SISMEMBER for membership check in Redis

Introduction

SISMEMBER helps you quickly check if a value is inside a set. It tells you yes or no.

Check if a user is in a list of online users.

Verify if a product ID is in a set of featured items.

See if an email address is in a blacklist.

Confirm if a tag exists in a set of tags for a blog post.

Determine if a session ID is active in a set of sessions.

Syntax

Redis

SISMEMBER key member

key is the name of the set you want to check.

member is the value you want to find in the set.

Examples

Check if 'user123' is in the 'online_users' set.

Redis

SISMEMBER online_users user123

Check if product ID '98765' is in the 'featured_products' set.

Redis

SISMEMBER featured_products 98765

Check if the email '[e-mail]' is in the 'blacklist' set.

Redis

SISMEMBER blacklist [e-mail]

Sample Program

First, add three colors to the 'colors' set. Then check if 'green' is in the set (should be yes). Finally, check if 'yellow' is in the set (should be no).

Redis

SADD colors red green blue
SISMEMBER colors green
SISMEMBER colors yellow

Important Notes

SISMEMBER returns 1 if the member exists, 0 if it does not.

Sets in Redis do not allow duplicate members.

Use SISMEMBER for fast membership checks without scanning the whole set.

Summary

SISMEMBER checks if a value is in a Redis set.

Returns 1 for yes, 0 for no.

Great for quick membership tests in lists stored as sets.

Practice

1. What does the Redis command SISMEMBER do?

easy

A. Adds a value to a set

B. Removes a value from a set

C. Checks if a value is a member of a set and returns 1 or 0

D. Returns all members of a set

Solution

1. Step 1: Understand SISMEMBER purpose

   The SISMEMBER command checks membership of a value in a Redis set.

2. Step 2: Identify return values

   It returns 1 if the value is in the set, otherwise 0.

3. Final Answer:

   Checks if a value is a member of a set and returns 1 or 0 -> Option C

4. Quick Check:

   SISMEMBER returns 1 or 0 [OK]

Hint: SISMEMBER returns 1 if present, else 0 [OK]

Common Mistakes:

• Confusing SISMEMBER with SADD or SREM
• Expecting SISMEMBER to return the value itself
• Thinking SISMEMBER returns all set members

2. Which of the following is the correct syntax to check if 'apple' is in the set 'fruits' using Redis?

easy

A. SISMEMBER(fruits, apple)

B. SISMEMBER 'fruits' 'apple'

C. SISMEMBER fruits, apple

D. SISMEMBER fruits apple

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use space-separated arguments without commas or parentheses.

2. Step 2: Match correct syntax

   Correct syntax is SISMEMBER key member, so SISMEMBER fruits apple is correct.

3. Final Answer:

   SISMEMBER fruits apple -> Option D

4. Quick Check:

   Redis commands use spaces, no commas or quotes needed [OK]

Hint: Use space-separated arguments without quotes or commas [OK]

Common Mistakes:

• Adding commas between arguments
• Using parentheses like a function call
• Adding quotes around arguments unnecessarily

3. Given the Redis set 'colors' contains {"red", "blue", "green"}, what is the output of SISMEMBER colors yellow?

medium

A. 0

B. 1

C. red

D. Error

Solution

1. Step 1: Check if 'yellow' is in the set 'colors'

   The set contains "red", "blue", and "green" but not "yellow".

2. Step 2: Understand SISMEMBER return value

   SISMEMBER returns 1 if member exists, else 0. Since "yellow" is not in the set, it returns 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Member absent = 0 [OK]

Hint: If value not in set, SISMEMBER returns 0 [OK]

Common Mistakes:

• Assuming it returns the value itself
• Confusing 0 with an error
• Thinking it returns the whole set

4. You run the command SISMEMBER colors without specifying a member. What will happen?

medium

A. Syntax error

B. Returns 1

C. Returns 0

D. Returns all members

Solution

1. Step 1: Check command argument requirements

   SISMEMBER requires exactly two arguments: the set key and the member to check.

2. Step 2: Analyze missing argument effect

   Omitting the member argument causes a syntax error in Redis.

3. Final Answer:

   Syntax error -> Option A

4. Quick Check:

   Missing arguments cause syntax error [OK]

Hint: SISMEMBER needs two arguments; missing one causes error [OK]

Common Mistakes:

• Expecting default member to be checked
• Thinking it returns 0 if member missing
• Assuming it returns all members

5. You have a Redis set 'users' with members {"alice", "bob", "carol"}. You want to check if 'dave' is a member and add him only if he is not already present. Which sequence of commands correctly does this?

hard

A. SISMEMBER users dave
SREM users dave

B. SISMEMBER users dave
SADD users dave

C. SADD users dave
SISMEMBER users dave

D. SREM users dave
SADD users dave

Solution

1. Step 1: Check membership before adding

   Use SISMEMBER to see if 'dave' is already in 'users'.

2. Step 2: Add 'dave' only if not present

   If SISMEMBER returns 0, then use SADD to add 'dave'.

3. Final Answer:

   SISMEMBER users dave SADD users dave -> Option B

4. Quick Check:

   Check membership then add if absent [OK]

Hint: Check with SISMEMBER before SADD to avoid duplicates [OK]

Common Mistakes:

• Adding before checking membership
• Removing member instead of adding
• Checking membership after adding