Question

I added six data to sorted set. Then I use zinterstore to get expected result. but there is 0 result.

zadd la.41 41 kfc
zadd la.42 42 mdl
zadd la.43 43 apple

zadd lo.77 77 apple
zadd lo.78 78 mdl
zadd lo.79 79 kfc

zinterstore close 6 la.41 la.42 la.43 lo.77 lo.78 lo.79

expected result:

kfc
mdl
apple

actual result:

0

why ? thanks.

Answer 1

It looks like you either you’re misunderstanding ZADD and creating more sets than you mean to, or you’re using intersection when you should be using union. Let’s look at both:

Creating fewer sets

One potential problem is that you’re creating six sets, and instead you might want to create two sets. In set notation, you’re doing this:

la.41 = {kfc}
la.42 = {mdl}
la.43 = {apple}

When instead you might want to be doing this:

la = {kfc, mdl, apple}

If this is what you’re intending to do, then you’ve misunderstood the ZADD command.

ZADD takes a key, a score, and a member. You can think of the key as the name of the set, the member as the thing you’re adding to the set, and the score as the member’s position in the set.

For example, if I run the command ZADD foo 1 a then I have created a set called foo which has the member a with a score of 1. Ignoring the score for a second, the situation is now this:

foo = {a}

If I now run ZADD foo 2 b then I’ve added the member b to the existing set foo:

foo = {a, b}

You are using a different key in each of your ZADD commands, so you’re creating a new set with each command. If you use the same key, and therefore the same set:

ZADD la 41 kfc
ZADD la 42 mdl

You’ll end up creating a single set:

la = {kfc, mdl}

Union versus intersection

The alternative is that you do want to create six sets, but you want the union and not the intersection.

The union of two sets is a set containing all of the elements that were present in either of the original sets:

{1, 2} ∪ {2, 3} = {1, 2, 3}

The intersection of two sets is a set containing all of the elements that were present in both of the original sets:

{1, 2} ∩ {2, 3} = {2}

You’re creating six sets, which have no common elements and then taking the intersection of them:

{kfc} ∩ {mdl} ∩ {apple} ∩ {kfc} ∩ {mdl} ∩ {apple} = {}

If you were taking the union instead you would end up with all of the elements in the resulting set:

{kfc} ∪ {mdl} ∪ {apple} ∪ {kfc} ∪ {mdl} ∪ {apple} = {kfc, mdl, apple}

You can achieve this by using the ZUNIONSTORE Redis command instead of ZINTERSTORE.