PostgreSQL is an object-relational database management system (ORDBMS) based on POSTGRES, Version 4.2, developed at the University of California at Berkeley Computer Science Department. POSTGRES pioneered many concepts that only became available in some commercial database systems much later.
PostgreSQL is an open-source descendant of this original Berkeley code. It supports a large part of the SQL standard and offers many modern features:
- complex queries
- foreign keys
- updatable views
- transactional integrity
- multiversion concurrency control
Also, PostgreSQL can be extended by the user in many ways, for example by adding new
- data types
- aggregate functions
- index methods
- procedural languages
To follow along with this tutorial, you will need one Ubuntu 18.04 server that has been configured by following our Initial Server Setup for Ubuntu 18.04 guide. After completing this prerequisite tutorial, your server should have a non-root user with sudo permissions and a basic firewall.
Step 1 — Installing PostgreSQL
Ubuntu’s default repositories contain Postgres packages, so you can install these using the
apt packaging system.
Since this is your first time using
apt in this session, refresh your local package index. Then, install the Postgres package along with a
-contrib package that adds some additional utilities and functionality:
sudo apt update sudo apt install postgresql postgresql-contrib
Now that the software is installed, we can go over how it works and how it may be different from similar database management systems you may have used.
Step 2 — Using PostgreSQL Roles and Databases
By default, Postgres uses a concept called “roles” to handle in authentication and authorization. These are, in some ways, similar to regular Unix-style accounts, but Postgres does not distinguish between users and groups and instead prefers the more flexible term “role”.
Upon installation, Postgres is set up to use ident authentication, meaning that it associates Postgres roles with a matching Unix/Linux system account. If a role exists within Postgres, a Unix/Linux username with the same name is able to sign in as that role.
The installation procedure created a user account called postgres that is associated with the default Postgres role. In order to use Postgres, you can log into that account.
There are a few ways to utilize this account to access Postgres.
Switching Over to the postgres Account
Switch over to the postgres account on your server by typing:
sudo -i -u postgres
You can now access a Postgres prompt immediately by typing:
This will log you into the PostgreSQL prompt, and from here you are free to interact with the database management system right away.
Exit out of the PostgreSQL prompt by typing:
This will bring you back to the
postgres Linux command prompt.
Accessing a Postgres Prompt Without Switching Accounts
You can also run the command you’d like with the postgres account directly with
For instance, in the last example, you were instructed to get to the Postgres prompt by first switching to the postgres user and then running
psql to open the Postgres prompt. You could do this in one step by running the single command
psql as the postgres user with
sudo, like this:
sudo -u postgres psql
This will log you directly into Postgres without the intermediary
bash shell in between.
Again, you can exit the interactive Postgres session by typing:
Many use cases require more than one Postgres role. Read on to learn how to configure these.
Step 3 — Creating a New Role
Currently, you just have the postgres role configured within the database. You can create new roles from the command line with the
createrole command. The
--interactive flag will prompt you for the name of the new role and also ask whether it should have superuser permissions.
If you are logged in as the postgres account, you can create a new user by typing:
If, instead, you prefer to use
sudo for each command without switching from your normal account, type:
sudo -u postgres createuser --interactive
The script will prompt you with some choices and, based on your responses, execute the correct Postgres commands to create a user to your specifications.
OutputEnter name of role to add: sammy Shall the new role be a superuser? (y/n) y
You can get more control by passing some additional flags. Check out the options by looking at the
Your installation of Postgres now has a new user, but you have not yet added any databases. The next section describes this process.
Step 4 — Creating a New Database
Another assumption that the Postgres authentication system makes by default is that for any role used to log in, that role will have a database with the same name which it can access.
This means that, if the user you created in the last section is called sammy, that role will attempt to connect to a database which is also called “sammy” by default. You can create the appropriate database with the
If you are logged in as the postgres account, you would type something like:
If, instead, you prefer to use
sudo for each command without switching from your normal account, you would type:
sudo -u postgres createdb sammy
This flexibility provides multiple paths for creating databases as needed.
Step 5 — Opening a Postgres Prompt with the New Role
To log in with
ident based authentication, you’ll need a Linux user with the same name as your Postgres role and database.
If you don’t have a matching Linux user available, you can create one with the
adduser command. You will have to do this from your non-root account with
sudo privileges (meaning, not logged in as the postgres user):
sudo adduser sammy
Once this new account is available, you can either switch over and connect to the database by typing:
sudo -i -u sammy psql
Or, you can do this inline:
sudo -u sammy psql
This command will log you in automatically, assuming that all of the components have been properly configured.
If you want your user to connect to a different database, you can do so by specifying the database like this:
psql -d postgres
Once logged in, you can get check your current connection information by typing:
OutputYou are connected to database "sammy" as user "sammy" via socket in "/var/run/postgresql" at port "5432".
This is useful if you are connecting to non-default databases or with non-default users.
Step 6 — Creating and Deleting Tables
Now that you know how to connect to the PostgreSQL database system, you can learn some basic Postgres management tasks.
First, create a table to store some data. As an example, a table that describes some playground equipment.
The basic syntax for this command is as follows:
CREATE TABLE table_name ( column_name1 col_type (field_length) column_constraints, column_name2 col_type (field_length), column_name3 col_type (field_length) );
As you can see, these commands give the table a name, and then define the columns as well as the column type and the max length of the field data. You can also optionally add table constraints for each column.
You can learn more about how to create and manage tables in Postgres here.
For demonstration purposes, create a simple table like this:
CREATE TABLE playground ( equip_id serial PRIMARY KEY, type varchar (50) NOT NULL, color varchar (25) NOT NULL, location varchar(25) check (location in ('north', 'south', 'west', 'east', 'northeast', 'southeast', 'southwest', 'northwest')), install_date date );
These commands will create a table that inventories playground equipment. This starts with an equipment ID, which is of the
serial type. This data type is an auto-incrementing integer. You’ve also given this column the constraint of
primary key which means that the values must be unique and not null.
For two of the columns (
install_date), the commands do not specify a field length. This is because some column types don’t require a set length because the length is implied by the type.
The next two commands create columns for the equipment
color respectively, each of which cannot be empty. The command after these creates a
location column and create a constraint that requires the value to be one of eight possible values. The last command creates a date column that records the date on which you installed the equipment.
You can see your new table by typing:
Output List of relations Schema | Name | Type | Owner --------+-------------------------+----------+------- public | playground | table | sammy public | playground_equip_id_seq | sequence | sammy (2 rows)
Your playground table is here, but there’s also something called
playground_equip_id_seq that is of the type
sequence. This is a representation of the
serial type which you gave your
equip_id column. This keeps track of the next number in the sequence and is created automatically for columns of this type.
If you want to see just the table without the sequence, you can type:
Output List of relations Schema | Name | Type | Owner --------+------------+-------+------- public | playground | table | sammy (1 row)
Step 7 — Adding, Querying, and Deleting Data in a Table
Now that you have a table, you can insert some data into it.
As an example, add a slide and a swing by calling the table you want to add to, naming the columns and then providing data for each column, like this:
INSERT INTO playground (type, color, location, install_date) VALUES ('slide', 'blue', 'south', '2017-04-28'); INSERT INTO playground (type, color, location, install_date) VALUES ('swing', 'yellow', 'northwest', '2018-08-16');
You should take care when entering the data to avoid a few common hangups. For one, do not wrap the column names in quotation marks, but the column values that you enter do need quotes.
Another thing to keep in mind is that you do not enter a value for the
equip_id column. This is because this is automatically generated whenever a new row in the table is created.
Retrieve the information you’ve added by typing:
SELECT * FROM playground;
Output equip_id | type | color | location | install_date ----------+-------+--------+-----------+-------------- 1 | slide | blue | south | 2017-04-28 2 | swing | yellow | northwest | 2018-08-16 (2 rows)
Here, you can see that your
equip_id has been filled in successfully and that all of your other data has been organized correctly.
If the slide on the playground breaks and you have to remove it, you can also remove the row from your table by typing:
DELETE FROM playground WHERE type = 'slide';
Query the table again:
SELECT * FROM playground;
Output equip_id | type | color | location | install_date ----------+-------+--------+-----------+-------------- 2 | swing | yellow | northwest | 2018-08-16 (1 row)
You notice that your slide is no longer a part of the table.
Step 8 — Adding and Deleting Columns from a Table
After creating a table, you can modify it to add or remove columns relatively easily. Add a column to show the last maintenance visit for each piece of equipment by typing:
ALTER TABLE playground ADD last_maint date;
If you view your table information again, you will see the new column has been added (but no data has been entered):
SELECT * FROM playground;
Output equip_id | type | color | location | install_date | last_maint ----------+-------+--------+-----------+--------------+------------ 2 | swing | yellow | northwest | 2018-08-16 | (1 row)
Deleting a column is just as simple. If you find that your work crew uses a separate tool to keep track of maintenance history, you can delete of the column by typing:
ALTER TABLE playground DROP last_maint;
This deletes the
last_maint column and any values found within it, but leaves all the other data intact.
Step 9 — Updating Data in a Table
So far, you’ve learned how to add records to a table and how to delete them, but this tutorial hasn’t yet covered how to modify existing entries.
You can update the values of an existing entry by querying for the record you want and setting the column to the value you wish to use. You can query for the “swing” record (this will match every swing in your table) and change its color to “red”. This could be useful if you gave the swing set a paint job:
UPDATE playground SET color = 'red' WHERE type = 'swing';
You can verify that the operation was successful by querying the data again:
SELECT * FROM playground;
Output equip_id | type | color | location | install_date ----------+-------+-------+-----------+-------------- 2 | swing | red | northwest | 2010-08-16 (1 row)
As you can see, your slide is now registered as being red.