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Database/Redis.md

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+# [Redis](https://redis.io/)
+
+Redis is in the family of databases called **key-value stores**.
+
+The essence of a key-value store is the ability to store some data, called a value, inside a key. This data can later be retrieved only if we know the exact key used to store it.
+
+Often Redis it is called a *data structure* server because it has outer key-value shell, but each value can contain a complex data structure, such as a string, a list, a hashes, or ordered data structures called sorted sets as well as probabilistic data structures like *hyperloglog*.
+
+## [Redis Commands](https://redis.io/commands)
+
+### Server Startup
+
+```bash
+redis-server  # start the server
+redis-cli
+```
+
+### [Key-Value Pairs](https://redis.io/commands#generic)
+
+```sh
+SET <key> <value> [ EX <seconds> ] # store a key-value pair, TTL optional
+GET <key>  # read a key content
+EXISTS <key>  # check if a key exists
+DEL <key>  # delete a key-value pair
+
+INCR <key>  # atomically increment a number stored at a given key
+INCRBY <key> <amount>  # increment the number contained inside a key by a specific amount
+DECR <key>
+DECRBY <key> <amount>
+
+# re-setting the key will make it permanent (TTL -1)
+EXPIRE <key> <seconds>  # make the key expire after <second> seconds
+TTL <key>  # see remaining seconds before expiry
+PEXPIRE <key> <seconds>  # make the key expire after <second> milli-seconds
+PTTL <key>  # see remaining milli-seconds before expiry
+PERSIST <key>  # make the key permanent
+```
+
+### [Lists](https://redis.io/commands#list)
+
+A list is a series of ordered values.
+
+```sh
+RPUSH <key> <value1> <value2> ... # add one or more values to the end of the list
+LPUSH <key> <value1> <value2> ...  # add one or more values to the start of a list
+
+LLEN  # number of items in the list
+LRANGE <key> <start_index> <end_index>  # return a subset of the list, end index included. Negative indexes caout backwards from the end
+
+LPOP  # remove and return the first item fro the list
+RPOP  # remove and return the last item fro the list
+```
+
+### [Sets](https://redis.io/commands#set)
+
+A set is similar to a list, except it does not have a specific order and each element may only appear once.
+
+```sh
+SADD <key> <value1> <value2> ... # add one or more values to the set (retunr 0 if values are alredy inside)
+SREM <key> <value>  # remove the given member from the set, return 1 or 0 to signal if the member was actually there or not.
+SPOP <key> <value>  # remove and return value from the set
+
+SISMEMBER <key> <value>  # test if value is in the set
+SMEMBERS <key>  # lis of all set items
+
+SUINION <key1> <key2> ...  # combine two or more sets and return the list of all elements.
+```
+
+### [Sorted Sets](https://redis.io/commands#sorted_set)
+
+Sets are a very handy data type, but as they are unsorted they don't work well for a number of problems. This is why Redis 1.2 introduced Sorted Sets.
+
+A sorted set is similar to a regular set, but now each value has an associated score. This score is used to sort the elements in the set.
+
+```sh
+ZADD <key> <score> <value>  # add a value with it's score
+
+ZRANGE <key> <start_index> <end_index>  # return a subset of the sortedSet
+
+...
+```
+
+### [Hashes](https://redis.io/commands#hash)
+
+Hashes are maps between string fields and string values, so they are the perfect data type to represent objects.
+
+```sh
+HSET <key> <field> <value> [ <field> <value> ... ] # set the string of a hash field
+HSETNX <key> <field> <value>  # set the value of a hash field, only if the field does not exist
+
+HEXISTS <key> <field>  # determine if a hash field exists
+
+HLEN <key>  # get the number of fields in a hash
+HSTRLEN <key> <field>  # get the length of the value of a hash field
+HGETALL <key>  # get all fields and values in a hash
+HGET <key> <field>  # get data on a single field
+HKEYS <key>  # get all the fields in a hash
+HVALS <key>  # get all the values in a hash
+
+HDEL <key> <field_1> <field_2> ...  # delete one or more field hases
+
+HMGET <key> <field> [<field> ...]  # get the values of all the given hash fields
+HMSET <key> <field> <value> [<field> <value> ...]  # set multiple hash fields to multiple values
+
+HINCRBY <key> <field> <amount>  # increment the integer value of a hash field by the given number
+HINCRBYFLOAT <key> <field> <amount> # increment the float value of a hash field by the given amount
+
+HSCAN <key> <cursor> [MATCH <pattern>] [COUNT <count>]  # incrementally iterate hash fields and associated values
+```