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+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 (Redis 3.0.2 or greater)
+
+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. Never add elements.
+- **NX**: Don't update already existing elements. Always add 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.
+
+## Range of integer scores that can be expressed precisely
+
+Redis 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][tdtss].
+
+[tdtss]: /topics/data-types#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`).
+
+@return
+
+@integer-reply, specifically:
+
+- The number of elements added to the sorted set, not including elements already
+  existing for which the score was updated.
+
+If the `INCR` option is specified, the return value will be @bulk-string-reply:
+
+- The new score of `member` (a double precision floating point number)
+  represented as string, or `nil` if the operation was aborted (when called with
+  either the `XX` or the `NX` option).
+
+@history
+
+- `>= 2.4`: Accepts multiple elements. In Redis versions older than 2.4 it was
+  possible to add or update a single member per call.
+
+@examples
+
+```cli
+ZADD myzset 1 "one"
+ZADD myzset 1 "uno"
+ZADD myzset 2 "two" 3 "three"
+ZRANGE myzset 0 -1 WITHSCORES
+```