ZADD for adding scored members in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command ZADD do?

The ZADD command adds one or more members with scores to a sorted set in Redis. It keeps the set ordered by score.

beginner

How do you add a member with score 10 to a sorted set named players using ZADD?

Use the command: ZADD players 10 member_name. This adds member_name with score 10 to the players sorted set.

intermediate

What happens if you add a member that already exists in the sorted set with a new score using ZADD?

The member's score is updated to the new score you provide. The sorted set reorders itself based on the updated score.

intermediate

Can ZADD add multiple members at once? How?

Yes. You can add multiple members by listing score-member pairs: ZADD key score1 member1 score2 member2 ....

intermediate

What does the return value of ZADD represent?

It returns the number of new members added to the sorted set, not counting members whose scores were updated.

What type of Redis data structure does ZADD work with?

A Hashes

B Lists

C Sorted sets

D Strings

ZADD adds members with scores to sorted sets only.

What happens if you use ZADD to add a member that already exists with a different score?

A The member is ignored

B The member's score is updated

C An error is returned

D The member is duplicated

The existing member's score is updated to the new score.

Which of these is the correct syntax to add two members with scores to a sorted set named game?

A ZADD game member1 10 member2 20

B ZADD game 10 20 member1 member2

C ZADD game member1 10, member2 20

D ZADD game 10 member1 20 member2

The correct syntax is ZADD key score1 member1 score2 member2.

What does ZADD return after adding members?

A Number of new members added

B The lowest score in the set

C The highest score in the set

D Total number of members in the set

ZADD returns how many new members were added, excluding updated ones.

If you want to add a member with score 5 to a sorted set called scores, which command is correct?

A ZADD scores 5 member

B ZADD scores member 5

C ZADD scores 5:member

D ZADD scores (5, member)

The score comes before the member name in the ZADD command.

Explain how the ZADD command works in Redis and what it is used for.

Think about adding and ordering members by numbers.

Describe the syntax of the ZADD command and how to add multiple members at once.

Remember the order: score then member, repeated for each.

Practice

1. What does the Redis command ZADD do?

easy

A. Adds members with scores to a sorted set

B. Deletes members from a sorted set

C. Retrieves all members from a sorted set

D. Creates a new key with a string value

Solution

1. Step 1: Understand the purpose of ZADD

   The ZADD command is used to add members with associated scores to a sorted set in Redis.

2. Step 2: Compare with other commands

   Deleting members is done by ZREM, retrieving members by ZRANGE, and creating string keys by SET, so these are not correct.

3. Final Answer:

   Adds members with scores to a sorted set -> Option A

4. Quick Check:

   ZADD adds scored members [OK]

Hint: ZADD always adds scored members to sorted sets [OK]

Common Mistakes:

• Confusing ZADD with ZREM or ZRANGE
• Thinking ZADD creates string keys
• Assuming ZADD deletes members

2. Which of the following is the correct syntax to add a member 'player1' with score 100 to a sorted set 'game_scores' using ZADD?

easy

A. ZADD game_scores 100 player1

B. ZADD 100 game_scores player1

C. ZADD game_scores player1 100

D. ZADD player1 100 game_scores

Solution

1. Step 1: Recall ZADD syntax

   The correct syntax is: ZADD key score member. So the key comes first, then the score, then the member.

2. Step 2: Match the syntax to options

   ZADD game_scores 100 player1 matches this order: ZADD game_scores 100 player1. Others have wrong order.

3. Final Answer:

   ZADD game_scores 100 player1 -> Option A

4. Quick Check:

   ZADD key score member order [OK]

Hint: Remember: ZADD key score member [OK]

Common Mistakes:

• Swapping score and member positions
• Putting member before key
• Using wrong argument order

3. What will be the result of the following commands?

ZADD leaderboard 50 Alice 70 Bob 60 Charlie
ZRANGE leaderboard 0 -1 WITHSCORES

medium

A. ["Bob", "70", "Charlie", "60", "Alice", "50"]

B. ["Alice", "50", "Charlie", "60", "Bob", "70"]

C. ["Alice", "50", "Bob", "70", "Charlie", "60"]

D. ["Charlie", "60", "Alice", "50", "Bob", "70"]

Solution

1. Step 1: Understand ZADD and ZRANGE behavior

   ZADD adds members with scores. ZRANGE with 0 -1 returns all members sorted by score ascending, WITHSCORES returns scores alongside.

2. Step 2: Sort members by score ascending

   Scores: Alice=50, Charlie=60, Bob=70. Sorted ascending: Alice, Charlie, Bob.

3. Final Answer:

   ["Alice", "50", "Charlie", "60", "Bob", "70"] -> Option B

4. Quick Check:

   ZRANGE sorts ascending by score [OK]

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

Common Mistakes:

• Assuming ZRANGE sorts by insertion order
• Mixing up ascending and descending order
• Ignoring WITHSCORES option

4. Identify the error in this Redis command: ZADD myset player1 100

medium

A. ZADD cannot add members with numeric scores

B. The key name 'myset' is invalid

C. The score and member are reversed; score must come before member

D. The command is correct and will work as expected

Solution

1. Step 1: Check ZADD argument order

   ZADD syntax requires the key first, then the score, then the member. Here, 'player1' is before '100', which is reversed.

2. Step 2: Validate other options

   Key name 'myset' is valid, numeric scores are allowed, so options B and C are incorrect. The command is not correct, so D is wrong.

3. Final Answer:

   The score and member are reversed; score must come before member -> Option C

4. Quick Check:

   ZADD requires score before member [OK]

Hint: Score always comes before member in ZADD [OK]

Common Mistakes:

• Swapping score and member positions
• Thinking key name is invalid
• Believing numeric scores are disallowed

5. You want to add multiple players with their scores to a sorted set 'ranking' in one command. Which command correctly adds 'Anna' with 85, 'Ben' with 90, and 'Cara' with 80?

hard

A. ZADD ranking Anna 85 Ben 90 Cara 80

B. ZADD ranking Cara 80 85 Anna 90 Ben

C. ZADD ranking 85 Anna Cara 80 90 Ben

D. ZADD ranking 85 Anna 90 Ben 80 Cara

Solution

1. Step 1: Recall ZADD syntax for multiple members

   ZADD syntax allows multiple score-member pairs: ZADD key score1 member1 score2 member2 ...

2. Step 2: Match pairs correctly

   ZADD ranking 85 Anna 90 Ben 80 Cara lists pairs as 85 Anna, 90 Ben, 80 Cara, matching the players and scores in the order given.

3. Final Answer:

   ZADD ranking 85 Anna 90 Ben 80 Cara -> Option D

4. Quick Check:

   ZADD multiple pairs as score member [OK]

Hint: List pairs as score then member repeatedly [OK]

Common Mistakes:

• Swapping member and score order
• Mixing up pairs order
• Using inconsistent score-member pairs