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

Concept Flow - Key condition expressions

Start Query

↓

Specify Partition Key Condition

↓

Optionally Specify Sort Key Condition

↓

Evaluate Conditions Against Partition Items

↓

Return Matching Items

↓

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.

Practice

1. What is the required part of a key condition expression in DynamoDB?

easy

A. Sort key condition

B. Partition key condition

C. Both partition and sort key conditions

D. Neither partition nor sort key condition

Solution

1. Step 1: Understand key condition expression components

   Key condition expressions must always include the partition key condition to identify the partition to query.

2. Step 2: Identify optional parts

   The sort key condition is optional and used to refine the query within the partition.

3. Final Answer:

   Partition key condition -> Option B

4. Quick Check:

   Partition key condition = required [OK]

Hint: Partition key condition is always required in key condition expressions [OK]

Common Mistakes:

• Thinking sort key condition is mandatory
• Using only sort key condition without partition key
• Confusing key condition with filter expressions

2. Which of the following is the correct syntax for a key condition expression to find items with partition key 'UserID' equal to '123'?

easy

A. UserID = '123'

B. partition_key = 'UserID' AND sort_key = '123'

C. UserID == 123

D. Key('UserID').eq('123')

Solution

1. Step 1: Identify correct key condition syntax

   In DynamoDB, key condition expressions use attribute names and operators like '=' with values in quotes for strings.

2. Step 2: Evaluate each option

   UserID = '123' uses correct syntax: UserID = '123'. partition_key = 'UserID' AND sort_key = '123' uses wrong attribute names and syntax. UserID == 123 uses '==' which is invalid. Key('UserID').eq('123') is not valid DynamoDB syntax.

3. Final Answer:

   UserID = '123' -> Option A

4. Quick Check:

   Correct syntax uses '=' and quotes for strings [OK]

Hint: Use single '=' and quotes for string values in key condition expressions [OK]

Common Mistakes:

• Using '==' instead of '='
• Confusing attribute names with keywords
• Using function syntax not supported in key conditions

3. Given a DynamoDB table with partition key 'UserID' and sort key 'Timestamp', what will this key condition expression return?
UserID = 'user1' AND Timestamp BETWEEN 100 AND 200

medium

A. All items with UserID 'user1' only

B. All items with Timestamp between 100 and 200 regardless of UserID

C. All items with UserID 'user1' and Timestamp between 100 and 200 inclusive

D. Syntax error due to BETWEEN usage

Solution

1. Step 1: Analyze partition key condition

   The expression requires UserID to be 'user1', so only items with this partition key are considered.

2. Step 2: Analyze sort key condition with BETWEEN

   The sort key Timestamp must be between 100 and 200 inclusive, filtering items within that range.

3. Final Answer:

   All items with UserID 'user1' and Timestamp between 100 and 200 inclusive -> Option C

4. Quick Check:

   Partition key + BETWEEN on sort key filters correctly [OK]

Hint: BETWEEN filters sort key range within partition key [OK]

Common Mistakes:

• Ignoring partition key condition
• Thinking BETWEEN applies to partition key
• Assuming syntax error with BETWEEN

4. You wrote this key condition expression:
begins_with(UserID, :uid) AND Timestamp = :ts
But your query returns an error. What is the likely cause?

medium

A. Using begins_with on the partition key

B. Missing partition key condition

C. begins_with function used on sort key is valid, so no error

D. Incorrect use of AND operator

Solution

1. Step 1: Identify key condition expression rules

   Partition key condition must use '=' operator; functions like begins_with are not allowed on partition key.

2. Step 2: Check usage of begins_with

   begins_with is valid only on sort key, not partition key. If used on partition key, it causes an error.

3. Final Answer:

   Using begins_with on the partition key -> Option A

4. Quick Check:

   begins_with only valid on sort key [OK]

Hint: begins_with only works on sort key, not partition key [OK]

Common Mistakes:

• Using begins_with on partition key
• Omitting partition key condition
• Misusing AND operator syntax

5. You want to query a DynamoDB table with partition key 'Category' and sort key 'Date'. You need all items where Category is 'Books' and Date starts with '2023-06'. Which key condition expression should you use?

hard

A. begins_with(Category, 'Books') AND Date = '2023-06'

B. Category = 'Books' AND Date BETWEEN '2023-06-01' AND '2023-06-30'

C. Category = 'Books' OR begins_with(Date, '2023-06')

D. Category = 'Books' AND begins_with(Date, '2023-06')

Solution

1. Step 1: Identify partition key condition

   Category must be exactly 'Books', so use equality operator on partition key.

2. Step 2: Use begins_with on sort key

   Date starts with '2023-06' means use begins_with function on sort key Date.

3. Step 3: Evaluate options

   Category = 'Books' AND begins_with(Date, '2023-06') correctly combines equality on partition key and begins_with on sort key. Category = 'Books' AND Date BETWEEN '2023-06-01' AND '2023-06-30' uses BETWEEN but requires exact date range, which is more complex. Category = 'Books' OR begins_with(Date, '2023-06') uses OR which is invalid in key condition expressions. begins_with(Category, 'Books') AND Date = '2023-06' incorrectly uses begins_with on partition key.

4. Final Answer:

   Category = 'Books' AND begins_with(Date, '2023-06') -> Option D

5. Quick Check:

   Partition key = value AND begins_with on sort key [OK]

Hint: Use '=' for partition key and begins_with() for sort key prefix [OK]

Common Mistakes:

• Using OR instead of AND
• Using begins_with on partition key
• Using BETWEEN without exact date range