Top-N queries in Redis - Step-by-Step Execution

Concept Flow - Top-N queries

Start with dataset

↓

Sort data by score or value

↓

Select top N items

↓

Return top N results

↓

End

Top-N queries sort data by a score or value and then pick the top N items to return.

Execution Sample

Redis

ZADD leaderboard 100 "Alice" 200 "Bob" 150 "Carol"
ZREVRANGE leaderboard 0 2 WITHSCORES

Add scores for players and get top 3 players with their scores.

Execution Table

Step	Command	Action	Data State	Output
1	ZADD leaderboard 100 "Alice"	Add Alice with score 100	{"leaderboard": {"Alice": 100}}	OK
2	ZADD leaderboard 200 "Bob"	Add Bob with score 200	{"leaderboard": {"Alice": 100, "Bob": 200}}	OK
3	ZADD leaderboard 150 "Carol"	Add Carol with score 150	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	OK
4	ZRANGE leaderboard -3 -1 WITHSCORES	Get all 3 members sorted by score ascending	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	["Alice", "100", "Carol", "150", "Bob", "200"]
5	ZREVRANGE leaderboard 0 2 WITHSCORES	Get top 3 members sorted by score descending	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	["Bob", "200", "Carol", "150", "Alice", "100"]
6	End	Top-N query complete	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	Top 3 players with scores returned

💡 All top 3 players retrieved; query ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
leaderboard	{}	{"Alice": 100}	{"Alice": 100, "Bob": 200}	{"Alice": 100, "Bob": 200, "Carol": 150}	{"Alice": 100, "Bob": 200, "Carol": 150}

Key Moments - 2 Insights

Why does ZRANGE with -3 -1 return all players sorted ascending, not top scores?

ZRANGE returns elements in ascending order by score. To get top scores descending, use ZREVRANGE as shown in step 5.

How does Redis store scores and members in sorted sets?

Redis stores members with their scores in a sorted set, allowing fast retrieval by score order, as seen in the leaderboard variable changes.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what order are the players returned?

A Descending by score: Bob, Carol, Alice

B Random order

C Ascending by score: Alice, Carol, Bob

D Alphabetical order

💡 Hint

Check the output column at step 4 showing scores from 100 to 200.

At which step does the leaderboard contain all three players?

A After Step 1

B After Step 3

C After Step 2

D After Step 4

💡 Hint

Look at variable_tracker for leaderboard state after each step.

If you want the top 2 players by highest score, which command would you use?

A ZREVRANGE leaderboard 0 1

B ZRANGE leaderboard 0 1

C ZRANGE leaderboard -2 -1

D ZREVRANGE leaderboard -2 -1

💡 Hint

Refer to step 5 where ZREVRANGE returns highest scores first.

Concept Snapshot

Top-N queries in Redis use sorted sets.
Add members with scores using ZADD.
Use ZREVRANGE to get top N by highest scores.
ZRANGE returns sorted ascending by score.
Top-N queries help find best scoring items quickly.

Full Transcript

This visual execution shows how Redis handles Top-N queries using sorted sets. We add players with scores using ZADD commands. The leaderboard variable tracks the stored data. Using ZRANGE returns members sorted by score ascending, while ZREVRANGE returns them descending, which is useful for Top-N queries. The execution table traces each command, data state, and output. Key moments clarify common confusions about sorting order and data storage. The quiz tests understanding of order, data state, and correct commands for Top-N retrieval.

Practice

1. What Redis command is used to get the top N highest scores from a sorted set?

easy

A. ZRANGE

B. ZREVRANGE

C. ZADD

D. ZREM

Solution

1. Step 1: Understand sorted set ordering

   Redis sorted sets store elements with scores. To get highest scores, we want descending order.

2. Step 2: Identify the correct command

   ZREVRANGE returns elements in descending order by score, perfect for top N queries.

3. Final Answer:

   ZREVRANGE -> Option B

4. Quick Check:

   Top N highest = ZREVRANGE [OK]

Hint: Use ZREVRANGE for top scores in descending order [OK]

Common Mistakes:

• Using ZRANGE which returns lowest scores first
• Confusing ZADD (adds elements) with query commands
• Using ZREM which removes elements

2. Which of the following is the correct syntax to get the top 3 highest scoring members from sorted set players?

easy

A. ZREVRANGE players 1 3

B. ZRANGE players 0 2

C. ZREVRANGE players 0 2

D. ZRANGE players -3 -1

Solution

1. Step 1: Understand index ranges in ZREVRANGE

   Indexes start at 0. To get top 3, use 0 to 2 inclusive.

2. Step 2: Choose correct command and range

   ZREVRANGE returns highest scores first. So ZREVRANGE players 0 2 gets top 3.

3. Final Answer:

   ZREVRANGE players 0 2 -> Option C

4. Quick Check:

   Top 3 highest = ZREVRANGE players 0 2 [OK]

Hint: Use 0 to N-1 indexes with ZREVRANGE for top N [OK]

Common Mistakes:

• Using ZRANGE which returns lowest scores
• Starting index at 1 instead of 0
• Using negative indexes incorrectly

3. Given a sorted set scores with members and scores: {"alice": 50, "bob": 70, "carol": 60}, what is the result of ZREVRANGE scores 0 1 WITHSCORES?

medium

A. ["carol", "60", "alice", "50"]

B. ["alice", "50", "carol", "60"]

C. ["bob", "70", "alice", "50"]

D. ["bob", "70", "carol", "60"]

Solution

1. Step 1: Order members by descending score

   Scores descending: bob(70), carol(60), alice(50).

2. Step 2: Select top 2 with scores

   ZREVRANGE 0 1 returns first two: bob and carol with their scores.

3. Final Answer:

   ["bob", "70", "carol", "60"] -> Option D

4. Quick Check:

   Top 2 with scores = bob(70), carol(60) [OK]

Hint: ZREVRANGE 0 N-1 gets top N in descending order [OK]

Common Mistakes:

• Mixing ascending and descending order
• Forgetting WITHSCORES returns pairs
• Selecting wrong index range

4. You run ZREVRANGE leaderboard 0 4 but get an empty list. What is the most likely cause?

medium

A. The sorted set leaderboard does not exist or is empty

B. The command syntax is incorrect

C. ZREVRANGE only works with hashes, not sorted sets

D. The indexes 0 to 4 are out of range for the sorted set

Solution

1. Step 1: Check command syntax validity

   ZREVRANGE syntax is correct for sorted sets, so syntax is not the issue.

2. Step 2: Understand empty result causes

   If the sorted set does not exist or is empty, ZREVRANGE returns an empty list.

3. Final Answer:

   The sorted set leaderboard does not exist or is empty -> Option A

4. Quick Check:

   Empty result means no data in sorted set [OK]

Hint: Empty list means sorted set missing or empty [OK]

Common Mistakes:

• Assuming syntax error without checking data
• Thinking ZREVRANGE works on hashes
• Believing index out of range causes error instead of empty list

5. You want to get the top 5 players with the lowest scores from a sorted set game_scores. Which command should you use?

hard

A. ZRANGE game_scores 0 4

B. ZRANGE game_scores -5 -1

C. ZREVRANGE game_scores -5 -1

D. ZREVRANGE game_scores 0 4

Solution

1. Step 1: Identify order for lowest scores

   ZRANGE returns elements in ascending order by score, so lowest scores first.

2. Step 2: Select correct range for top 5

   Indexes 0 to 4 select first 5 elements, which are lowest scores.

3. Final Answer:

   ZRANGE game_scores 0 4 -> Option A

4. Quick Check:

   Lowest 5 scores = ZRANGE 0 4 [OK]

Hint: Use ZRANGE 0 N-1 for lowest N scores [OK]

Common Mistakes:

• Using ZREVRANGE which returns highest scores
• Using negative indexes incorrectly
• Confusing ascending and descending order