ZREM for removal in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command ZREM do?

The ZREM command removes one or more members from a sorted set in Redis.

beginner

How does ZREM affect the sorted set if the member does not exist?

If the member does not exist in the sorted set, ZREM does nothing and returns 0.

beginner

What is the return value of ZREM?

ZREM returns the number of members that were removed from the sorted set.

intermediate

Can ZREM remove multiple members at once?

Yes, ZREM can remove multiple members in a single command by listing them all.

intermediate

What happens if you remove all members from a sorted set using ZREM?

If all members are removed, the sorted set becomes empty and Redis deletes the key automatically.

What does the Redis command ZREM myset member1 do?

A Removes 'member1' from the sorted set 'myset'

B Adds 'member1' to the sorted set 'myset'

C Returns the score of 'member1' in 'myset'

D Deletes the entire sorted set 'myset'

ZREM removes specified members from a sorted set. Here, it removes 'member1' from 'myset'.

If you run ZREM myset memberX but 'memberX' is not in 'myset', what happens?

A Redis returns an error

B Redis removes a random member

C Redis returns 0 and does nothing

D Redis deletes the entire set

If the member does not exist, ZREM returns 0 and does not change the set.

What does ZREM myset member1 member2 do?

A Adds 'member1' and 'member2' to 'myset'

B Removes both 'member1' and 'member2' from 'myset'

C Returns the scores of 'member1' and 'member2'

D Removes only 'member1' from 'myset'

ZREM can remove multiple members at once by listing them all.

What is the return value of ZREM?

A The number of members removed

B The total number of members in the set

C The score of the removed member

D Always 1

ZREM returns how many members were actually removed from the set.

What happens if you remove all members from a sorted set using ZREM?

A The sorted set remains empty but the key stays

B The sorted set resets scores to zero

C Redis throws an error

D Redis deletes the key automatically

When a sorted set becomes empty, Redis deletes the key automatically.

Explain how the Redis command ZREM works and what it returns.

Think about removing items from a list and what feedback you get.

Describe what happens to a sorted set key in Redis when all its members are removed using ZREM.

Consider what happens to an empty container in real life.

Practice

1. What does the Redis command ZREM do?

easy

A. Removes specified members from a sorted set

B. Adds new members to a sorted set

C. Returns the score of a member in a sorted set

D. Deletes the entire sorted set

Solution

1. Step 1: Understand the purpose of ZREM

   The command ZREM is used to remove one or more members from a sorted set in Redis.

2. Step 2: Compare with other commands

   Adding members is done by ZADD, getting scores by ZSCORE, and deleting the whole set by DEL, so these are not correct.

3. Final Answer:

   Removes specified members from a sorted set -> Option A

4. Quick Check:

   ZREM removes members = B [OK]

Hint: ZREM always removes members from sorted sets [OK]

Common Mistakes:

• Confusing ZREM with ZADD
• Thinking ZREM deletes the whole set
• Assuming ZREM returns member scores

2. Which of the following is the correct syntax to remove members 'a' and 'b' from a sorted set named 'myzset' using ZREM?

easy

A. ZREM myzset (a,b)

B. ZREM myzset {a,b}

C. ZREM myzset ['a','b']

D. ZREM myzset a b

Solution

1. Step 1: Recall ZREM syntax

   ZREM command syntax is: ZREM key member [member ...]. Members are listed separated by spaces.

2. Step 2: Analyze options

   ZREM myzset a b uses space-separated members correctly. Options B, C, and D use invalid syntax with braces, brackets, or parentheses which Redis does not accept.

3. Final Answer:

   ZREM myzset a b -> Option D

4. Quick Check:

   Correct ZREM syntax = A [OK]

Hint: List members separated by spaces after key in ZREM [OK]

Common Mistakes:

• Using brackets or braces around members
• Separating members with commas
• Passing members as a single string

3. Given a sorted set 'myzset' with members and scores: {"a":1, "b":2, "c":3}, what will be the result of ZREM myzset b d followed by ZCARD myzset?

medium

A. 1

B. 2

C. 3

D. 0

Solution

1. Step 1: Understand ZREM effect

   ZREM removes specified members if they exist. 'b' exists and will be removed. 'd' does not exist and is ignored without error.

2. Step 2: Calculate remaining members

   Initially 3 members: a, b, c. After removal of 'b', members left are 'a' and 'c', so 2 remain.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Removed 1 member, 2 remain = C [OK]

Hint: ZREM ignores missing members, count remaining with ZCARD [OK]

Common Mistakes:

• Assuming error on removing non-existent member
• Counting removed non-existent member
• Confusing ZREM with ZADD effect

4. You run the command ZREM myzset without specifying any members. What will happen?

medium

A. Syntax error due to missing members

B. Returns 0 and does nothing

C. Removes the first member by default

D. Removes all members from 'myzset'

Solution

1. Step 1: Check ZREM command requirements

   ZREM requires at least one member argument to remove. Omitting members is invalid syntax.

2. Step 2: Result of missing members

   Running ZREM without members causes a syntax error, Redis will not execute the command.

3. Final Answer:

   Syntax error due to missing members -> Option A

4. Quick Check:

   ZREM needs members, missing causes syntax error = D [OK]

Hint: Always specify members to remove with ZREM [OK]

Common Mistakes:

• Assuming ZREM clears entire set without members
• Expecting default removal of first member
• Thinking it returns 0 silently

5. You have a sorted set 'myzset' with members: {"x":10, "y":20, "z":30}. You want to remove all members with scores less than 25 using ZREM. Which approach correctly achieves this?

hard

A. Use ZREM myzset x y

B. Use ZREM myzset with a score range argument

C. Use ZREMRANGEBYSCORE myzset -inf 24

D. Use ZREM myzset z

Solution

1. Step 1: Understand ZREM limitations

   ZREM removes specified members by name, it cannot remove by score range directly.

2. Step 2: Identify members to remove

   Members x (score 10) and y (score 20) are less than 25, so to remove them by score range, use ZREMRANGEBYSCORE myzset -inf 24.

3. Step 3: Analyze options

   A correctly uses ZREMRANGEBYSCORE to remove by score range. B is invalid syntax. C removes members by name but requires manual identification. D removes wrong member.

4. Final Answer:

   Use ZREMRANGEBYSCORE myzset -inf 24 -> Option C

5. Quick Check:

   Removing by score range requires ZREMRANGEBYSCORE = A [OK]

Hint: Use ZREMRANGEBYSCORE to remove members by score range [OK]

Common Mistakes:

• Trying to remove by score with ZREM
• Using invalid syntax with ZREM
• Removing wrong members by name