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Condition Expressions
To manipulate data in an Amazon DynamoDB table, you use the PutItem,
UpdateItem, and DeleteItem operations. (You can also use
BatchWriteItem to perform multiple PutItem or
DeleteItem operations in a single call.)
For these data manipulation operations, you can specify a condition expression to determine which items should be modified. If the condition expression evaluates to true, the operation succeeds; otherwise, the operation fails.
The following are some AWS Command Line Interface (AWS CLI) examples of using condition
expressions. These
examples are based on the ProductCatalog table, which was introduced in Specifying Item Attributes. The partition
key for this table is Id; there is no sort key. The following
PutItem operation creates a sample ProductCatalog item that
the examples refer to.
aws dynamodb put-item \ --table-name ProductCatalog \ --item file://item.json
The arguments for --item are stored in the item.json file.
(For simplicity, only a few item attributes are used.)
{ "Id": {"N": "456" }, "ProductCategory": {"S": "Sporting Goods" }, "Price": {"N": "650" } }
Topics
Preventing Overwrites of an Existing Item
The PutItem operation overwrites an item with the same key (if it exists). If
you want to avoid this, use a condition expression. This allows the write to proceed
only if the item in question does not already have the same key.
aws dynamodb put-item \ --table-name ProductCatalog \ --item file://item.json \ --condition-expression "attribute_not_exists(Id)"
If the condition expression evaluates to false, DynamoDB returns the following error
message:
The conditional request failed.
Note
For more information about attribute_not_exists and other functions,
see Comparison Operator and Function Reference.
Checking for Attributes in an Item
You can check for the existence (or nonexistence) of any attribute. If the condition expression evaluates to true, the operation succeeds; otherwise, it fails.
The following example uses attribute_not_exists to delete a product only if it
does not have a Price attribute.
aws dynamodb delete-item \ --table-name ProductCatalog \ --key '{"Id": {"N": "456"}}' \ --condition-expression "attribute_not_exists(Price)"
DynamoDB also provides an attribute_exists function. The following example deletes
a product only if it has received poor reviews.
aws dynamodb delete-item \ --table-name ProductCatalog \ --key '{"Id": {"N": "456"}}' \ --condition-expression "attribute_exists(ProductReviews.OneStar)"
Note
For more information about attribute_not_exists, attribute_exists
and other functions, see Comparison Operator and Function Reference.
Conditional Deletes
To perform a conditional delete, you use a DeleteItem operation with a
condition expression. The condition expression must evaluate to true in order for
the
operation to succeed; otherwise, the operation fails.
Consider the item from Condition Expressions.
{ "Id": { "N": "456" }, "Price": { "N": "650" }, "ProductCategory": { "S": "Sporting Goods" } }
Suppose that you wanted to delete the item, but only under the following conditions:
-
The
ProductCategoryis either "Sporting Goods" or "Gardening Supplies." -
The
Priceis between 500 and 600.
The following example tries to delete the item.
aws dynamodb delete-item \ --table-name ProductCatalog \ --key '{"Id":{"N":"456"}}' \ --condition-expression "(ProductCategory IN (:cat1, :cat2)) and (Price between :lo and :hi)" \ --expression-attribute-values file://values.json
The arguments for --expression-attribute-values are stored in the
values.json file.
{ ":cat1": {"S": "Sporting Goods"}, ":cat2": {"S": "Gardening Supplies"}, ":lo": {"N": "500"}, ":hi": {"N": "600"} }
Note
In the condition expression, the : (colon character) indicates an
expression attribute value—a placeholder for an
actual value. For more information, see Expression Attribute Values.
For more information about IN, AND, and other keywords, see Comparison Operator and Function Reference.
In this example, the ProductCategory comparison evaluates to true, but the
Price comparison evaluates to false. This causes the condition
expression to evaluate to false and the DeleteItem operation to
fail.
Conditional Updates
To perform a conditional update, you use an UpdateItem operation with a
condition expression. The condition expression must evaluate to true in order for
the
operation to succeed; otherwise, the operation fails.
Note
UpdateItem also supports update expressions,
where you specify the modifications you want to make to an
item. For more information, see Update Expressions.
Suppose that you started with the item shown in Condition Expressions.
{ "Id": { "N": "456"}, "Price": {"N": "650"}, "ProductCategory": {"S": "Sporting Goods"} }
The following example performs an UpdateItem operation. It tries to reduce the
Price of a product by 75—but the condition expression prevents
the update if the current Price is below 500.
aws dynamodb update-item \ --table-name ProductCatalog \ --key '{"Id": {"N": "456"}}' \ --update-expression "SET Price = Price - :discount" \ --condition-expression "Price > :limit" \ --expression-attribute-values file://values.json
The arguments for --item are stored in the values.json
file.
{ ":discount": { "N": "75"}, ":limit": {"N": "500"} }
If the starting Price is 650, the UpdateItem operation reduces the
Price to 575. If you run the UpdateItem operation again, the
Price is reduced to 500. If you run it a third time, the condition
expression evaluates to false, and the update fails.
Note
In the condition expression, the : (colon character) indicates an
expression attribute value—a placeholder for an
actual value. For more information, see Expression Attribute Values.
For more information about ">" and other operators, see Comparison Operator and Function Reference.
