PostgreSQL 101: Getting Started! (Part 1)

PostgreSQL

An object-relational database system

I. Installation

A. Mac OSX:

brew install postgresql

B. Ubuntu

sudo apt-get update
sudo apt-get install postgresql postgresql-contrib

II. Console Commands

A. Connecting to PostgreSQL Server

To connect to the PostgreSQL server with as user postgres:

psql -U postgres

By default, psql connects to a PostgreSQL server running on localhost at port 5432. To connect to a different port and/or host. Add the -p and -h tag:

psql -U postgres -p 12345 -h 192.32.123.32

Once in, you may navigate via the following commands:

• \l – list databases
• \c – change databases
• \d – list tables
• \df – list functions
• \df – list functions with definitions
• \q – quit

III. Database Creation

CREATE DATABASE < database name >;

# Creates database with name: test_db
CREATE DATABASE test_db

IV. Database Drop

DROP DATABASE < database name >;

 # Drops database with name: test_db
DROP DATABASE test_db

V. Table Creation

CREATE TABLE programs(
  programid SERIAL PRIMARY KEY,
  degree CHARACTER VARYING,
  program CHARACTER VARYING
);

CREATE TABLE students(
  studentid SERIAL PRIMARY KEY,
  student_number CHARACTER VARYING UNIQUE,
  first_name CHARACTER VARYING,
  last_name CHARACTER VARYING,
  programid INTEGER REFERENCES programs,
  insertedon TIMESTAMP WITHOUT TIME ZONE DEFAULT now()
);

A. Column Data Types

• SERIAL
• CHARACTER VARYING
• CHARACTER(10)
• INTEGER
• TIMESTAMP WITHOUT TIME ZONE

B. Common Added Options

• PRIMARY KEY
• UNIQUE
• DEFAULT

VI. CRUD Operations

A. Insertion of Rows

Template:

INSERT INTO table_name(column1, column2, column3...)
VALUES(value1, value2, value3...);

Sample:

INSERT INTO programs(degree, program)
VALUES('BS', 'Computer Science');

INSERT INTO programs(degree, program)
VALUES('BS', 'Business Administration and Accountancy');

INSERT INTO students(student_number, first_name, last_name, programid)
VALUES('2010-00031', 'Juan', 'Cruz', 1);

INSERT INTO students(student_number, first_name, last_name, programid)
VALUES('2010-00032', 'Pedro', 'Santos', 2);

B. Read/Lookup of Row

i. Get All Rows

SELECT * FROM students;

ii. Get Rows Satisfying Certain Conditions

# Gets row/s with studentid = 1

SELECT * FROM students where studentid = 1;

# Gets row/s where the last_name starts with 'cru' (non case sensitive)

SELECT * FROM students where last_name ilike 'cru%';

# Gets row/s where the student_number column is either 2010-0033, '2010-30011', or '2010-18415'

SELECT * FROM students where student_number in ('2010-00033', '2010-30011', '2010-18415');

iii. Get Specific Columns from Resulting Rows

# Selects the lastname and firstname from the students table

SELECT last_name, firstname from students;

# Selects the program column from rows of the programs table satisfying the condition and then prepending the given string

SELECT 'BUSINESS PROGRAM: ' || program from programs where program ilike '%business%';

C. Update of Row

i. Update all Rows

UPDATE students SET last_name = 'Cruz';

ii. Update Rows Satisfying Conditions

UPDATE students SET last_name = 'Santos' where studentid = 1;

UPDATE programs SET degree = 'BA' where programid NOT IN (2);

D. Deletion of Row

i. Delete all Rows

 DELETE FROM students

ii. Delete Rows Satisfying Conditions

DELETE FROM students WHERE studentid NOT IN (1,2)

VII. Queries

A. Joins

i. Inner Join

Syntax:

SELECT * FROM table_1 JOIN table_2 using (common_column_name);

Example:

SELECT student_number, program FROM students JOIN programs using (programid);

ii. Left Join

Syntax:

SELECT * FROM table_1 LEFT JOIN table_2 on table_1.column_name = table_2.column_name;

Example:

We insert a student row without a program

INSERT INTO students(student_number, first_name, last_name)
VALUES('2010-35007', 'Juana', 'Change');

Doing a left join would still return the recently inserted row but with empty Programs-related fields.

SELECT * FROM students LEFT join programs on students.programid = programs.programid;

iii. Right Join

Syntax:

SELECT * FROM table_1 RIGHT JOIN table_2 on table_1.column_name = table_2.column_name;

Example:

We insert a program row without any students attached

INSERT INTO programs(degree, program)
VALUES('BS', 'Information Technology');

Doing a right join would still return the recently inserted row but with empty Students-related fields.

SELECT * FROM students RIGHT join programs on students.programid = programs.programid;

B.Where

Specify conditions by which rows from the query will be filtered.

SELECT * from students where programid IS NOT NULL;

C. Group By

Allows use of aggregate functions with the attributes provided to the GROUP BY clause as basis for aggregations

SELECT program, COUNT(*) FROM students
JOIN programs USING (programid) GROUP BY program;

Above example counts students per program.

D. Having

Similar to WHERE but applies the condition to the groups produced with GROUP BY.

SELECT program, COUNT(*) FROM students
JOIN programs USING (programid) GROUP BY program HAVING COUNT(*) > 1;

E. Union

Joins resulting datasets from multiple queries.

select * from students where programid in (1, 2)

UNION

select * from students;

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