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Create PostgreSQL Database In Linux And Connect To R

PostgreSQL is a commonly used database language for creating and managing large amounts of data effectively.

Here, you will see how to:

1) create a PostgreSQL database using the Linux terminal

2) connect the PostgreSQL database to R using the “RpostgreSQL” library

Creating our PostgreSQL database

In this example, we are going to create a simple database containing a table of dates, cities, and average temperature in degrees (Celsius).

We will name our database weather, and our table cities.

Once we open a Linux terminal, we enter the following:

sudo -u postgres createdb weather
sudo -u postgres psql weather

This creates our weather database. We then enter \password to set a password:

\password

Once prompted, we will enter a password rainbow, or whatever password you prefer:

Enter new password: rainbow

Enter it again: rainbow 

We then enter psql to initiate PostgreSQL and \conninfo to test the connection:

stocks=# psql

stocks-# \conninfo

You are connected to database "weather" as user "postgres" via socket in "/var/run/postgresql" at port "5432".

Create Table for Database

Once we have verified our connection, we can now create the table. As mentioned, we will name our table cities, and include the fields datecity_name, and averagetemp_celsius. We will define each variable as DATEVARCHAR(50), and DECIMAL(7,2) respectively.

CREATE TABLE cities (date DATE,     
city_name VARCHAR(50),
averagetemp_celsius DECIMAL(7,2) );

Once that’s done, we can then insert the appropriate values into our table:

INSERT INTO cities VALUES ('02/08/2017', 'New York', 25), 
('02/08/2017','Los Angeles',28),
('03/08/2017','London',18),
('03/08/2017','Paris',22),
('03/08/2017','Los Angeles',28),
('04/08/2017','Berlin',18),
('04/08/2017','Tokyo',25),
('05/08/2017','Zurich',24),
('05/08/2017','Shanghai',29);

Connect database to R using RPostgreSQL

Firstly, we install the RPostgreSQL package:

install.packages("RPostgreSQL")
require("RPostgreSQL")

Then, we define the database password within R:
pw <- {   "rainbow" } 

We define our driver and set up the connection:

drv <- dbDriver("PostgreSQL")  
con <- dbConnect(drv, dbname = "weather",
host = "localhost",
port = 5432,
user = "postgres", password = pw)

rm(pw)
We verify that the “cities” table exists:
dbExistsTable(con, "cities")
Once verified, we read our table into R using dbReadTable:
myTable <- dbReadTable(con,c("cities"))
attach(myTable)

And there’s our table – we’ve successfully connected our PostgreSQL database to R! 

dbSendQuery: Editing database directly from R

Now that we've created our database, what if we wish to edit directly from R? For instance, suppose that we have a new row of values that we would like to input. Instead of having to access the PostgreSQL database directly every time, we would like to send a query from R that does this for us.

Let us suppose that we wish to insert the following three values: '06/08/2017', 'Sao Paulo', '16'.

To do this, we create a variable res that uses a dbSendQuery command to send an INSERT INTO query as follows:

res <- dbSendQuery(con, statement=paste("INSERT INTO 
cities (date,
city_name,
averagetemp_celsius)
VALUES ('06/08/2017', 'Sao Paulo', '16')"));

Once we have sent the query, we now see that upon opening the associated data frame in R, our entry for Sao Paulo has been included and our query has been committed!

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