Set Operations

Server-side set algebra

Redis can compute intersection, union, and difference between two or more Sets entirely on the server. You never need to pull all members to the client and compute locally.

SINTER — intersection

SINTER key [key ...] returns only the members that appear in every listed Set. Classic use case: mutual friends.

127.0.0.1:6379> SADD user:1:follows "alice" "bob" "carol" "dave"
(integer) 4
127.0.0.1:6379> SADD user:2:follows "bob" "carol" "eve" "frank"
(integer) 4
127.0.0.1:6379> SINTER user:1:follows user:2:follows
1) "bob"
2) "carol"

SADD user:1:follows "alice" "bob" "carol" "dave"
SADD user:2:follows "bob" "carol" "eve" "frank"
SINTER user:1:follows user:2:follows

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SUNION — union

SUNION key [key ...] returns all members from any of the listed Sets, deduplicated. Use case: combined audience across campaigns.

127.0.0.1:6379> SADD campaign:A "user:1" "user:2" "user:3"
(integer) 3
127.0.0.1:6379> SADD campaign:B "user:2" "user:4" "user:5"
(integer) 3
127.0.0.1:6379> SUNION campaign:A campaign:B
1) "user:1"
2) "user:2"
3) "user:3"
4) "user:4"
5) "user:5"

SADD campaign:A "user:1" "user:2" "user:3"
SADD campaign:B "user:2" "user:4" "user:5"
SUNION campaign:A campaign:B

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SDIFF — difference

SDIFF key [key ...] returns members in the first Set that do not appear in any subsequent Set. Use case: find users in one group but not another.

127.0.0.1:6379> SADD plan:premium "user:1" "user:2" "user:3"
(integer) 3
127.0.0.1:6379> SADD notified:promo "user:2" "user:3"
(integer) 2
127.0.0.1:6379> SDIFF plan:premium notified:promo
1) "user:1"

SADD plan:premium "user:1" "user:2" "user:3"
SADD notified:promo "user:2" "user:3"
SDIFF plan:premium notified:promo

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SINTERSTORE, SUNIONSTORE, SDIFFSTORE

The *STORE variants write the result to a destination key instead of returning it. The destination is a regular Set you can inspect later.

127.0.0.1:6379> SADD group:eng "alice" "bob" "carol"
(integer) 3
127.0.0.1:6379> SADD group:ops "carol" "dave"
(integer) 2
127.0.0.1:6379> SINTERSTORE overlap:eng-ops group:eng group:ops
(integer) 1
127.0.0.1:6379> SMEMBERS overlap:eng-ops
1) "carol"
127.0.0.1:6379> SUNIONSTORE all:staff group:eng group:ops
(integer) 4
127.0.0.1:6379> SMEMBERS all:staff
1) "alice"
2) "bob"
3) "carol"
4) "dave"

SADD group:eng "alice" "bob" "carol"
SADD group:ops "carol" "dave"
SINTERSTORE overlap:eng-ops group:eng group:ops
SMEMBERS overlap:eng-ops
SUNIONSTORE all:staff group:eng group:ops
SMEMBERS all:staff

Copy commands ⧉

🟥 Set algebra runs server-side — no data transfer needed

With SINTER, SUNION, and SDIFF, Redis does all the computation in-process. For large Sets (millions of members), this avoids transferring the entire collection to your application server just to intersect or diff it locally. The *STORE variants go further: the result is materialized as a new key that other clients can read immediately.

Which command returns members present in ALL listed sets?

SUNION SDIFF SINTER SMEMBERS

What does SDIFF key1 key2 return?

Members in key2 but not key1 Members in key1 but not key2 Members in both key1 and key2 All members from key1 and key2

What is the effect of SINTERSTORE destination key1 key2?

Reads the intersection into a local variable Computes the intersection and stores it in destination Copies key1 and key2 into destination Returns the intersection without storing it

SUNION across two sets with some overlapping members returns how many copies of a shared member?

Two Zero One It depends on insertion order

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