In Databases By Leigh Halliday December 07, 2014 Leigh Halliday

The Basics of PSQL

Overview

PSQL is the command line tool for accessing the PostgreSQL database. I recommend anyone using Postgres to at least learn the basics of how to navigate around and feel comfortable working in this tool. Even for those used to only dealing with data through an ORM like ActiveRecord in Rails, or for those who prefer Navicat for querying data, learning a simple tool like PSQL will go a long way. The next time you're on the server and there is no GUI in site, you'll feel comfortable instead of stressed.

Connecting and Exiting

To connect to PSQL, unsurprisingly, you enter the command psql from the command line. Once you've done that, you'll be plopped into a screen that looks like this:

psql (9.3.5)
Type "help" for help.

lhalliday=# 

This is where it all begins and where we'll enter all the commands from here on out. At any point if you wish to exit the PSQL shell, simply type \q.

Navigating Databases

In PostgreSQL, there is a hierarchy to the data:

  • Databases
    • Tables
      • Columns

You can think of databases of a bucket for all the data relating to a website. To see which databases are available to you, type the \l command. You'll see something similar to the following:

lhalliday=# \l
                                       List of databases
       Name        |   Owner   | Encoding |   Collate   |    Ctype    |    Access privileges    
-------------------+-----------+----------+-------------+-------------+-------------------------
 flipgive_dev      | lhalliday | UTF8     | en_CA.UTF-8 | en_CA.UTF-8 | 
 flipgive_test1    | lhalliday | UTF8     | en_CA.UTF-8 | en_CA.UTF-8 | 
 idioma            | lhalliday | UTF8     | en_CA.UTF-8 | en_CA.UTF-8 | 
 idioma_test       | lhalliday | UTF8     | en_CA.UTF-8 | en_CA.UTF-8 | 

To choose which database you'd like to work with you use the \c db_name command.

lhalliday=# \c paisita
You are now connected to database "paisita" as user "lhalliday".
paisita=# 

Navigating Tables

Now that we're inside the paisita database. Let's see which tables are available to us. To do that we use the \dt command.

paisita=# \dt
                 List of relations
 Schema |         Name          | Type  |   Owner   
--------+-----------------------+-------+-----------
 public | categories            | table | lhalliday
 public | category_translations | table | lhalliday
 public | idioma_phrases        | table | lhalliday
 public | post_translations     | table | lhalliday
 public | posts                 | table | lhalliday
 public | schema_migrations     | table | lhalliday
 public | setting_translations  | table | lhalliday
 public | settings              | table | lhalliday
 public | uploads               | table | lhalliday
 public | user_translations     | table | lhalliday
 public | users                 | table | lhalliday
(11 rows)

To see the details of a table, you simply type \d table_name.

paisita=# \d users
                                                Table "public.users"
             Column              |            Type             |                     Modifiers                      
---------------------------------+-----------------------------+----------------------------------------------------
 id                              | integer                     | not null default nextval('users_id_seq'::regclass)
 name                            | character varying           | not null
 email                           | character varying           | not null
 linkedin                        | character varying           | 
 twitter                         | character varying           | 
 crypted_password                | character varying           | not null
 salt                            | character varying           | not null
 remember_me_token               | character varying           | 
 remember_me_token_expires_at    | timestamp without time zone | 
 reset_password_token            | character varying           | 
 reset_password_token_expires_at | timestamp without time zone | 
 reset_password_email_sent_at    | timestamp without time zone | 
 upload_id                       | integer                     | 
 github                          | character varying           | 
 snippet                         | text                        | 
Indexes:
    "users_pkey" PRIMARY KEY, btree (id)
    "index_users_on_remember_me_token" btree (remember_me_token)
    "index_users_on_reset_password_token" btree (reset_password_token)

To select or perform SQL within the console, you simply write it as normal, remembering to end your statements with a ;

paisita=# select count(*) from users;
 count 
-------
     1
(1 row)