SQL tips

You can use the INNER JOIN operator to combine data from two or more tables based on a common column or set of columns. Here's an example

Retrieve all orders with customer and product details
SELECT o.order_id, c.customer_name, p.product_name, o.order_date
FROM orders AS o
JOIN customers AS c ON o.customer_id = c.customer_id
JOIN products AS p ON o.product_id = p.product_id

You can use the LEFT JOIN operator to include data from one table even if there is no corresponding data in another table.

This is useful when you need to include all data from one table, even if there are missing values in another table. 

-- Retrieve all customers and their orders (even if they haven't placed an order)
SELECT c.customer_id, c.customer_name, o.order_id
FROM customers AS c
LEFT JOIN orders AS o ON c.customer_id = o.customer_id

-- Without aliases
SELECT employees.employee_name, departments.department_name
FROM employees
JOIN departments ON employees.department_id = departments.department_id

-- With aliases
SELECT emp.employee_name, dept.department_name
FROM employees AS emp
JOIN departments AS dept ON emp.department_id = dept.department_id

-- Imagine youwould like to select 20 columns from a 23-column table.
-- Instead of writing the following code:
SELECT
post. columnl , post. column2
, post. column3
, post. column4
, post. column5
• ••
, post. column20
FROM
posting as post

-- Do this:
SELECT
post.* EXCEPT (post.column21, post.column22,post.column23)
FROM
posting as post