Learning SQL: SELECT and SELECT DISTINCT

What is the SELECT Command?

The SELECT statement is one of the key tools in SQL for getting data from a database. Whether you’re working with big datasets or just doing simple queries, knowing how to use SELECT properly makes interacting with databases much easier.

Basic of the SELECT command

The SELECT statement lets you retrieve data from a database. You can control exactly what information is returned by choosing which columns you want. Here’s the basic syntax:

SELECT column1, column2, ...
FROM table_name;

For example, if you have a customers table and you want to retrieve their first name, last name, and email, the query would be:

SELECT first_name, last_name, email
FROM customers;

This query fetches the first_name, last_name, and email columns from the customers table for every record.

Additionally, If you need all the columns from a table, you can use SELECT *:

SELECT *
FROM customers;

This query retrieves every column from the customers table. However, it’s a best practice to avoid using SELECT * unless necessary, as it can slow down your queries and return more data than required.

Filtering Result with SELECT DISTINCT

Sometimes, you’ll encounter duplicate data you want to remove. This is where the DISTINCT keyword comes into play. When you use SELECT DISTINCT, SQL ensures that unique values are returned in the result set.

SELECT DISTINCT column1, column2, ...
FROM table_name;

Let’s say you’re querying an orders table where each customer can place multiple orders. If you want a list of customers who have placed orders, but you don’t want duplicate entries for the same customer, you can run the following query:

SELECT DISTINCT customer_id
FROM orders;

This query will return only the unique customer IDs from the orders table, even if some customers have placed multiple orders.

You can also apply DISTINCT to multiple columns. In this case, SQL will return unique combinations of the specified columns.

For example, if you want to see all unique customer-product pairs from the orders table:

SELECT DISTINCT customer_id, product_id
FROM orders;

Recap

The SELECT statement is fundamental to querying databases, while the DISTINCT keyword helps filter the results by removing duplicate values. Whether you’re pulling large datasets or working with unique customer information, understanding when and how to use SELECT and SELECT DISTINCT will make your queries more efficient and your data cleaner.

Resources

PostgreSQL SELECT

PostgreSQL SELECT DISTINCT

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