What do you know about sampling and joins in…

Selections (queries SELECT) retrieve data from one or more tables. Joins combine rows from two or more tables based on a related column between them.

Types of joins:

INNER JOIN: Returns rows with matches in both tables.
LEFT JOIN (LEFT OUTER JOIN): Returns all rows from the left table and matching rows from the right. If no matches, NULL is returned for the right table.
RIGHT JOIN (RIGHT OUTER JOIN): Returns all rows from the right table and matching rows from the left. If no matches, NULL is returned for the left table.
FULL JOIN (FULL OUTER JOIN): Returns all rows where there is a match in at least one of the tables. If no matches, NULL is returned for missing fields. (Not supported by all DBMSs, for example, MySQL uses UNION to emulate this).
CROSS JOIN: Returns the Cartesian product of rows from both tables (each row of the first table is joined with each row of the second).

Syntax examples:

-- INNER JOIN
SELECT *
FROM table1
INNER JOIN table2 ON table1.id = table2.foreign_id;

-- LEFT JOIN
SELECT customer.name, order.order_id
FROM customer
LEFT JOIN order ON customer.id = order.customer_id;

Keywords for selections:

SELECT: Defines columns to retrieve.
FROM: Specifies the table(s).
WHERE: Filters rows based on a condition.
GROUP BY: Groups rows with the same values into a summary row.
HAVING: Filters groups after GROUP BY.
ORDER BY: Sorts the result.
LIMIT / OFFSET: Limits the number of returned rows.

Optimizing selections and joins is important for database performance. Indexes, correct data type definitions, avoiding SELECT * when possible, and optimizing WHERE conditions are used.

In PHP, extensions like PDO or MySQLi are used for working with selections and joins, providing APIs for executing SQL queries.

What do you know about sampling and joins in databases?