join questions

In MySQL, JOIN operations are used to combine rows from two or more tables based on related columns. Use `INNER JOIN` to return records with matching values in both tables, `LEFT JOIN` to return all records from the left table and matched records from the right table, and `RIGHT JOIN` for the opposite. For example, `SELECT * FROM orders INNER JOIN customers ON orders.customer_id = customers.id` retrieves orders with customer details.

`Array.prototype.join` joins all elements of an array into a string, with elements separated by a specified separator. The default separator is a comma if none is provided. 

const arr = ['a', 'b', 'c'];
console.log(arr.join('-')); // 'a-b-c'

`Array.prototype.join` joins all elements of an array into a string, with elements separated by a specified separator. The default separator is a comma if none is provided. 

const arr = ['a', 'b', 'c'];
console.log(arr.join('-')); // 'a-b-c'

`Array.prototype.join` joins all elements of an array into a string separated by a specified separator. The default separator is a comma. 

const arr = ['a', 'b', 'c'];
const joined = arr.join('-');
console.log(joined); // 'a-b-c'

`INNER JOIN` returns rows where there is a match in both joined tables, while `LEFT JOIN` (or `LEFT OUTER JOIN`) returns all rows from the left table and matched rows from the right table. If there’s no match, NULL values are returned for the right table's columns. For example: `SELECT * FROM a INNER JOIN b ON a.id = b.a_id;` versus `SELECT * FROM a LEFT JOIN b ON a.id = b.a_id;`.