Why sorted sets combine uniqueness with ordering in Redis - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a member with a score to a sorted set in Redis.

Redis

ZADD myset 10 [1]

Drag options to blanks, or click blank then click option'

Amember1

BmemberX

CmemberA

DmemberZ

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the member before the score.

Using a command other than ZADD.

The command ZADD myset 10 memberA adds the member memberA with score 10 to the sorted set myset.

2fill in blank

medium

Complete the code to retrieve members from the sorted set ordered by score ascending.

Redis

ZRANGE myset 0 [1]

Drag options to blanks, or click blank then click option'

A-1

B5

C10

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a positive number that limits the output.

Confusing the order of arguments.

The command ZRANGE myset 0 -1 returns all members from the sorted set myset ordered by score from lowest to highest.

3fill in blank

hard

Fix the error in the command to add a member with score 15 to the sorted set.

Redis

ZADD myset 15 [1]

Drag options to blanks, or click blank then click option'

Ascore15

Bmember15

C15

Dmember

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the member before the score.

Using a number as the member name.

The correct syntax is ZADD myset score member. So the member name should be after the score.

4fill in blank

hard

Fill both blanks to retrieve members with scores between 5 and 20 inclusive.

Redis

ZRANGEBYSCORE myset [1] [2]

Drag options to blanks, or click blank then click option'

A5

B20

C(5

D(20

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses which exclude the boundary scores.

Swapping the min and max values.

The command ZRANGEBYSCORE myset 5 20 returns members with scores from 5 to 20 inclusive.

5fill in blank

hard

Fill all three blanks to increment the score of 'memberA' by 5 in the sorted set.

Redis

ZINCRBY [1] [2] [3]

Drag options to blanks, or click blank then click option'

Amyset

B5

CmemberA

DmemberB

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the increment and member arguments.

Using the wrong key name.

The command ZINCRBY myset 5 memberA increases the score of memberA by 5 in the sorted set myset.

Practice

1. What is the main reason Redis sorted sets combine uniqueness with ordering?

easy

A. To store unique members with associated scores for sorted retrieval

B. To allow duplicate members with different scores

C. To store unordered lists of members

D. To only store scores without members

Solution

1. Step 1: Understand uniqueness in sorted sets

   Sorted sets store each member only once, ensuring uniqueness.

2. Step 2: Understand ordering in sorted sets

   Each member has a score that determines its order in the set.

3. Final Answer:

   To store unique members with associated scores for sorted retrieval -> Option A

4. Quick Check:

   Uniqueness + ordering = unique members with scores [OK]

Hint: Remember: unique members + scores = sorted sets [OK]

Common Mistakes:

• Thinking duplicates are allowed
• Confusing sorted sets with lists
• Ignoring the role of scores

2. Which of the following is the correct Redis command to add a member with a score to a sorted set?

easy

A. ZADD myset member1 10

B. SADD myset 10 member1

C. ZADD myset 10 member1

D. ADD myset 10 member1

Solution

1. Step 1: Recall Redis sorted set add syntax

   The correct syntax is ZADD key score member.

2. Step 2: Match syntax with options

   ZADD myset 10 member1 matches ZADD myset 10 member1 correctly.

3. Final Answer:

   ZADD myset 10 member1 -> Option C

4. Quick Check:

   ZADD key score member = correct syntax [OK]

Hint: ZADD uses score before member name [OK]

Common Mistakes:

• Using SADD instead of ZADD
• Swapping score and member order
• Using ADD command which doesn't exist

3. Given the commands:
ZADD myset 5 apple
ZADD myset 10 banana
ZADD myset 7 cherry
What will ZRANGE myset 0 -1 WITHSCORES return?

medium

A. ["apple", "5", "banana", "10", "cherry", "7"]

B. ["apple", "5", "cherry", "7", "banana", "10"]

C. ["banana", "10", "cherry", "7", "apple", "5"]

D. ["cherry", "7", "apple", "5", "banana", "10"]

Solution

1. Step 1: Understand ZADD inserts with scores

   Members apple=5, banana=10, cherry=7 are added.

2. Step 2: ZRANGE returns members sorted by score ascending

   Order is apple(5), cherry(7), banana(10).

3. Final Answer:

   ["apple", "5", "cherry", "7", "banana", "10"] -> Option B

4. Quick Check:

   ZRANGE sorts by score ascending [OK]

Hint: ZRANGE returns members sorted by score ascending [OK]

Common Mistakes:

• Assuming ZRANGE sorts by insertion order
• Confusing WITHSCORES output format
• Mixing up ascending and descending order

4. You run these commands:
ZADD myset 1 apple
ZADD myset 2 apple
ZRANGE myset 0 -1
But the output shows only one 'apple'. What is the issue?

medium

A. The second ZADD command has wrong syntax

B. Redis sorted sets allow duplicates, so this is a bug

C. ZRANGE command is incorrect and filters duplicates

D. Sorted sets do not allow duplicate members; score is updated instead

Solution

1. Step 1: Understand uniqueness in sorted sets

   Sorted sets keep unique members only; adding same member updates score.

2. Step 2: Analyze commands effect

   Second ZADD updates 'apple' score from 1 to 2, no duplicate added.

3. Final Answer:

   Sorted sets do not allow duplicate members; score is updated instead -> Option D

4. Quick Check:

   Same member updates score, no duplicates [OK]

Hint: Same member updates score, no duplicates added [OK]

Common Mistakes:

• Expecting duplicates in sorted sets
• Thinking ZRANGE filters duplicates
• Assuming syntax error caused issue

5. You want to keep a leaderboard where players have unique names and scores that can change. Which Redis data structure best fits this need and why?

hard

A. Sorted sets, because they store unique members with scores and keep them ordered

B. Lists, because they allow duplicates and maintain insertion order

C. Hashes, because they store key-value pairs without ordering

D. Sets, because they store unique members but no scores

Solution

1. Step 1: Identify requirements for leaderboard

   Need unique player names, scores, and sorted order by score.

2. Step 2: Match Redis data structures to requirements

   Sorted sets store unique members with scores and keep order automatically.

3. Step 3: Exclude other options

   Lists allow duplicates and no score ordering; hashes lack ordering; sets lack scores.

4. Final Answer:

   Sorted sets, because they store unique members with scores and keep them ordered -> Option A

5. Quick Check:

   Leaderboard needs unique + ordered scores = sorted sets [OK]

Hint: Leaderboard needs unique + ordered scores: use sorted sets [OK]

Common Mistakes:

• Choosing lists or sets ignoring ordering or scores
• Confusing hashes with sorted sets
• Not considering uniqueness requirement