To access the Postgres server, open a terminal and run $ psql. You should have a prompt that looks something like this:
The learn_sd at the beginning of the prompt is the name of the default database we created with our username. To keep things easy to read, I'll shorten the prompt to just:
Let's create a real database of our own. In psql, type:
> CREATE DATABASE test_database;
This is our first SQL statement. SQL keywords are case insensitive but are conventionally typed in all caps. All SQL statements must end with a semicolon. If you press enter and nothing happens, check to see if the prompt has changed from =# to -# - if so, you forgot to put a semicolon at the end of your statement. Just type one and press enter, and your statement should run.
Let's list out all of the databases in our Postgres server:
This is a command to psql, the program that is interacting with our Postgres server, and not a SQL statement. psql commands start with a backslash and don't end with a semicolon.
There are a couple databases used as templates for other databases, template0 and template1. You shouldn't ever delete those. Postgres also makes a default database called postgres , and if you're on a Mac and created a database with your username, that one will be there, too. And now you should have a database called test_database.
To create tables and columns in our database, we need to connect to it:
> \c test_database
Note: Our prompt now shows that we're connected to the test_database
To see a list of psql commands, we can type # \? . We can press the down arrow to scroll through them, or q to quit before the end.
> CREATE TABLE contacts (name varchar, age int, birthday timestamp);
This creates a table called contacts with a column for name, age, and birthday. Just like Ruby objects have a class, SQL columns have a data type. In this case, name has the data type varchar, which is means a varying number of characters (this is more or less the SQL equivalent of a string); age is an int, or integer; and birthday is a timestamp, which includes a date and time.
SQL has many data types, but here are the most common ones you'll use: int, float, varchar, text (for long blocks of text), timestamp, and boolean. For a full list, see the PostgreSQL data type documentation .
To list the columns in our new table, type # \d contacts.
To see a list of all the tables in your database, type # \dt.
If you mess something up and need to delete a table, type # DROP TABLE table_name;, where table_name is the name of the table.
Here's how to add a column to a table:
> ALTER TABLE contacts ADD family boolean;
Drop a column like this:
> ALTER TABLE contacts DROP family;
It's a good idea to always have a unique, automatically-incrementing ID number for each record in your database. Here's how to add such a field:
> ALTER TABLE contacts ADD id serial PRIMARY KEY;
The serial data type is an autoincrementing integer.
Now that we have tables with columns, let's put some data in them:
> INSERT INTO contacts (name, age, birthday) VALUES ('Wes', 43, '1969-05-01');
Notice how we need to put quotes around varchars and timestamps, but not around ints. Also, remember that with SQL, we need to use single quotes - double quotes won't work.
Often, we'll need to get back the ID from an insert, so that we know how to find the row we just inserted. Here's how to return the ID from an INSERT statement:
> INSERT INTO contacts ( name , age , birthday ) VALUES ( 'Wes' , 43 , '1969-05-01' ) RETURNING id ;
Now that you have data in your database, let's get it out. Reading data from a database is called querying. Here's a query for you to try:
> SELECT name FROM contacts;
This gives you a list of all the names in your contacts table.
You can select multiple columns from a table:
> SELECT name, birthday FROM contacts;
If you want to select all of the columns from a table, there's a shortcut:
> SELECT * FROM contacts;
You can limit your SELECTs to only return rows that match certain criteria:
> SELECT * FROM contacts WHERE age >= 18;
This gives us all of the adults in contacts.
A WHERE clauses accepts the following operators: =, !=, >, <, >= , <=, BETWEEN, LIKE, and IN. You can also prepend any operator with NOT. Here are examples of the last couple operators:
> SELECT * FROM contacts WHERE age BETWEEN 13 AND 17; > SELECT * FROM contacts WHERE name BETWEEN 'Judith' AND 'Wilma'; > SELECT * FROM contacts WHERE name LIKE 'We%'; > SELECT * FROM contacts WHERE birthday IN ('1969-01-01', '1999-01-01');
For the LIKE operator, the % is a wildcard, meaning it can stand for any number of any characters.
What if we need to change data in our database? There are a lot of fancy ways to do this, but let's focus on the simplest and most common: select a record by its primary key, and update its data:
> UPDATE contacts SET name = 'Wes Anderson' WHERE id = 1;
Now, let's delete a record:
> DELETE FROM contacts WHERE id = 1;
We're done playing around with this database now, so let's drop it. First, we'll need to connect to our original, default database. If you don't remember what it was called, just type \list. For me, it's called learn_sd:
> \c learn_sd > DROP DATABASE test_database;
Finally, to quit psql, just run:
We have already seen how to read data using select statements, so in this section we will treat how to input, update and finally delete data.
We can add information to a table like this:
INSERT INTO persons (given_name, last_name, dob, address, email) VALUES ('Korben', 'Dallas', '2097-06-01', '97 Main City, 987142', 'firstname.lastname@example.org'), ('Leloo', 'Dallas', '2097-04-16', '97 Main City, 987142', 'email@example.com');
We can update data that is already in a table like this:
UPDATE persons SET family_name = 'Granthem' WHERE family_name = 'Dallas';
SQL queries affect as many rows that match in the WHERE clause. In this case, we change all last names of 'Dallas' to 'Granthem'.
We can delete one or more rows of data from a table like this:
DELETE FROM persons WHERE family_name = 'Granthem';
And now they are gone.
CREATE DATABASE contact;
Note that the name of the database is singular.
Check that the database was created:
To store information in a database, first we need tables to store them in. Let's make a table called "contacts":
CREATE TABLE contacts ( id serial PRIMARY KEY, given_name varchar, last_name varchar, dob date, address varchar, email varchar);
This line is doing a few things. It is creating columns with values like given_name, dob, and email, and saying that only things like "varchars" -- or text of varying characters -- or dates can be stored in them.
id serial PRIMARY KEY portion creates a numeric ID for every new entry, starting at the number 1 and incrementing with each entry. ID's are useful because they are unique; if we have two contacts with the same name, we can use their different ID's to tell them apart in our code. For more on primary key see 5.3.4. Primary Keys_
In Postgres you can examine the tables schema by issuing the following commands:
\d contacts Table "public.contacts" Column | Type | Modifiers ------------+-------------------+------------------------------------------------------- id | integer | not null default nextval('contacts_id_seq'::regclass) given_name | character varying | last_name | character varying | dob | date | address | character varying | email | character varying | Indexes: "contacts_pkey" PRIMARY KEY, btree (id)
You can also examine the database schema, which allows you to see all the tables within your particular database
\dt List of relations Schema | Name | Type | Owner --------+----------+-------+------- public | contacts | table | postgres (1 row)
Now that we've decided on the kind of information we're going to store in our table, let's create some information. We can add information to a table like this:
INSERT INTO contacts (given_name, last_name, dob, address, email) VALUES ('Korben', 'Dallas', '2097-06-01', '97 Main City, 987142', 'firstname.lastname@example.org'), ('Leloo', 'Dallas', '2097-04-16', '97 Main City, 987142', 'email@example.com');
In order to see all of our new information, type
SElECT * FROM contacts; into the terminal.
We can add new columns to a table after the fact, as well:
ALTER TABLE contacts ADD adult boolean;
Here we are specifying the table that we want to change, and then naming the new column along with its type.
To see the change use
\d contacts and you will see the new column in the table schema.
\d contacts Table "public.contacts" Column | Type | Modifiers ------------+-------------------+------------------------------------------------------- id | integer | not null default nextval('contacts_id_seq'::regclass) given_name | character varying | last_name | character varying | dob | date | address | character varying | email | character varying | adult | boolean | Indexes: "contacts_pkey" PRIMARY KEY, btree (id)
If we decide that a particular column is unnecessary or not very useful, we can also remove it:
ALTER TABLE contacts DROP adult;
Like adding a column, the
ALTER TABLE command also takes a
DROP parameter, followed by the name of the column we want to drop.
DROP TABLE contacts;
will delete a table and all the data in it (with one caveat, this will only happen if the data can be deleted which may not be possible due to foreign key constraints).
To see the table was dropped use
\dt to check the contact database.
\dt No relations found.
If I start building a database table meant to contain contact information, the table schema could look something like this:
Contacts -------- name: varchar email: varchar phone: varchar
Here is Joe's data:
name email phone ---- ----- ----- Joe firstname.lastname@example.org 858-555-1212
Now Joe gets another phone:
Joe email@example.com 619-222-3333
Joe firstname.lastname@example.org 619-444-5555
This leads to replication of data and eventual problems if Joe ever wants to update his email because it is recorded in multiple places.
One way to address this data replication problem is to add columns for every possible mode of communication:
Contacts -------- name: varchar email: varchar phone: varchar fax: varchar pager: varchar landline: varchar cell: varchar google_voice: varchar
However, this looks like it will be a waste of space -- Joe doesn't have a fax or pager -- and there is no guarantee that we have enough or the right columns. What if someone has a second cell phone?
The solution is to add another table, meaning entity, that contains phones.
Contacts -------- id: integer name: varchar email: varchar
Phones ------ id: integer contact_id: integer number: varchar description: varchar
Before we link up our tables in the database, we need to think about the kind of relationshp the Contacts table has with the Phones table. Is it a one-to-one relationship? No, that would be more like the relationship between a Contact and their date of birth, where one person only ever has one birthday. Is it a many-to-many relationship? Not quite, our phones don't have multiple owners. In our database, Phones and Contacts have a one-to-many relationship: a Contact has many Phones.
SELECT * FROM contacts;
id name email --- ----- ----- 1 Joe email@example.com 2 Jenny firstname.lastname@example.org
SELECT * FROM phones;
id contact_id number description -- ---------- ------------ ----------- 1 1 858-555-1212 Cell 2 2 8675309 Home 3 1 619-222-3333 Landline
Notice that the
contact_id column in the phones table contains
id values that correspond to Joe and Jenny in the contacts table.
CREATE DATABASE contact;
Note that the name of the database is singular.
CREATE TABLE contacts (id serial PRIMARY KEY, name varchar, email varchar); CREATE TABLE phones (id serial PRIMARY KEY, contact_id integer REFERENCES contacts (id) NOT NULL, number varchar, description varchar);
Note that the table names are plural.
REFERENCES contacts (id) NOT NULL means that the
contact_id has to be provided (not
NULL) and has to be a value from the
id column in the
contacts table. Together these guarantee that the phones are always associated with a contact.
INSERT INTO contacts (name, email) VALUES ('Joe', 'email@example.com'), ('Jenny', 'firstname.lastname@example.org'); INSERT INTO phones (contact_id, number, description) VALUES (1, '858-555-1212', 'Cell'), (2, '8675309', 'Home'), (1, '619-222-3333', 'Landline');
Note that the primary key
ids are inserted automagically so we do not have to enter them.
However, for the
contact_id we have to use the
id for the corresponding person.
Get all of Joes information:
SELECT id FROM contacts WHERE contacts.name = 'Joe'; SELECT * FROM phones WHERE phones.contact_id = 1; -- value from first SELECT
The value for the
contact_id is the
id from the first
Get all of Joes phone information:
SELECT * FROM phones WHERE contact_id IN (SELECT id FROM contacts WHERE name = 'Joe');
Getting everybodys information - simplest join possible:
SELECT * FROM contacts, phones WHERE contacts.id = phones.contact_id;
Get only Joes phone numbers:
SELECT phones.number FROM contacts, phones WHERE contacts.id = phones.contact_id AND contacts.name = 'Joe';
Or using a
SELECT p.number FROM contacts c JOIN phones p ON c.id = p.contact_id WHERE c.name = 'Joe';
SELECT referencing two table:
SELECT * FROM contacts, phones;
returns the full cartesian product (every combination of rows) of the tables involved:
id | name | email | id | contact_id | number | description ----+-------+----------------+----+------------+--------------+------------- 1 | Joe | email@example.com | 1 | 1 | 858-555-1212 | Cell 1 | Joe | firstname.lastname@example.org | 2 | 2 | 8675309 | Home 1 | Joe | email@example.com | 3 | 1 | 619-222-3333 | Landline 2 | Jenny | firstname.lastname@example.org | 1 | 1 | 858-555-1212 | Cell 2 | Jenny | email@example.com | 2 | 2 | 8675309 | Home 2 | Jenny | firstname.lastname@example.org | 3 | 1 | 619-222-3333 | Landline
Note: The first
id column is from the
contacts table, and the second is from the
This result set is not really useful because, for instance, Jenny has nothing to do with Joe's landline (last row).
We are really only interested in records where all the information belongs to the same person. That is what we specify with the
SELECT * FROM contacts c JOIN phones p ON c.id = p.contact_id;
The result is a much smaller table:
id | name | email | id | contact_id | number | description ----+-------+----------------+----+------------+--------------+------------- 1 | Joe | email@example.com | 1 | 1 | 858-555-1212 | Cell 2 | Jenny | firstname.lastname@example.org | 2 | 2 | 8675309 | Home 1 | Joe | email@example.com | 3 | 1 | 619-222-3333 | Landline
Notice how the contacts'
id and the
contact_id are the same in each row; signifying that the data belongs together.
And from this we can further select, for instance, only rows associated with Joe:
SELECT * FROM contacts c JOIN phones p ON c.id = p.contact_id WHERE name = 'Joe';
The result is an even smaller table:
id | name | email | id | contact_id | number | description ----+-------+----------------+----+------------+--------------+------------- 1 | Joe | firstname.lastname@example.org | 1 | 1 | 858-555-1212 | Cell 1 | Joe | email@example.com | 3 | 1 | 619-222-3333 | Landline
serial PRIMARY KEY, given_name, family_name, address, dob, email using appropriate datatypes found here.
CREATE TABLE members ( id serial PRIMARY KEY, surname varchar, first_name varchar, address varchar, zipcode integer, telephone varchar, recommended_by integer REFERENCES members(id), join_date timestamp without time zone );
Now that the database is created and tables are in place, we need to fill those tables with some data so we have something to query.
\.at the end.
JOINs on the Foreign Keys
Produce a list of start times for bookings by members named 'David Farrell'?
Hint: Remember that a
JOIN is selecting all records from Table A and Table B, where the join condition is met.
Produce a list of the start times for bookings for tennis courts, for the date '2012-09-21'? Return a list of start time and facility name pairings, ordered by the time.
BETWEENand specify the date (with time as HR:MN:SC).
Produce a list of all members who have used a tennis court? Include in your output the name of the court, and the name of the member formatted as first name, surname. Ensure no duplicate data, and order by the first name.
FROM ... ... JOIN ... ... ON ... = ... JOIN ... ... ON ... = ... WHERE ... IN ...
Produce a number of how many times Nancy Dare has used the pool table facility?
Produce a list of how many times Nancy Dare has visited each country club facility.
Produce a list of all members who have recommended another member? Ensure that there are no duplicates in the list, and that results are ordered by (surname, firstname).
JOINThe tables we are joining don't have to be different tables. We can join a table with itself. This is called a self join. In this case we have to use aliases for the table otherwise PostgreSQL will not know which id column of which table instance we mean.
FROM tacos ... JOIN tacos ... ON ... = ...
Output a list of all members, including the individual who recommended them (if any), without using any
JOINs? Ensure that there are no duplicates in the list, and that member is formatted as one column and ordered by member.
SELECT DISTINCT ... || ' ' || ... AS ...,
SELECT DISTINCT ... || ' ' || ... AS ...., (SELECT ... || ' ' || ... AS .... FROM ... ... WHERE ... = ... ) FROM ... ...
Data licensed under https://creativecommons.org/licenses/by-sa/3.0/
Today's Tentative Schedule
9:15am - Stand Up
9:30am - Understanding the Web followed by challenges
12:00 noon - Lunch
1:00pm - Demonstration: How the Internet Works
4.30pm - Review
5:00pm - Class Ends