Added two books

Backend/PostgreSQL/PostgreSQL.txt

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+> SEMI COLON IS IMPORTANT
+
+## To start the SQL server
+
+1. psql -U postgres -h localhost
+2. Enter the password
+
+## Create a new User for Database
+
+1. CREATE USER <user_name> WITH PASSWORD  <'password'>
+
+## Create a new User for Database and grant all privilegs
+
+1. CREATE DATABASE <database name>
+2. GRANT ALL PRIVILAGES ON DATABASE <database name> to <user_name>
+
+## Basic Commands
+
+1.  /l → Lists all Databases
+2. DROP → { DROP DATABASE <database_name> } → Deletes the database
+3. \c →  { \c <database_name> } Connect the database
+
+## Table operations
+
+1. CREATE TABLE  <table_name> ( 
+- <column_name -1> <Type> <primary key , not null etc> ,
+
+    ID INT NOT NULL , 
+
+    NAME CHAR[50],
+
+    AGE INT,
+
+    ADDRESS TEXT
+
+     );
+
+1. \d → Lists all tables within a database
+2. \d <table_name> → description of entire table

Backend/PostgreSQL/README.md

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+## Redis Basics
+
+Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache, and message broker. Redis provides data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes, and streams. Redis has built-in replication, Lua scripting, LRU eviction, transactions, and different levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic partitioning with Redis Cluster.
+
+## To start the Redis server
+
+1. redis-server
+2. redis-cli
+
+## Notes
+
+{key :"value"} → value is always a string (key value pair)
+
+Numbers ⇒ "10020"
+
+Objects ⇒ {key : "{ a : '1' }" }
+
+Array ⇒ {key : "[1,2,3,4]" }
+
+---
+
+## Basic Commands
+
+1. SET → SET <key> <value>
+2. GET → GET <key>
+3. DEL → DEL <key>
+4. EXISTS → EXISTS <key>
+5. KEYS \* → displays all available keys
+6. FLUSHALL → Deletes everything
+7. TTL → Time to live { TTL <key> }
+8. EXPIRE → EXPIRE <key> <duration(in sec)>
+9. SETEX → SETEX <key> <seconds> <value>
+
+---
+
+## ARRAY OPERATIONS
+
+1. LPUSH → insert from left side → LPUSH <key> <Elements....>
+2. RPUSH → insert from right side → RPUSH <key> <Elements....>
+3. LPOP → Delete from left → LPOP <key> <count>
+4. RPOP →Delete from right → RPOP <key> <count>
+5. LRANGE → Iterating through the array → LRANGE <start index {eg: 0}> <end index {eg : -1}>
+6. SADD → Allows only unique String elements in the array ( tip : if you have to insert words with space write it within double quotes else it will take it as 2 words ; by default it takes input as Strings
+
+   { SADD <key> <members ..>
+
+7. SMEMBERS → SMEMBERS <key>
+
+---
+
+## HASHES (kind of objects)
+
+1. HSET → HSET <key> <field i.e. attribute name eg roll , name etc. > <value>
+2. HGET → HGET <key> <field>
+3. HGETALL → HGET <key>
+4. HDEL → HDEL <key> <field>
+5. HEXISTS → HEXISTS <key> <field> → returns 0 or 1
+
+## PostgreSQL Commands
+
+> SEMI COLON IS IMPORTANT
+
+## To start the SQL server
+
+1. psql -U postgres -h localhost
+2. Enter the password
+
+## Create a new User for Database
+
+1. CREATE USER <user_name> WITH PASSWORD <'password'>
+
+## Create a new User for Database and grant all privilegs
+
+1. CREATE DATABASE <database_name>
+2. GRANT ALL PRIVILEGES ON DATABASE <database_name> to <user_name>
+
+## Basic Commands
+
+1.  /l → Lists all Databases
+2.  DROP → { DROP DATABASE <database_name> } → Deletes the database
+3.  \c → { \c <database_name> } Connect the database
+
+## Table operations
+
+1. CREATE TABLE <table_name> (
+
+- <column_name -1> <Type> <primary key , not null etc> ,
+
+  ID INT NOT NULL ,
+
+  NAME CHAR[50],
+
+  AGE INT,
+
+  ADDRESS TEXT
+
+  );
+
+1. \d → Lists all tables within a database
+2. \d <table_name> → description of entire table