What is a relational database?
Data stored as row records in tables. Imagine a spreadsheet with column
headers describing the contents of each column, and each row is a
record.
A database can contain many tables. A table can contain many rows. A row
can contain many columns.
Records are related to those in different tables through common columns
that are present in both tables.
For example, an Employee table might have the following columns in
each record:
Employee
EmployeeID FirstName LastName DepartmentID
And a Department table might have the following columns in each
record:
Department
DepartmentID DepartmentName
Notice that both Employee and Department have a DepartmentID
column. This common column relates the two tables and can be used to
join them together with a query.
The structure described by the table definitions is known as the
schema.
Compare to NoSQL databases that work with key/value pairs or are
document stores.
Relational vs NoSQL
NoSQL is a term that refers to non-relational databases, most usually
document store databases. (Though it can apply to almost any kind of
non-relational database.)
MongoDB is a great example of a NoSQL database.
When Do You Use NoSQL Versus a Relational Database?
Unfortunately, there are no definitive rules on when to choose one or
the other.
Do you need ACID-compliance? Consider a relational database.
Does your schema (structure of data) change frequently? Consider NoSQL.
Does absolute consistency in your data matter, e.g. a bank, inventory
management system, employee management, academic records, etc.? Consider
a relational database.
Do you need easy-to-deploy high-availability? Consider NoSQL.
Do you need transactions to happen atomically? (The ability to update
multiple records simultaneously?) Consider a relational database.
Do you need read-only access to piles of data? Consider NoSQL.
PostgreSQL
PostgreSQL is a venerable relational database that is freely available
and world-class.
SQL, Structured Query Language
SQL ("sequel") is the language that people use for interfacing with
relational databases.
Create a table with CREATE TABLE
A database is made up of a number of tables. Let's create a table using
SQL in the shell. Be sure to end the command with a semicolon ;.
(Note: SQL commands are often capitalized by convention, but can be
lowercase.)
$ psql
psql (10.1)
Type "help" for help.
dbname=> CREATE TABLE Employee (ID INT, LastName VARCHAR(20));
Use the \dt command to show which tables exist:
dbname=> CREATE TABLE Employee (ID INT, LastName VARCHAR(20));
CREATE TABLE
dbname=> \dt
List of relations
Schema | Name | Type | Owner
--------+----------+-------+-------
public | employee | table | beej
(1 row)
Use the \d command to see what columns a table has:
dbname=> \d Employee
Table "public.employee"
Column | Type | Collation | Nullable | Default
--------------+-----------------------+-----------+----------+---------
id | integer | | |
lastname | character varying(20) | | |
Create a row with INSERT
dbname=> INSERT INTO Employee (ID, LastName) VALUES (10, 'Tanngnjostr');
INSERT 0 1
You can omit the column names if you're putting data in every column:
dbname=> INSERT INTO Employee VALUES (10, 'Tanngnjostr');
INSERT 0 1
Run some more inserts into the table:
INSERT INTO Employee VALUES (11, 'Alice');
INSERT INTO Employee VALUES (12, 'Bob');
INSERT INTO Employee VALUES (13, 'Charlie');
INSERT INTO Employee VALUES (14, 'Dave');
INSERT INTO Employee VALUES (15, 'Eve');
Read rows with SELECT
You can query the table with SELECT.
Query all the rows and columnts:
dbname=> SELECT * FROM Employee;
id | lastname
----+-------------
10 | Tanngnjostr
11 | Alice
12 | Bob
13 | Charlie
14 | Dave
15 | Eve
(6 rows)
With SELECT, * means "all columns".
You can choose specific columns:
dbname=> SELECT LastName FROM Employee;
lastname
-------------
Tanngnjostr
Alice
Bob
Charlie
Dave
Eve
(6 rows)
And you can search for specific rows with the WHERE clause:
dbname=> SELECT * FROM Employee WHERE ID=12;
id | lastname
----+----------
12 | Bob
(1 row)
dbname=> SELECT * FROM Employee WHERE ID=14 OR LastName='Bob';
id | lastname
----+----------
12 | Bob
14 | Dave
(2 rows)
Finally, you can rename the output columns, if you wish:
SELECT id AS Employee ID, LastName AS Name
FROM Employee
WHERE ID=14 OR LastName='Bob';
Employee ID | Name
-------------+----------
12 | Bob
14 | Dave
Update rows with UPDATE
The UPDATE command can update one or many rows. Restrict which rows
are updated with a WHERE clause.`
dbname=> UPDATE Employee SET LastName='Harvey' WHERE ID=10;
UPDATE 1
dbname=> SELECT * FROM Employee WHERE ID=10;
id | lastname
----+----------
10 | Harvey
(1 row)
You can update multiple columns at once:
dbname=> UPDATE Employee SET LastName='Octothorpe', ID=99 WHERE ID=14;
UPDATE 1
Delete rows with DELETE
Delete from a table with the DELETE command. Use a WHERE clause to
restrict the delete.
CAUTION! If you don't use a WHERE clause, all rows will be deleted
from the table!
Delete some rows:
dbname=> DELETE FROM Employee WHERE ID >= 15;
DELETE 2
Delete ALL rows (Danger, Will Robinson!):
dbname=> DELETE FROM Employee;
DELETE 4
Deleting entire tables with DROP
If you want to get rid of an entire table, use DROP.
WARNING! There is no going back. Table will be completely blown
away. Destroyed ...by the Empire.
dbname=> DROP TABLE Employee;
DROP TABLE
The WHERE Clause
You've already seen some examples of how WHERE affects SELECT,
UPDATE, and DELETE.
Normal operators like <, >, =, <=, >= are available.
For example:
sql
SELECT * from animals
WHERE age >= 10;
AND, OR, and Parentheses
You can add more boolean logic with AND, OR, and affect precedence
with parentheses:
sql
SELECT * from animals
WHERE age >= 10 AND type = 'goat';
sql
SELECT * from animals
WHERE age >= 10 AND (type = 'goat' OR type = 'antelope');
LIKE
The LIKE operator can be used to do pattern matching.
sql
_ -- Match any single character
% -- Match any sequence of characters
To select all animals that start with ab:
sql
SELECT * from animal
WHERE name LIKE 'ab%';
Column Data Types
You probably noticed a few data types we specified with CREATE TABLE,
above. PostgreSQL has a lot of data
types.
This is an incomplete list of some of the more common types:
sql
VARCHAR(n) -- Variable character string of max length n
BOOLEAN -- TRUE or FALSE
INTEGER -- Integer value
INT -- Same as INTEGER
DECIMAL(p,s) -- Decimal number with p digits of precision
-- and s digits right of the decimal point
REAL -- Floating point number
DATE -- Holds a date
TIME -- Holds a time
TIMESTAMP -- Holds an instant of time (date and time)
BLOB -- Binary object
ACID and CRUD
These are two common database terms.
ACID
Short for Atomicity, Consistency, Isolation, Durability. When
people mention "ACID-compliance", they're generally talking about the
ability of the database to accurately record transactions in the case of
crash or power failure.
Atomicity: all transactions will be "all or nothing".
Consistency: all transactions will leave the database in a consistent
state with all its defined rules and constraints.
Isonlation: the results of concurrent transactions is the same as if
those transactions had been executed sequentially.
Durability: Once a transaction is committed, it will remain committed,
despite crashes, power outages, snow, and sleet.
CRUD
Short for Create, Read, Update, Delete. Describes the four basic
functions of a data store.
In a relational database, these functions are handled by INSERT,
SELECT, UPDATE, and DELETE.
NULL and NOT NULL
Columns in records can sometimes have no data, referred to by the
special keyword as NULL. Sometimes it makes sense to have NULL
columns, and sometimes it doesn't.
If you explicitly want to disallow NULL columns in your table, you can
create the columns with the NOT NULL constraint:
CREATE TABLE Employee (
ID INT NOT NULL,
LastName VARCHAR(20));
COUNT
You can select a count of items in question with the COUNT operator.
For example, count the rows filtered by the WHERE clause:
`sql
SELECT COUNT(*) FROM Animals WHERE legcount >= 4;
count
5
`
Useful with GROUP BY, below.
ORDER BY
ORDER BY which sorts SELECT results for you. Use DESC to sort in
reverse order.
`sql
SELECT * FROM Pets
ORDER BY age DESC;
| name | age |
|---|---|
| Rover | 9 |
| Zaphod | 4 |
| Mittens | 3 |
`
GROUP BY
When used with an aggregating function like COUNT, can be
used to produce groups of results.
Count all the customers in certain countries:
`sql
SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country;
| COUNT(CustomerID) | Country |
|---|---|
| 1123 | USA |
| 734 | Germany |
etc.
`
Keys: Primary, Foreign, and Composite
Primary Key
Rows in a table often have one column that is called the primary key.
The value in this column applies to all the rest of the data in the
record. For example, an EmployeeID would be a great primary key,
assuming the rest of the record held employee information.
Employee
ID (Primary Key) LastName FirstName DepartmentID
To create a table and specify the primary key, use the NOT NULL and
PRIMARY KEY constraints:
sql
CREATE TABLE Employee (
ID INT NOT NULL PRIMARY KEY,
LastName VARCHAR(20),
FirstName VARCHAR(20),
DepartmentID INT);
You can always search quickly by primary key.
Foreign Keys
If a key refers to a primary key in another table, it is called a
foreign key (abbreviated "FK"). You are not allowed to make changes to
the database that would cause the foreign key to refer to a non-existent
record.
The database uses this to maintain referential integrity.
Create a foreign key using the REFERENCES constraint. It specifies the
remote table and column the key refers to.
`sql
CREATE TABLE Department (
ID INT NOT NULL PRIMARY KEY,
Name VARCHAR(20));
CREATE TABLE Employee (
ID INT NOT NULL PRIMARY KEY,
LastName VARCHAR(20),
FirstName VARCHAR(20),
DepartmentID INT REFERENCES Department(ID));
`
In the above example, you cannot add a row to Employee until that
DepartmentID already exists in Department's ID.
Also, you cannot delete a row from Department if that row's ID was a
DepartmentID in Employee.
Composite Keys
Keys can also consist of more than one column. Composite keys can be
created as follows:
sql
CREATE TABLE example (
a INT,
b INT,
c INT,
PRIMARY KEY (a, c));
Auto-increment Columns
These are columns that the database manages, usually in an
ever-increasing sequence. It's perfect for generation unique, numeric
IDs for primary Keys.
In some databases (e.g MySQL) this is done with an
AUTO_INCREMENT
keyword. PostgreSQL is different.
In PostgreSQL, use the SERIAL keyword to auto-generate sequential
numeric IDs for records.
sql
CREATE TABLE Company (
ID SERIAL PRIMARY KEY,
Name VARCHAR(20));
When you insert, do not specify the ID column. You must however,
give a column name list that includes the remaining column names you are
inserting data for. The ID column will be automatically generated by the
database.
sql
INSERT INTO Company (Name) VALUES ('My Awesome Company');
Joins
This concept is extremely important to understanding how to use
relational databases!
When you have two (or more) tables with data you wish to retrieve from
both, you do so by using a join. These come in a number of varieties,
some of which are covered here.
When you're using SELECT to make the join between two tables, you can
specify the tables specific columns are from by using the . operator.
This is especially useful when columns have the same name in the
different tables:
sql
SELECT Animal.name, Farm.name
FROM Animal, Farm
WHERE Animal.FarmID = Farm.ID;
Tables to use in these examples:
`sql
CREATE TABLE Department (
ID INT NOT NULL PRIMARY KEY,
Name VARCHAR(20));
CREATE TABLE Employee (
ID INT NOT NULL PRIMARY KEY,
Name VARCHAR(20),
DepartmentID INT);
INSERT INTO Department VALUES (10, 'Marketing');
INSERT INTO Department VALUES (11, 'Sales');
INSERT INTO Department VALUES (12, 'Entertainment');
INSERT INTO Employee VALUES (1, 'Alice', 10);
INSERT INTO Employee VALUES (2, 'Bob', 12);
INSERT INTO Employee VALUES (3, 'Charlie', 99);
`
NOTE: Importantly, department ID 11 is not referred to from
Employee, and department ID 99 (Charlie) does not exist in
Department. This is instrumental in the following examples.
Inner Join, The Most Common Join
This is the most commonly-used join, by far, and is what people mean
when they just say "join" with no further qualifiers.
This will return only the rows that match the requirements from both
tables.
For example, we don't see "Sales" or "Charlie" in the join because
neither of them match up to the other table:
dbname=> SELECT Employee.ID, Employee.Name, Department.Name
FROM Employee, Department
WHERE Employee.DepartmentID = Department.ID;
id | name | name
----+-------+---------------
1 | Alice | Marketing
2 | Bob | Entertainment
(2 rows)
Above, we used a WHERE clause to perform the inner join. This is
absolutely the most common way to do it.
There is an alternative syntax, below, that is barely ever used.
dbname=> SELECT Employee.ID, Employee.Name, Department.Name
FROM Employee INNER JOIN Department
ON Employee.DepartmentID = Department.ID;
id | name | name
----+-------+---------------
1 | Alice | Marketing
2 | Bob | Entertainment
(2 rows)
Left Outer Join
This join works like an inner join, but also returns all the rows from
the "left" table (the one after the FROM clause). It puts NULL
in for the missing values in the "right" table (the one after the
LEFT JOIN clause.)
Example:
dbname=> SELECT Employee.ID, Employee.Name, Department.Name
FROM Employee LEFT JOIN Department
ON Employee.DepartmentID = Department.ID;
id | name | name
----+---------+---------------
1 | Alice | Marketing
2 | Bob | Entertainment
3 | Charlie |
(3 rows)
Notice that even though Charlie's department isn't found in Department, his record is still listed with a NULL department name.
Right Outer Join
This join works like an inner join, but also returns all the rows from
the "right" table (the one after the RIGHT JOIN clause). It puts
NULL in for the missing values in the "right" table (the one after the
FROM clause.)
dbname=> SELECT Employee.ID, Employee.Name, Department.Name
FROM Employee RIGHT JOIN Department
ON Employee.DepartmentID = Department.ID;
id | name | name
----+-------+---------------
1 | Alice | Marketing
2 | Bob | Entertainment
| | Sales
(3 rows)
Notice that even though there are no employees in the Sales department,
the Sales name is listed with a NULL employee name.
Full Outer Join
This is a blend of a Left and Right Outer Join. All information from
both tables is selected, with NULL filling the gaps where necessary.
dbname=> SELECT Employee.ID, Employee.Name, Department.Name
FROM Employee
FULL JOIN Department
ON Employee.DepartmentID = Department.ID;
id | name | name
----+---------+---------------
1 | Alice | Marketing
2 | Bob | Entertainment
3 | Charlie |
| | Sales
(4 rows)
Indexes
When searching through tables, you use a WHERE clause to narrow things
down. For speed, the columns mentioned in the WHERE clause should
either be a primary key, or a column for which an index has been
built.
Indexes help speed searches. In a large table, searching over an
unindexed column will be slow.
Example of creating an index on the Employee table from the
Keys section:
dbname=> CREATE INDEX ON Employee (LastName);
CREATE INDEX
dbname=> \d Employee
Table "public.employee"
Column | Type | Collation | Nullable | Default
--------------+-----------------------+-----------+----------+---------
id | integer | | not null |
lastname | character varying(20) | | |
firstname | character varying(20) | | |
departmentid | integer | | |
Indexes:
"employee_pkey" PRIMARY KEY, btree (id)
"employee_lastname_idx" btree (lastname)
Foreign-key constraints:
"employee_departmentid_fkey" FOREIGN KEY (departmentid) REFERENCES department(id)
Transactions
In PostgreSQL, you can bundle a series of statements into a
transaction. The transaction is executed atomically, which means
either the entire transaction occurs, or none of the transaction occurs.
There will never be a case where a transaction partially occurs.
Create a transaction by starting with a BEGIN statement, followed by
all the statements that are to be within the transaction.
START TRANSACTIONis generally synonymous withBEGINin SQL.
To execute the transaction ("Let's do it!"), end with a COMMIT
statement.
To abort the transaction and do nothing ("On second thought,
nevermind!") end with a ROLLBACK statement. This makes it like
nothing within the transaction ever happened.
Usually transactions happen within a program that checks for sanity and
either commits or rolls back.
Pseudocode making DB calls that check if a rollback is necessary:
`javascript
db("BEGIN"); // Begin transaction
db(UPDATE accounts SET balance = balance - 100.00);
WHERE name = 'Alice'
let balance = db("SELECT balance WHERE name = 'Alice'");
// Don't let the balance go below zero:
if (balance < 0) {
db("ROLLBACK"); // Never mind!! Roll it all back.
} else {
db("COMMIT"); // Plenty of cash
}
`
In the above example, the UPDATE and SELECT must happen at the same
time (atomically) or else another process could sneak in between and
withdraw too much money. Because it needs to be atomic, it's wrapped in
a transaction.
If you just enter a single SQL statement that is not inside a BEGIN
transaction block, it gets automatically wrapped in a BEGIN/COMMIT
block. It is a mini transaction that is COMMITted immediately.
Not all SQL databases support transactions, but most do.
The EXPLAIN Command
The EXPLAIN command will tell you how much time the database is
spending doing a query, and what it's doing in that time.
It's a powerful command that can help tell you where you need to add
indexes, change structure, or rewrite queries.
dbname=> EXPLAIN SELECT * FROM foo;
QUERY PLAN
---------------------------------------------------------
Seq Scan on foo (cost=0.00..155.00 rows=10000 width=4)
(1 row)
For more information, see the PostgreSQL EXPLAIN documentation
Quick and Dirty DB Design
Designing a non-trivial database is a difficult, learned skill best left to
professionals. Feel free to do small databases with minimal training, but if you
get in a professional situation with a large database that needs to be designed,
you should consult with people with strong domain knowledge.
That said, here are a couple pointers.
In general, all your tables should have a unique
PRIMARY KEYfor each row.
It's common to useSERIALorAUTO_INCREMENTto make this happen.Keep an eye out for commonly duplicated data. If you are duplicating text data
across several records, consider that maybe it should be in its own table and
referred to with a foreign key.Watch out for unrelated data in the same record. If it's a record in the
Employeetable but it hasDepartment_Addressas a column, that probably
belongs in aDepartmenttable, referred to by a public key.
But if you really want to design database, read on to the Normalization and
Normal Forms section.
Normalization and Normal Forms
[This topic is very deep and this section cannot do it full justice.]
Normalization is the process of designing or refactoring your tables
for maximum consistency and minimum redundancy.
With NoSQL databases, we're used to denormalized data that is stored
with speed in mind, and not so much consistency (sometimes NoSQL
databases talk about eventual consistency).
Non-normalized tables are considered an anti-pattern in relational databases.
There are many normal forms. We'll talk about First, Second, and Third
normal forms.
Anomalies
One of the reasons for normalizing tables is to avoid anomalies.
Insert anomaly: When we cannot insert a row into the table because
some of the dependent information is not yet known. For example, we
cannot create a new class record in the school database, because the
record requires at least one student, and none have enrolled yet.
Update anomaly: When information is duplicated in the database and
some rows are updated but not others. For example, say a record contains
a city and a zipcode, but then the post office changes the zipcode. If
some of the records are updated but not others, some cities will have
the old zipcodes.
Delete anomaly: The opposite of an insert anomaly. When we delete some
information and other related information must also be deleted against
our will. For example, deleting the last student from a course causes
the other course information to be also deleted.
By normalizing your tables, you can avoid these anomalies.
First Normal Form (1NF)
When a database is in first normal form, there is a primary key for each
row, and there are no repeating sets of columns that should be in their
own table.
Unnormalized (column titles on separate lines for clarity):
Farm
ID
AnimalName1 AnimalBreed1 AnimalProducesEggs1
AnimalName2 AnimalBreed2 AnimalProducesEggs2
1NF:
Farm
ID
Animal
ID FarmID[FK Farm(ID)] Name Breed ProducesEggs
Use a join to select all the animals in the farm:
sql
SELECT Name, Farm.ID FROM Animal, Farm WHERE Farm.ID = Animal.FarmID;
Second Normal Form (2NF)
To be in 2NF, a table must already be in 1NF.
Additionally, all non-key data must fully relate to the key data in the table.
In the farm example, above, Animal has a Name and a key FarmID, but
these two pieces of information are not related.
We can fix this by adding a table to link the other two tables together:
2NF:
Farm
ID
FarmAnimal
FarmID[FK Farm(ID)] AnimalID[FK Animal(ID)]
Animal
ID Name Breed ProducesEggs
Use a join to select all the animals in the farms:
sql
SELECT Name, Farm.ID
FROM Animal, FarmAnimal, Farm
WHERE Farm.ID = FarmAnimal.FarmID AND
Animal.ID = FarmAnimal.AnimalID;
Third Normal Form (3NF)
A table in 3NF must already be in 2NF.
Additionally, columns that relate to each other AND to the key need to
be moved into their own tables. This is known as removing transitive
dependencies.
In the Farm example, the columns Breed and ProducesEggs are related.
If you know the breed, you automatically know if it produces eggs or
not.
3NF:
Farm
ID
FarmAnimal
FarmID[FK Farm(ID)] AnimalID[FK Animal(ID)]
BreedEggs
Breed ProducesEggs
Animal
ID Name Breed[FK BreedEggs(Breed)]
Use a join to select all the animals names that produce eggs
in the farm:
sql
SELECT Name, Farm.ID
FROM Animal, FarmAnimal, BreedEggs, Farm
WHERE Farm.ID = FarmAnimal.FarmID AND
Animal.ID = FarmAnimal.AnimalID AND
Animal.Breed = BreedEggs.Breed AND
BreedEggs.ProducesEggs = TRUE;
More reading:
Node-Postgres
This is a library that allows you to interface with PostgreSQL through
NodeJS.
Its documentation is exceptionally good.
Security
PostgreSQL Password
You might have noticed that you don't need a password to access your
database that you created. This is because PostgreSQL by default uses
something called peer authentication
mode.
In a nutshell, it makes sure that you are logged in as yourself before
you access your database. If a different user tries to access your
database, they will be denied.
If you need to set up password access, see client authentication in the
PostgreSQL
manual
Writing Client Software
When writing code that accesses databases, there are a few rules you
should follow to keep things safe.
Don't store database passwords or other sensitive information in your
code repository. Store dummy credentials instead.When building SQL queries in code, use parameterized
queries.
You build your query with parameter placeholders for where the query
arguments will go.
This is your number-one line of defense against SQL injection
attacks.
It's a seriously noob move to not use parameterized queries.
Other Relational Databases
There are tons of them by Microsoft, Oracle, etc. etc.
Other popular open source databases in widespread use are:
Assignment: Install PostgreSQL
IMPORTANT! These instructions assume you haven't already installed
PostgreSQL. If you have already installed it, skip this section or
Google for how to upgrade your installation.
Mac with Homebrew
- Open a terminal
-
Install PostgreSQL:
brew install postgresqlIf you get install errors at this point relating to the link phase
failing or missing permissions, look back in the output and see
what file it failed to write.For example, if it's failing to write something in
/usr/local/share/man-something, try setting the ownership on
those directories to yourself.Example (from the command line):
$ sudo chown -R $(whoami) /usr/local/share/manThen try to install again.
-
Start the database process
-
If you want to start it every time you log in, run:
brew services start postgresql
-
* If you want to just start it one time right now, run:
pg_ctl -D /usr/local/var/postgres start
- Create a database named the same as your username:
createdb $(whoami)- Optionally you can call it anything you want, but the shell defaults to looking for a database named the same as your user.
This database will contain tables.
Then start a shell by running psql and see if it works. You should see
this prompt:
$ psql
psql (10.1)
Type "help" for help.
dbname=>
(Use psql databasename if you created the database under something
other than your username.)
Use \l to get a list of databases.
You can enter \q to exit the shell.
Windows
Reports are that one of the easiest installs is with
chocolatey. Might want to try that
first.
You can also download a Windows
installer from the
official site.
Another option is to use the Windows Subsystem for
Linux
and follow the Ubuntu instructions for installing
PostgreSQL.
Arch Linux
Arch requires a bit more hands-on, but not much more. Check this out if
you want to see a different Unix-y install procedure (or if you run
Arch).
Assignment: Create a Table and Use It
Launch the shell on your database, and create a table.
sql
CREATE TABLE Employee (ID INT, FirstName VARCHAR(20), LastName VARCHAR(20));
Insert some records:
sql
INSERT INTO Employee VALUES (1, 'Alpha', 'Alphason');
INSERT INTO Employee VALUES (2, 'Bravo', 'Bravoson');
INSERT INTO Employee VALUES (3, 'Charlie', 'Charleson');
INSERT INTO Employee VALUES (4, 'Delta', 'Deltason');
INSERT INTO Employee VALUES (5, 'Echo', 'Ecoson');
Select all records:
sql
SELECT * FROM Employee;
Select Employee #3's record:
sql
SELECT * FROM Employee WHERE ID=3;
Delete Employee #3's record:
sql
DELETE FROM Employee WHERE ID=3;
Use SELECT to verify the record is deleted.
Update Employee #2's name to be "Foxtrot Foxtrotson":
sql
UPDATE Employee SET FirstName='Foxtrot', LastName='Foxtrotson' WHERE ID=2;
Use SELECT to verify the update.
Assignment: NodeJS Program to Create and Populate a Table
Using Node-Postgres, write a program that
creates a table.
Run the following query from your JS code:
sql
CREATE TABLE IF NOT EXISTS Earthquake
(Name VARCHAR(20), Magnitude REAL)
Populate the table with the following data:
javascript
let data = [
["Earthquake 1", 2.2],
["Earthquake 2", 7.0],
["Earthquake 3", 1.8],
["Earthquake 4", 5.2],
["Earthquake 5", 2.9],
["Earthquake 6", 0.6],
["Earthquake 7", 6.6]
];
You'll have to run an INSERT statement for each one.
Open a PostgreSQL shell (psql) and verify the table exists:
user-> \dt
List of relations
Schema | Name | Type | Owner
--------+------------+-------+-------
public | earthquake | table | user
(1 row)
Also verify it is populated:
user-> SELECT * from Earthquake;
name | magnitude
--------------+-----------
Earthquake 1 | 2.2
Earthquake 2 | 7
Earthquake 3 | 1.8
Earthquake 4 | 5.2
Earthquake 5 | 2.9
Earthquake 6 | 0.6
Earthquake 7 | 6.6
(7 rows)
Hints:
Extra Credit:
- Add an ID column to help normalize the database. Make this column
SERIALto auto-increment. - Add Date, Lat, and Lon columns to record more information about the event.
Assignment: Command-line Earthquake Query Tool
Write a tool that queries the database for earthquakes that are at least
a given magnitude.
$ node earthquake 2.9
Earthquakes with magnitudes greater than or equal to 2.9:
Earthquake 2: 7
Earthquake 7: 6.6
Earthquake 4: 5.2
Earthquake 5: 2.9
Use ORDER BY Magnitude DESC to order the results in descending order
by magnitude.
Assignment: RESTful Earthquake Data Server
Use ExpressJS and write a webserver that
implements a RESTful API to access the earthquake data.
Endpoints:
/ (GET) Output usage information in HTML.
Example results:
html
<html>
<body>Usage: [endpoint info]</body>
</html>
/minmag (GET) Output JSON list of earthquakes that are larger than the
value specified in the mag parameter. Use form encoding to pass the
data.
Example results:
json
{
"results": [
{
"name": "Earthquake 2",
"magnitude": 7
},
{
"name": "Earthquake 4",
"magnitude": 5.2
}
]
}
Extra Credit:
/new (POST) Add a new earthquake to the database. Use form encoding to
pass name and mag. Return a JSON status message:
json
{ "status": "ok" }
or
json
{ "status": "error", "message": "[error message]" }
/delete (DELETE) Delete an earthquake from the database. Use form
encoding to pass name. Return status similar to /new, above.
UPDATE Employee SET FirstName='Foxtrot', LastName='Foxtrotson' WHERE ID=2;
Use SELECT to verify the update.
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