in DynamoDB.)

For example, the following AWS CLI example would fail because COMMENT is a reserved word.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "Comment"

To work around this, you can replace Comment with an expression attribute name such as #c. The # (pound sign) is required and indicates that this is a placeholder for an attribute name. The AWS CLI example would now look like the following.

aws dynamodb get-item \
     --table-name ProductCatalog \
     --key '{"Id":{"N":"123"}}' \
     --projection-expression "#c" \
     --expression-attribute-names '{"#c":"Comment"}'

If an attribute name begins with a number, contains a space or contains a reserved word, you must use an expression attribute name to replace that attribute's name in the expression.

In an expression, a dot (".") is interpreted as a separator character in a document path. However, DynamoDB also allows you to use a dot character and other special characters, such as a hyphen ("-") as part of an attribute name. This can be ambiguous in some cases. To illustrate, suppose that you wanted to retrieve the Safety.Warning attribute from a ProductCatalog item (see Referring to item attributes when using expressions in DynamoDB).

Suppose that you wanted to access Safety.Warning using a projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "Safety.Warning"

DynamoDB would return an empty result, rather than the expected string ("Always wear a helmet"). This is because DynamoDB interprets a dot in an expression as a document path separator. In this case, you must define an expression attribute name (such as #sw) as a substitute for Safety.Warning. You could then use the following projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#sw" \
    --expression-attribute-names '{"#sw":"Safety.Warning"}'

DynamoDB would then return the correct result.

If an attribute name contains a dot (".") or a hyphen ("-"), you must use an expression attribute name to replace that attribute's name in the expression.

Suppose that you wanted to access the nested attribute ProductReviews.OneStar. In an expression attribute name, DynamoDB treats the dot (".") as a character within an attribute's name. To reference the nested attribute, define an expression attribute name for each element in the document path:

• #pr — ProductReviews

• #1star — OneStar

You could then use #pr.#1star for the projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#pr.#1star"  \
    --expression-attribute-names '{"#pr":"ProductReviews", "#1star":"OneStar"}'

DynamoDB would then return the correct result.

Expression attribute names are helpful when you need to refer to the same attribute name repeatedly. For example, consider the following expression for retrieving some of the reviews from a ProductCatalog item.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "ProductReviews.FiveStar, ProductReviews.ThreeStar, ProductReviews.OneStar"

To make this more concise, you can replace ProductReviews with an expression attribute name such as #pr. The revised expression would now look like the following.

• #pr.FiveStar, #pr.ThreeStar, #pr.OneStar

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#pr.FiveStar, #pr.ThreeStar, #pr.OneStar" \
    --expression-attribute-names '{"#pr":"ProductReviews"}'

If you define an expression attribute name, you must use it consistently throughout the entire expression. Also, you cannot omit the # symbol.

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