Key condition expressions in DynamoDB - Step-by-Step Execution

Concept Flow - Key condition expressions

Start Query

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Specify Partition Key Condition

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Optionally Specify Sort Key Condition

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Evaluate Conditions Against Partition Items

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Return Matching Items

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End Query

The query starts by specifying the partition key condition, optionally adds a sort key condition, then evaluates these conditions against items in the partition to find and return matching items.

Execution Sample

DynamoDB

KeyConditionExpression: "UserId = :uid AND CreatedAt > :date"
ExpressionAttributeValues: {":uid": "123", ":date": "2023-01-01"}

This query finds items where UserId equals '123' and CreatedAt is after '2023-01-01'.

Execution Table

Step	Action	Condition Evaluated	Item Attribute Values	Condition Result	Items Returned
1	Start Query	UserId = :uid AND CreatedAt > :date	UserId=123, CreatedAt=2023-02-01	True	Item included
2	Evaluate next item	UserId = :uid AND CreatedAt > :date	UserId=123, CreatedAt=2022-12-31	False	Item excluded
3	Evaluate next item	UserId = :uid AND CreatedAt > :date	UserId=123, CreatedAt=2023-01-02	True	Item included
4	End Query	No more items			Return 2 items

💡 All items evaluated; only those matching both conditions are returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Current Item	None	UserId=123, CreatedAt=2023-02-01	UserId=123, CreatedAt=2022-12-31	UserId=123, CreatedAt=2023-01-02	None
Items Returned Count	0	1	1	2	2

Key Moments - 2 Insights

Why does the item with UserId=123 and CreatedAt=2022-12-31 get excluded?

Because although the partition key UserId matches, the sort key condition CreatedAt > :date (2023-01-01) is false, so the whole condition fails (see execution_table row 2).

Can we query without specifying the sort key condition?

Yes, specifying only the partition key condition is valid and returns all items with that partition key (not shown in this trace but implied in concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, how many items matched both conditions?

A 1

B 3

C 2

D 4

💡 Hint

Check the 'Items Returned' column in rows 1, 2, 3, and 4.

At which step does the condition evaluate to false due to the sort key?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Condition Result' column and the item attributes in step 2.

If the sort key condition was removed, how would the number of returned items change?

A It would stay the same

B It would increase

C It would decrease

D No items would be returned

💡 Hint

Removing the sort key condition means all items with UserId=123 match, see concept_flow.

Concept Snapshot

Key Condition Expressions in DynamoDB:
- Must include partition key equality condition.
- Optional sort key conditions use operators (=, <, >, BETWEEN, etc.).
- Query returns items matching all key conditions.
- Used to efficiently find items by primary key.
- ExpressionAttributeValues provide actual values for placeholders.

Full Transcript

This visual trace shows how DynamoDB processes a query with key condition expressions. The query requires a partition key condition and optionally a sort key condition. Items in the partition matching the partition key are checked against the sort key condition. Items matching both conditions are returned. For example, items with UserId equal to '123' and CreatedAt after '2023-01-01' are included. Items failing any condition are excluded. The trace tracks each item evaluated, the condition result, and the count of items returned. Key moments clarify why some items are excluded and that the sort key condition is optional. The quiz tests understanding of how many items match, when conditions fail, and the effect of removing the sort key condition.