Databases 101 (Relational Databases)

What is a Database?

A database is a program or library that helps you store data. They usually impose some sort of structure to the data to assist with querying and filtering. The most common structure databases use to store data is a table, and most databases will use multiple tables to store data.

A table is composed of rows representing an item, and columns represent some attribute of that item. Much like using a spreadsheet for a task list, where a row would be a task, and you may have a column for a task name, and a second column to determine whether it has been completed.

What is a Relational Database?

A relational database is a database in which data is organized into tables, usually with some form of relationship between them. For example, a table containing customer names and addresses, and a table containing sales transactions. Rather than repeat customer details every time a transaction is done, the data is stored once in one table, and then a unique reference is stored in the sales transactions table, linking each transaction to the customer. This approach makes a relational database very efficient for storing data, since reused data can be stored in a single table and referenced in other tables.

Data can be retrieved in many forms by combining multiple tables into new tables using operations like JOIN. The composed table will have new rows that are a combination of its parent tables.

Why We Use Databases?

We use databases to store data. They help provide us guarantees that data is stored and that it exists in the correct format. In addition most databases are heavily optimized making data retrieval very fast.

Because of these attributes, we use databases a lot. Most e-commerce sites use databases to keep inventory and sales records. Doctors offices use databases to store medical records, and the DMV uses databases to keep track of cars. Lawyers use databases to keep track of case law, and many websites use databases to store and organize content. Databases are everywhere, and you interact with them daily.

What is SQL?

SQL or Structured Query Language is a domain specific language used for accessing databases. It has a declarative syntax, meaning you declare the structure you want returned, the sources, and constraints in a single statement. The database’s query parser and planner will determine a how to retrieve your data from this statement.

SQL shell

Many relational databases provide an interactive CLI for interacting with the database. For example, MySQL provides the mysql command, Postgresql provides psql, and Oracle provides sqlplus. These programs give you the ability to compose queries, and diagnose issues with the software.

todo:: Add example of connecting with each command.

MySQL

To connect to a MySQL database from the CLI your command would usually take this form:

$ mysql -u username -p -h server-address
Password:

Of these flags, -u = username, -p = password, and -h = hostname. You may provide the password on the command prompt: -ppassword (no space after flag!). We strongly advise against this, as this may leave the password visible in your shell history.

Creating databases

Most database platforms allow you to create a new database using the CREATE DATABASE SQL query. It can be executed via a connection to the server, or via the SQL shell.

A MySQL example:

mysql> CREATE DATABASE example_database;

Specialized Create Database Commands

Some database platforms have specialized GUI tools, or CLI tools. For example Postgresql has a UNIX command createdb that creates databases. In many cases these commands are just wrappers around the standard SQL statements.

MySQL

$ mysqladmin create example_database

Postgresql

$ createdb example_database

Some platforms, like MySQL support multiple databases per instance, while other platforms like Oracle support one database per instance. You should check the documentation for your particular vendor to see what is supported.

Creating users

Users can be created in most databases using the the CREATE USER statement.

mysql> CREATE USER username;

Specialized Create User Commands

Some relational databases provide additional ways of creating users like specialized command line programs.

MySQL

MySQL does not support creation of users via the mysqladmin command.

Postgresql

$ createuser username

Create Tables

Tables are organized into rows and columns. Data is stored inside these tables. In order to host this information we need to create a table. We do this with the CREATE TABLE statement.

Standard Syntax is:

CREATE TABLE table_name (
column1 datatype(size),
column2 datatype(size),
column3 datatype(size)
);

Here is an example. This will create a table called “Users” with 2 columns.

CREATE TABLE Users (
name varchar(50),
address varchar(50)
);

Alter Table

The ALTER statement will allow you to modify the design of your current database. For example, if you have a table called “Users” which contains Names and Addresses, but you need need to add an age column, you could do so with:

ALTER TABLE Users ADD age int;

Standard Syntax is:

ALTER TABLE table_name ADD column_name datatype

If we need to modify a current table, we can also do so with the ALTER statement. If in the previous example you wanted to change the datatype of age from an integer to varchar, you would do so with:

ALTER TABLE Users MODIFY age varchar(50);

Standard Syntax is:

ALTER TABLE table_name MODIFY column_name datatype

If you now realize you don’t need the age column at all, you can drop it all together with:

ALTER TABLE Users DROP age;

Standard Syntax is:

ALTER TABLE table_name DROP column_name

Drop Table

If you have created a table and need to remove it from your database, you can with the DROP statement:

Standard Syntax is:

DROP table table_name

If we want to drop our table we created earlier (Users) we would use this command:

DROP table Users;

Data Type

There are different types of data that we will store in our database. It can be simple text or a number, sometimes it is a mix of these things. The table below lists some common Standard SQL commands that are also in MySQL. It is important to note this is not a complete list.

Data type Description
char(n) Fixed width character string, can hold letters and numbers-Max of 8,000 characters
varchar(n) Variable width character string, can hold letters and numbers- Max of 8,000 characters
text Variable width character string. Value is represented internally by a separately allocated object
int Allows whole numbers between -2,147,483,648 and 2,147,483,647
float A small number with a floating decimal point
decimal An exact numeric data value, allowing for a fixed decimal point
date Date formated as YYYY-MM-DD
datetime Date and time combination formated as YYYY-MM-DD HH:MM:SS
timestamp Stores a unique number that gets updated every time a row gets created or modified

Granting privileges

Privileges can be granted using the SQL GRANT statement. These statements are persisted by the RDBMS when issued. The typical command format is:

GRANT [PRIVILEGE] on [OBJECT] to [USER];

The standard SQL privileges are:

Privilege Description
ALL Allows user all privileges
ALTER Allows user to alter schema objects
CREATE Allows user to create schema object like tables
DELETE Allows user to delete from an object
EXECUTE Allows user to execute a store procedure or function
INSERT Allows user to add new data to an object
REFERENCES Allows user to create a referential table constraint
SELECT Allows user to read from an object
TRIGGER Allows user to create a trigger
TEMPORARY Allows user to create temporary tables
UPDATE Allows user to update existing data in an object

Below is an example granting a user SELECT privileges on a table

GRANT SELECT ON TABLE example_table TO user;

You can also grant multiple privileges in a single statement.

GRANT SELECT,INSERT ON TABLE example_table TO user;

Many databases stray from the SQL standard here, and it is important to read your database’s documentation when granting privileges. There may be additional privileges not listed here and syntax can vary significantly.

Removing Privileges

Privileges are removed with the SQL REVOKE statement. It follows a similar command format like grant:

REVOKE [PRIVILEGE] on [OBJECT] FROM [USER];

Basic normalized schema design

A normalized schema is a database with a table and column structure designed to reduce data redundancy. Typically data is placed into tables with a unique identifier, or primary key, and then is referenced by id in any tables that wish to use that data.

Suppose we have two types of records in a database; one for a city’s population and one for a city’s average temperature. We could simply create the tables like so:

City Population:

City Population
San Francisco 812,826

City Temperature:

City Temperature
San Francisco 73 F

A normalized version of this would have three tables instead of two.

City ID and Name:

City_id Name
1 San Francisco

City ID and Temperature:

City_id Temperature
1 73 F

City ID and Population:

City_id Population
1 812,826

The advantage of this design is that it prevents you from having to enter data multiple times, and generally reduces the storage cost of a row. If San Francisco changed its name you would only need to update a single row instead of two tables like the first example. SQL allows you to replace the id with a name using a JOIN statement when the data is retrieved, making it functionally identical to the two table example.

Select, Insert, Update and Delete

SELECT

The SELECT statement is the standard way you read from a table in an SQL database. You can use it to retrieve a set of data, and perform aggregations on them. The standard syntax is:

SELECT [column1,column2|*] FROM [TABLE];

By adding a WHERE statement, you can have the database filter results:

SELECT user_id, user_name FROM users WHERE user_id = 1;

You can join tables using a JOIN statement. In this example we’re temporarily assigning an alias of ‘u’ for the table users and an alias of ‘a’ for the table addresses:

SELECT user_id, user_name FROM users u JOIN addresses a
ON u.user_id = a.user_id  WHERE user_id = 1;

Count the rows in a table by using an aggregation:

SELECT COUNT(1) FROM users;

Order by a column:

SELECT * FROM users ORDER BY user_name;

INSERT

The INSERT statement is used to add additional data into a table. It can be used to insert data either a row at a time or in bulk. The standard syntax is:

INSERT INTO table (column1, column2, column3) VALUES (value1, value2, value2)

The column list is optional, if you don’t specify which columns you’re inserting data into, you must provide data for all columns.

For example, to insert a single row:

INSERT INTO users (user_name,user_phone) VALUES ("Joe Bloggs","555-1234");

Or in bulk:

INSERT INTO users (user_name,user_phone)
VALUES ("John Smith","555-5555"),("Tom Jones","555-0987");

Inserting in bulk like that is typically much quicker than using separate queries as the query planner only has to execute once, and any indexes are updated at the end.

UPDATE

UPDATE is the SQL statement for updating existing data in a table. It should almost always be used with a conditional statement. The standard syntax is:

UPDATE [TABLE] SET [COLUMN] = {expression}, {COLUMN2={expression}, ...}
[WHERE condition]
[ORDER BY ...]
[LIMIT count];

Without a WHERE condition, the statement will apply to all the rows in a table.

Here is a simple example of an UPDATE statement:

UPDATE users SET user_name = 'Jim Smith' WHERE user_name = 'James Smith';

DELETE

DELETE is the SQL statement for removing rows from a table. The standard syntax is:

DELETE FROM [TABLE]
[WHERE condition]
[ORDER BY ...]
[LIMIT count] ;

Note

Without a WHERE condition the statement will apply to all the rows of a table.

Here is a simple example of a DELETE statement:

DELETE FROM users WHERE user_name = 'James Smith';

Pro Tips

  • Before doing a write query, run it as a read query first to make sure you are retrieveing exactly what you want. If your query is:

    UPDATE users SET disabled=1 WHERE id=1;

    Run this first to validate you will be affecting the proper record:

    SELECT disabled FROM users WHERE id=1;

  • use a LIMIT on UPDATE and DELETE FROM queries to limit damage imposed by an erroneous query

    UPDATE users SET disabled=1 WHERE id=1 LIMIT 1;

    DELETE FROM users WHERE id=1 LIMIT 1;

  • If your database supports transactions, run START TRANSACTION first then run your query and check what it has done. If you’re happy with what you see then run COMMIT and finally STOP TRANSACTION. If you realize you’ve made a mistake, you can run ROLLBACK and any changes you’ve made will be undone.