Are you interested in learning how to use SQL but don't know where to start? Don't worry, you're not alone.
SQL, or Structured Query Language, is a programming language that is used to manage and manipulate data in relational databases. While SQL can seem intimidating to non-technical people, the road to learning SQL can be broken down into manageable steps.
In this blog post, we'll guide you through the process of learning SQL in a way that is welcoming and approachable for people who have no coding background.
Before diving into SQL, it's important to understand some basic concepts.
First, what is a database?
A database is a collection of data that is organized in a specific way to allow for efficient storage, retrieval, and management of data. A relational database is a type of database that organizes data into tables, with each table representing a different type of information.
Second, what is SQL used for?
SQL is used to query, or ask questions of, a database. It allows users to retrieve, update, and manipulate data stored in a relational database.
SQL syntax may seem overwhelming at first, but it's actually quite straightforward once you understand the basics. SQL statements are made up of keywords, commands, and clauses that are used to tell the database what to do. Here are some of the most commonly used SQL commands:
SELECT : retrieves data from one or more tables
FROM : specifies the table or tables to retrieve data from
WHERE : filters data based on a specified condition
ORDER BY : sorts data in ascending or descending order based on a specified column
JOIN : combines data from two or more tables based on a specified column
The first query is a simple way to retrieve all the information from a database table.
Syntax:
SELECT * FROM table_name;
This SELECT statement uses the * symbol to represent “all the columns”. After that, the FROM clause indicates the table to be read.
Example:
SELECT * FROM customers;
This will return all rows and columns from the "customers" table.
You can also only show a few columns from a table.
Syntax:
SELECT column1, column2, column3,... FROM table_name;
Let’s suppose we want a E-mail list with the columns " first_name ", " last_name ", and " email ". The query is very simple:
Example:
SELECT first_name, last_name, email FROM customers;
We specified the columns we want to extract from the table: " first_name ", " last_name ", and " email ".. In the second line of the query, we use the FROM clause to indicate the table to read.
Usually you don’t want to see all the data in a database. You want to see only the rows that match certain criteria: products from one product family. In SQL, this is called filtering the data. The WHERE clause filters the data you want from the rest of the data in the table. It extracts records that match certain conditions.
Syntax:
SELECT * FROM table_name WHERE condition;
For example, suppose we want a list of " customers " who lives in " New York City "
Example:
SELECT * FROM customers WHERE city = 'New York';
This will return all rows from the "customers" table where the "city" column is equal to "New York". The WHERE clause indicates that we only want to obtain those city = 'New York' is TRUE . Other records will not be part of the result. In other words, we are filtering records from the table customers.
Often you want to see query results in a certain order, such as products from the cheapest to the most expensive or product names in alphabetical order. The clause ORDER BY is used to define how the records will be ordered in the query result.
Syntax:
SELECT * FROM table_name ORDER BY column_name [ASC|DESC];
Example:
SELECT * FROM customers ORDER BY last_name ASC;
This will return all rows from the "customers" table sorted in ascending order by the "last_name" column. (ASC - Ascending and DESC - Descending).
You can join tables using JOIN, including INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, and CROSS JOIN .
Syntax:
SELECT * FROM table1 JOIN table2 ON table1.column_name = table2.column_name;
In this example, we want to join data from the tables customer and city. INNER JOIN needs to come after FROM and the name of the first table, customer. After INNER JOIN, place the name of the second table, city. The records with data from both tables are matched by ON with the condition to join. The records in the table city are matched to the records from the table customer if they have the same value in the column id in the table customer and in the column customer_id in the table city.
Example:
SELECT customers.first_name, orders.order_date FROM customers JOIN orders ON customers.customer_id = orders.customer_id;
One of the best ways to learn SQL is to practice using a sample database. Many online resources offer free sample databases that you can use to practice your SQL skills. Some popular sample databases include:
Northwind: a sample database that includes information about a fictional company's products, orders, and customers.
Download here.
Chinook: a sample database that includes information about a fictional music store's inventory, customers, and sales.
Download here.
Sakila: a sample database that includes information about a fictional DVD rental store's inventory, customers, and rentals.
Download here.
Using a sample database will allow you to practice writing SQL queries without having to worry about creating your own data.
Once you have a good understanding of SQL syntax and have practiced with a sample database, you can start to optimize your queries. Optimization refers to the process of making your SQL queries more efficient, which can help improve the performance of your database.
Some tips for optimizing your SQL queries include:
Avoid using SELECT * (which retrieves all columns from a table) and instead specify only the columns you need.
Use indexes to speed up queries that involve large amounts of data.
Avoid using subqueries (queries within queries) unless necessary, as they can be slow and resource-intensive.
SQL is a powerful tool for managing and manipulating data, and there is always more to learn. As you become more comfortable with SQL, consider learning more advanced topics such as:
Stored procedures: pre-written SQL code that can be reused in multiple queries
Triggers: code that is automatically executed when a certain event occurs, such as when data is inserted or updated in a table
Views: virtual tables that are created based on the results of a query, allowing for easier data analysis and reporting.
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Best For- Intermediate Level
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Publisher: O′Reilly
Latest Edition: 3rd edition
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1. SQL quiz by w3schools
2. Datalemur by Nick Singh here
Learning SQL may seem daunting at first, but by breaking it down into manageable steps and practicing with sample databases, you can quickly become proficient in querying and manipulating.
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