ZADD for adding scored members in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this ZADD command?

Given an empty sorted set named 'players', what will be the output after running this command?

ZADD players 10 "Alice" 20 "Bob" 15 "Charlie"

What does the command return?

Redis

ZADD players 10 "Alice" 20 "Bob" 15 "Charlie"

A 1

B 0

C 3

D Error: wrong number of arguments

💡 Hint

ZADD returns the number of new elements added to the sorted set.

Since the sorted set 'players' was empty, all three members are new and added. So, the command returns 3.

📝 Syntax

intermediate

Which ZADD command syntax is correct?

Which of the following ZADD commands is syntactically correct to add two members with scores to a sorted set named 'scores'?

A ZADD scores 100 "John" 200 "Jane"

B ZADD scores 100, "John", 200, "Jane"

C ZADD scores 100 John 200 Jane

D ZADD scores "John" 100 "Jane" 200

💡 Hint

The syntax is: ZADD key score1 member1 [score2 member2 ...]

Option A follows the correct syntax: score followed by member, repeated for each pair. Option A swaps score and member. Option A misses quotes around members (optional but recommended). Option A uses commas which are invalid in Redis commands.

❓ optimization

advanced

Optimizing multiple ZADD operations

You want to add 1000 members with scores to a sorted set 'rankings'. Which approach is more efficient?

A Use a single ZADD command with all 1000 score-member pairs.

B Run 1000 separate ZADD commands, each adding one member.

C Use 10 ZADD commands, each adding 100 members.

D Add members one by one using ZINCRBY with score increments.

💡 Hint

Minimize the number of commands sent to Redis for better performance.

Sending one ZADD command with all members reduces network overhead and is more efficient than multiple commands.

🔧 Debug

advanced

Why does this ZADD command fail?

You run this command:

ZADD leaderboard 50 "player1" 40

and get an error. What is the cause?

A Score must be an integer, 50 is invalid

B Missing member name after score 40

C Quotes around member names are not allowed

D Leaderboard key does not exist

💡 Hint

Each score must be followed by a member name.

The command provides a score (40) without a corresponding member name, causing a syntax error.

🧠 Conceptual

expert

What happens when adding a member with an existing name but different score?

Consider a sorted set 'tasks' with member 'task1' having score 5. What will be the result of:

ZADD tasks 10 "task1"

What does Redis do?

A Returns an error because 'task1' already exists

B Adds a new member 'task1' with score 10 and returns 1

C Ignores the command and returns 0

D Updates 'task1' score to 10 and returns 0

💡 Hint

ZADD updates the score if the member exists.

Redis updates the score of existing members and returns 0 because no new member was added.