Specifies how to perform the update. Valid values are PUT (default), DELETE, and ADD. The behavior depends on whether the specified primary key already exists in the table.
If an item with the specified Key is found in the table: PUT - Adds the specified attribute to the item. If the attribute already
exists, it is replaced by the new value.
DELETE - If no value is specified, the attribute and its value are removed
from the item. The data type of the specified value must match the existing value's data type.
If a set of values is specified, then those values are subtracted from the old set. For example, if the attribute value was the set [a,b,c] and the DELETE action specified [a,c], then the final attribute value would be [b]. Specifying an empty set is an error.
ADD - If the attribute does not already exist, then the attribute and
its values are added to the item. If the attribute does exist, then the behavior of ADD depends on the data type of the attribute:
If the existing attribute is a number, and if Value is also a number,
then the Value is mathematically added to the existing attribute. If Value is a negative number, then it is subtracted from the existing attribute.
If you use ADD to increment or decrement a number value for an item that
doesn't exist before the update, DynamoDB uses 0 as the initial value.
In addition, if you use ADD to update an existing item, and intend to increment or decrement an attribute value which does not yet exist, DynamoDB uses 0 as the initial value. For example, suppose that the item you want to update does not yet have an attribute named itemcount, but you decide to ADD the number 3 to this attribute anyway, even though it currently does not exist. DynamoDB will create the itemcount attribute, set its initial value to 0, and finally add 3 to it. The result will be a new itemcount attribute in the item, with a value of 3.
If the existing data type is a set, and if the Value is also a set, then
the Value is added to the existing set. (This is a set operation, not mathematical addition.) For example, if the attribute value was the set [1,2], and the ADD action specified , then the final attribute value would be [1,2,3]. An error occurs if an Add action is specified for a set attribute and the attribute type specified does not match the existing set type.
Both sets must have the same primitive data type. For example, if the existing data type is a set of strings, the Value must also be a set of strings. The same holds true for number sets and binary sets.
This action is only valid for an existing attribute whose data type is
number or is a set. Do not use ADD for any other data types.
If no item with the specified Key is found: PUT - DynamoDB creates a new item with the specified primary key, and
then adds the attribute.
DELETE - Nothing happens; there is no attribute to delete. ADD - DynamoDB creates an item with the supplied primary key and number
(or set of numbers) for the attribute value. The only data types allowed are number and number set; no other data types can be specified.