Introduction to Relational Databases
Relational databases are an important tool in modern computer programming and data management. A relational database is a type of database that stores data in tables with rows and columns. The data in a relational database is organized into tables that are related to each other based on common fields.
The concept of a relational database was first introduced in the 1970s by Edgar F. Codd. He proposed a model for data storage that used a mathematical approach to data modeling, where data was stored in tables with columns and rows, and relationships between tables were established using keys.
Today, relational databases are widely used in a variety of applications, from small-scale databases used by individuals and small businesses to large-scale databases used by multinational corporations and governments.
In the following sections, we will explore the basics of relational databases, including their structure, design, and use. We will also cover some of the most common database management systems and query languages used in the industry.
What are relational databases?
A relational database is a type of database that organizes data into one or more tables or relations. In a relational database, data is stored in tables with columns and rows, and each table has a unique name. The columns in a table represent the different attributes of the data, while the rows represent the individual records. Relational databases are used to store, manage, and retrieve data in a structured way, making it easy to access and analyze large amounts of data. They are widely used in many different industries, including finance, healthcare, retail, and more.
How are they used in modern applications?
Relational databases are widely used in modern applications to store and manage structured data. They are especially useful in applications where data needs to be organized into tables, such as financial systems, inventory management systems, customer relationship management systems, and content management systems, among others.
Relational databases provide a structured and efficient way to store data and allow for easy querying, searching, sorting, and manipulation of data. They are also capable of handling large amounts of data and multiple concurrent users.
In modern applications, relational databases are often used in conjunction with other technologies such as web development frameworks, APIs, and cloud computing services to build robust and scalable systems.
What is SQL?
SQL (Structured Query Language) is a programming language used for managing and manipulating data in relational databases. SQL allows users to insert, update, delete, and query data in a database, and is used by many popular database management systems such as MySQL, Oracle, and Microsoft SQL Server. SQL is a standard language, meaning that it can be used across different platforms and database systems.
SELECT, FROM, WHERE, ORDER BY, LIMIT
SELECT, FROM, WHERE, ORDER BY, and LIMIT are some of the most common keywords used in SQL syntax:
- SELECT: used to select specific columns from a table.
- FROM: specifies the table(s) to be queried.
- WHERE: used to filter rows based on a specified condition.
- ORDER BY: used to sort the rows in a result set by one or more columns.
- LIMIT: used to limit the number of rows returned in a result set.
Together, these keywords can be used to perform a wide variety of queries on a database, ranging from simple searches to complex data aggregations and manipulations.
SELECT
SELECT is a keyword in SQL that is used to query data from a database. It is followed by a list of columns or expressions that are to be selected. For example, if you want to select all the columns from a table, you can use the following syntax:
SELECT * FROM table_name;
If you want to select specific columns from a table, you can list them after the SELECT keyword, separated by commas:
SELECT column1, column2, ... FROM table_name;
You can also use expressions in the SELECT statement, such as concatenating two columns:
SELECT column1 || ' ' || column2 as full_name FROM table_name;
This would concatenate the values in column1 and column2, separated by a space, and label the result as “full_name”.
FROM
The FROM clause in SQL is used to specify the table or tables from which you want to retrieve data. You can use the FROM clause to join multiple tables together to retrieve data that is spread across different tables. The syntax for the FROM clause is as follows:
SELECT column1, column2, ...
FROM table1, table2, ...
In this example, you would replace column1, column2, table1, and table2 with the appropriate column and table names for your query. You can include as many columns and tables as needed, separating each with a comma.
WHERE
In SQL, the WHERE clause is used to filter records that meet a specific condition. It allows you to specify a condition that must be met in order for a row to be selected. The WHERE clause is commonly used with the SELECT statement.
The syntax for the WHERE clause is as follows:
SELECT column1, column2, ...
FROM table_name
WHERE condition;
In this syntax, column1, column2, ... are the names of the columns you want to select from the table, table_name is the name of the table you want to select data from, and condition is the condition that must be met in order for a row to be selected.
For example, the following SQL statement selects all the records from the “customers” table where the “country” is “Mexico”:
SELECT *
FROM customers
WHERE country = 'Mexico';
This statement will return all the records from the “customers” table where the “country” is “Mexico”.
In SQL, the ORDER BY keyword is used to sort the result set in ascending or descending order. It is typically used in conjunction with the SELECT statement.
The basic syntax for the ORDER BY clause is as follows:
SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC/DESC;
In this syntax, column1, column2, etc. are the columns to be selected from the table, and table_name is the name of the table from which the data is being selected. The ORDER BY clause is used to sort the result set in ascending or descending order, based on one or more columns.
For example, the following SQL statement would select all columns from the “employees” table, and order the result set by the “last_name” column in ascending order:
SELECT * FROM employees
ORDER BY last_name ASC;
If you want to sort the result set in descending order, you can use the DESC keyword:
SELECT * FROM employees
ORDER BY last_name DESC;
You can also sort by multiple columns, by listing them after the ORDER BY keyword:
SELECT * FROM employees
ORDER BY last_name, first_name;
This would first sort the result set by the “last_name” column, and then by the “first_name” column.
LIMIT
The LIMIT statement is used to limit the number of rows returned by a query. It is useful when dealing with large tables or when you only need to display a small portion of the data.
The syntax for the LIMIT statement is as follows:
SELECT column1, column2, ...
FROM table_name
LIMIT [number of rows];
The [number of rows] is the number of rows you want to return in the result set. For example, the following query returns the first 10 rows from the customers table:
SELECT *
FROM customers
LIMIT 10;
This query will return the first 10 rows of the customers table, starting from the first row. If you want to skip the first few rows and start from a specific row, you can specify an offset value after the number of rows:
SELECT *
FROM customers
LIMIT 10 OFFSET 5;
This query will return 10 rows from the customers table, starting from the 6th row.
Use of wildcards (*)
In SQL, the asterisk (*) is used as a shorthand notation to represent all columns in a table.
For example, the following SQL statement selects all columns from the “students” table:
SELECT * FROM students;
This will return all the columns in the “students” table. The asterisk is useful when you want to retrieve all the data from a table without specifying the individual column names. However, it is generally recommended to explicitly specify the column names that you need, especially in larger tables, to reduce the amount of data retrieved and improve query performance.
Distinct keyword
The DISTINCT keyword is used in a SQL SELECT statement to return only unique values from a column or set of columns. When used, the result set will only contain one instance of each unique value in the specified columns.
For example, consider the following table of customer orders:
| OrderID | CustomerID | OrderDate |
|---|---|---|
| 1 | 1001 | 2022-01-01 |
| 2 | 1001 | 2022-01-05 |
| 3 | 1002 | 2022-01-10 |
| 4 | 1003 | 2022-01-15 |
| 5 | 1001 | 2022-02-01 |
If we want to get a list of all the unique customer IDs, we can use the following SQL query:
SELECT DISTINCT CustomerID
FROM Orders;
The result set would be:
| CustomerID |
|---|
| 1001 |
| 1002 |
| 1003 |
Note that the DISTINCT keyword is only used with the SELECT statement, and cannot be used with other SQL statements like INSERT, UPDATE, or DELETE.
Tutorial
MySQL
Here’s a tutorial on how to install a SQL database management system, specifically MySQL:
- Download MySQL Installer from the official website: https://dev.mysql.com/downloads/installer/
- Launch the downloaded installer and choose the option to install MySQL Server.
- During the installation process, you will be prompted to choose a default configuration. Choose the option that best suits your needs (e.g. Developer Default, Server Only, etc.).
- After choosing the default configuration, you will be asked to enter a root password for the MySQL server. This is an important step, as the root user has full privileges over the server.
- Once the installation is complete, you can launch the MySQL Command Line Client by opening the command prompt and typing “mysql -u root -p” followed by your root password.
- You can now start creating databases, tables, and managing your data using SQL commands.
Note that the above steps are specific to installing MySQL on Windows. The installation process may differ slightly depending on your operating system and the database management system you choose to install.
PostgreSQL
Step 1: Install PostgreSQL
The first step to using PostgreSQL is installing it. You can download PostgreSQL from the official website for your operating system (https://www.postgresql.org/download/).
Step 2: Create a database
Once you have PostgreSQL installed, you can create a new database using the createdb command. Open a terminal or command prompt and type:
createdb mydatabase
This will create a new database named mydatabase. You can replace mydatabase with any name you choose.
Step 3: Connect to the database
To connect to the database, you can use the psql command followed by the name of the database. For example:
psql mydatabase
This will connect to the mydatabase database and bring up a PostgreSQL shell prompt.
Step 4: Create a table
Once you’re connected to the database, you can create a new table. For example, let’s create a table to store information about users:
CREATE TABLE users (
id SERIAL PRIMARY KEY,
name VARCHAR(50) NOT NULL,
email VARCHAR(50) NOT NULL
);
This will create a new table named users with three columns: id, name, and email. The id column is a serial primary key, which means it will automatically generate a unique value for each new row.
Step 5: Insert data
Now that you have a table, you can insert data into it using the INSERT INTO statement. For example:
INSERT INTO users (name, email) VALUES ('John Smith', 'john@example.com');
This will insert a new row into the users table with the name “John Smith” and the email “john@example.com“.
Step 6: Query data
You can retrieve data from the users table using the SELECT statement. For example:
SELECT * FROM users;
This will retrieve all rows from the users table. You can also add conditions to your query to filter the results. For example:
SELECT * FROM users WHERE name = 'John Smith';
This will retrieve all rows from the users table where the name is “John Smith”.
These are just the basics of using PostgreSQL, but it should give you a good starting point to begin exploring the world of SQL databases.
Exercise
Write a basic SELECT statement to retrieve data from a table
Write a SELECT statement with WHERE clause to filter data
Use the ORDER BY clause to sort data in ascending and descending order
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