Redis ZUNION Command Explained

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In Redis, the ZUNION command returns the union of the specified sorted sets.

Syntax

The syntax goes like this:

ZUNION numkeys key [key ...] [WEIGHTS weight [weight ...]]
  [AGGREGATE <SUM | MIN | MAX>] [WITHSCORES]

The following examples demonstrate how the arguments are used.

Example

Suppose we create the following sorted sets:

ZADD cats 1 meow 2 fluffy 3 scratch

And:

ZADD dogs 1 bark 2 woof 3 fluffy 4 scratch

Let’s use the ZUNION command to return the union of those two sorted sets:

ZUNION 2 cats dogs

Result:

1) "bark"
2) "meow"
3) "woof"
4) "fluffy"
5) "scratch"

As expected, it returned the union of the two given sorted sets. We can see that both fluffy and scratch appear in both sorted sets, but each of these only appear once in the result.

Include the Scores

We can use the WITHSCORES argument to include the scores:

ZUNION 2 cats dogs WITHSCORES

Result:

 1) "bark"
 2) "1"
 3) "meow"
 4) "1"
 5) "woof"
 6) "2"
 7) "fluffy"
 8) "5"
 9) "scratch"
10) "7"

By default, the scores for each member are added together from their respective sorted sets. In this example, fluffy has a score of 5 (2 from cats plus 3 from dogs) and scratch has a score of 7 (3 from cats and 4 from dogs).

The AGGREGATE Argument

In the previous example we saw how the scores were added together to produce an aggregate score. We can change this with the AGGREGATE argument so that it returns either the minimum or maximum score from the original sorted set.

To do this we use the AGGREGATE keyword followed by the option we want to use.

We can use MIN for “minimum”:

ZUNION 2 cats dogs AGGREGATE MIN WITHSCORES

Result:

 1) "bark"
 2) "1"
 3) "meow"
 4) "1"
 5) "fluffy"
 6) "2"
 7) "woof"
 8) "2"
 9) "scratch"
10) "3"

And MAX for “maximum”:

ZUNION 2 cats dogs AGGREGATE MAX WITHSCORES

Result:

 1) "bark"
 2) "1"
 3) "meow"
 4) "1"
 5) "woof"
 6) "2"
 7) "fluffy"
 8) "3"
 9) "scratch"
10) "4"

We can also explicitly state SUM for the default behaviour of adding the scores:

ZUNION 2 cats dogs AGGREGATE SUM WITHSCORES

Result:

 1) "bark"
 2) "1"
 3) "meow"
 4) "1"
 5) "woof"
 6) "2"
 7) "fluffy"
 8) "5"
 9) "scratch"
10) "7"

The WEIGHTS Argument

We can use the WEIGHTS argument to specify a multiplication factor for each input sorted set. This means that the score of every element in the input sorted set is multiplied by this factor before being passed to the aggregation function.

We can apply a different weighting to each sorted set.

Example:

ZUNION 2 cats dogs WEIGHTS 100 100 WITHSCORES

Result:

 1) "bark"
 2) "100"
 3) "meow"
 4) "100"
 5) "woof"
 6) "200"
 7) "fluffy"
 8) "500"
 9) "scratch"
10) "700"

In this example, I specified that the members of both sets should be multiplied by 100 and the result reflects this.

Here’s an example of applying a different weighting to each set:

ZUNION 2 cats dogs WEIGHTS 100 200 WITHSCORES

Result:

 1) "meow"
 2) "100"
 3) "bark"
 4) "200"
 5) "woof"
 6) "400"
 7) "fluffy"
 8) "800"
 9) "scratch"
10) "1100"

This time each element in the cats sorted set was multiplied by 100, while each element in the dogs sorted set was multiplied by 200.

The default multiplication factor is 1.

When the Keys Don’t Exist

If the keys don’t exist we get an empty array:

ZUNION 3 oops1 oops2 oops3

Result:

(empty array)

But if only one key exists, and it contains a sorted set, we get the members of that sorted set:

ZUNION 3 cats oops2 oops3

Result:

1) "meow"
2) "fluffy"
3) "scratch"

We can use the EXISTS command to check for the existence of a key:

EXISTS oops1

Result:

(integer) 0

An integer reply of zero means that it doesn’t exist.

Wrong Data Type

If any of the keys contain the wrong data type, an error occurs:

ZUNION 2 countries places

Result:

(error) WRONGTYPE Operation against a key holding the wrong kind of value

In my case, the places key contains a list and so we get the error.

We can use the TYPE command to check the key’s data type:

TYPE places

Result:

list

As suspected, it’s a list.

Actually, the countries key contains a set (not a sorted set) but it isn’t the culprit. Let’s check its type:

TYPE countries

Result:

set

The reason I say it’s not the culprit is because we can use it as the sole argument without any errors occurring:

ZUNION 1 countries

Result:

1) "Australia"
2) "New Zealand"

Conversely, if we pass the places list as the sole argument, we get the error:

ZUNION 1 places

Result:

(error) WRONGTYPE Operation against a key holding the wrong kind of value