ZADD key [NX | XX] [GT | LT] [CH] [INCR] data
Available since
Time complexity
O(log(N)) for each item added, where N is the number of elements in the sorted set.
ACL categories
@write, @sortedset, @fast

Adds all the specified members with the specified scores to the sorted set stored at key. It is possible to specify multiple score / member pairs. If a specified member is already a member of the sorted set, the score is updated and the element reinserted at the right position to ensure the correct ordering.

If key does not exist, a new sorted set with the specified members as sole members is created, like if the sorted set was empty. If the key exists but does not hold a sorted set, an error is returned.

The score values should be the string representation of a double precision floating point number. +inf and -inf values are valid values as well.

ZADD options #

ZADD supports a list of options, specified after the name of the key and before the first score argument. Options are:

  • XX: Only update elements that already exist. Don’t add new elements.
  • NX: Only add new elements. Don’t update already existing elements.
  • LT: Only update existing elements if the new score is less than the current score. This flag doesn’t prevent adding new elements.
  • GT: Only update existing elements if the new score is greater than the current score. This flag doesn’t prevent adding new elements.
  • CH: Modify the return value from the number of new elements added, to the total number of elements changed (CH is an abbreviation of changed). Changed elements are new elements added and elements already existing for which the score was updated. So elements specified in the command line having the same score as they had in the past are not counted. Note: normally the return value of ZADD only counts the number of new elements added.
  • INCR: When this option is specified ZADD acts like ZINCRBY. Only one score-element pair can be specified in this mode.

Note: The GT, LT and NX options are mutually exclusive.

Range of integer scores that can be expressed precisely #

Redict sorted sets use a double 64-bit floating point number to represent the score. In all the architectures we support, this is represented as an IEEE 754 floating point number, that is able to represent precisely integer numbers between -(2^53) and +(2^53) included. In more practical terms, all the integers between -9007199254740992 and 9007199254740992 are perfectly representable. Larger integers, or fractions, are internally represented in exponential form, so it is possible that you get only an approximation of the decimal number, or of the very big integer, that you set as score.

Sorted sets 101 #

Sorted sets are sorted by their score in an ascending way. The same element only exists a single time, no repeated elements are permitted. The score can be modified both by ZADD that will update the element score, and as a side effect, its position on the sorted set, and by ZINCRBY that can be used in order to update the score relatively to its previous value.

The current score of an element can be retrieved using the ZSCORE command, that can also be used to verify if an element already exists or not.

For an introduction to sorted sets, see the data types page on sorted sets.

Elements with the same score #

While the same element can’t be repeated in a sorted set since every element is unique, it is possible to add multiple different elements having the same score. When multiple elements have the same score, they are ordered lexicographically (they are still ordered by score as a first key, however, locally, all the elements with the same score are relatively ordered lexicographically).

The lexicographic ordering used is binary, it compares strings as array of bytes.

If the user inserts all the elements in a sorted set with the same score (for example 0), all the elements of the sorted set are sorted lexicographically, and range queries on elements are possible using the command ZRANGEBYLEX (Note: it is also possible to query sorted sets by range of scores using ZRANGEBYSCORE).

Examples #

ZADD myzset 1 "one"
ZADD myzset 1 "uno"
ZADD myzset 2 "two" 3 "three"

History #

Accepts multiple elements.
Added the `XX`, `NX`, `CH` and `INCR` options.
Added the `GT` and `LT` options.

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