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‫So I've mentioned this before,

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‫but let's do a little zoom in

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‫on the conditional rights of DynamoDB.

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‫So this is for the write operations.

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‫So we have PutItem, UpdateItem,

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‫DeleteItem, and BatchWriteItem.

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‫And the idea is that you can specify a Condition expression

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‫that's going to determine which items should be modified.

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‫Now you have several conditions possible.

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‫We'll have a look at many of these in examples.

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‫So we have attributes_exists or attributes_not_exists,

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‫attribute_type, so this is to check whether

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‫or not an attribute_exists doesn't exist

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‫and is of the correct type,

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‫contains and begins_with, this is for string comparison.

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‫And then we have the IN keyword to check different values.

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‫For example, for product category

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‫to be in two different categories.

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‫And Price we have between :low and :high.

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‫But we can also, for example,

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‫do price over or price less than specific values.

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‫And finally if we want to check for string length,

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‫we have the size attributes.

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‫Okay, so you have seen Filter Expressions

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‫and you've seen Condition Expressions.

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‫So Filter Expressions filter the results of a read query.

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‫So Filter Expressions are related to read queries,

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‫whereas Condition Expressions are only for rights operations

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‫and they tell DynamoDB which rights should succeed or not.

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‫So let's do an example, for example, an Update Item.

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‫So here we update an item on the table product catalog

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‫and we say that we want to set the price to be equal

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‫to the current price minus discounts

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‫only if the price is over to a specific limit.

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‫That's our condition expression.

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‫Now to fill in what is the discount and what is the limit?

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‫We pass in the file://values.json

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‫which has the discount to be a number

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‫of 150 and the limit to be a number of 500.

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‫So that means that if we have this item

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‫in our DynamoDB table with a key 4, 5, 6

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‫if we apply this Update Item command is going to

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‫be transformed to be now having a price of 500.

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‫That's because indeed before the price was strictly

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‫over the limit of 500 because 650 is greater than 500.

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‫But if we apply yet again the same command now

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‫it's not going to succeed.

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‫The price will never go below 500 because, well,

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‫the condition expression would evaluate to being false.

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‫So as you can see, condition expressions vary

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‫whether or not the item has the value over 500 or not

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‫and they will determine whether or not

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‫the item will be updated.

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‫So if we have Delete Item

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‫if we can check for example, the attribute_not_exists.

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‫So this only succeeds

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‫if the attribute doesn't exist yet when there's no value.

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‫For example, we're saying hey, delete an item

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‫or delete many items if you have a batch right item

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‫we can do delete one item for our product catalog

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‫if the attribute price doesn't exist.

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‫That makes sense.

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‫If we have an item in our catalog but we don't have a price

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‫we may not be able to sell it.

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‫So we may want to clean up our table and delete it.

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‫Alternatively, you have attribute_exists.

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‫So it is the opposite of attributes not exist.

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‫And here we can check if something exists.

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‫For example,

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‫if we have ProductReviews.Onestar that exists

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‫that means that our product review

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‫our product actually has one star and

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‫so we can delete it altogether from our DynamoDB table

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‫but we don't have it then don't do anything.

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‫So you can start seeing the usefulness of it.

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‫So there's a good use case

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‫for attribute_not_exists and attribute_exists.

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‫So if we have attribute_not_exists

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‫and then we specify the partition key

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‫what's going to happen is that if the item is already

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‫in the database, then this condition will be evaluated false

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‫and then we don't overwrite the item.

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‫But if the item doesn't exist yet

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‫then we can write the item.

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‫So it's one way to make sure

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‫that we never overwrite existing data

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‫for this we do attribute_not_exists

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‫and then we specify the partition key.

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‫And in case we have the partition key and the sort key

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‫and we want to make sure to again never override something

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‫then you specify attributes_not_exists

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‫partition key and attributes_not_exists sort key

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‫and you're good to go. For other things,

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‫for example, to check on values, you can have a look

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‫at whether a product category is in different categories

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‫or if the price is between a low and a high price.

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‫And again, we can pass

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‫in this values.json file we're specifying category 1

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‫and category 2 as well

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‫as the low of 500 and the high of 600.

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‫And so when we apply Delete Item

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‫in case the item does belong to this range

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‫then we're good to go.

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‫But as we can see, for example,

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‫even though the products category Sporting Good is compliant

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‫with our condition,

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‫which is the first part of our condition,

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‫the price is 650, which is not between 500 and 600.

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‫And so the whole condition is false

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‫and the item will not be deleted in this case.

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‫You can also have a look at String Comparisons.

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‫So you have begins_with and contains.

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‫So here for example, we're saying, hey

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‫I want to make sure to delete items

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‫if it begins with http.://

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‫So we won't, don't want to keep any insecure images

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‫in our product catalog.

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‫So that's just some examples of course.

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‫But what I want to remember out of it is that

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‫Condition Expressions help you

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‫create conditions that will tell that DynamoDB

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‫whether or not to apply your right operations.

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‫Alright, let's see for this lecture

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‫I hope you liked it, and I will see you in the next lecture.