Explain the written SQL query: what do we retrieve, how do we retrieve it, why do we filter reaction not equal to 0, how do GROUP BY and ORDER BY work? Tell about JOIN types (LEFT, RIGHT, their differences).

Let's analyze a typical SQL query step by step:

• What we retrieve: usually specific fields are selected from tables, such as user data, their reactions, etc.
• How we retrieve: using the SELECT operator to specify the needed columns from tables joined via JOIN.
• Why filter reactions not equal to 0: suppose reaction 0 indicates no reaction or a neutral state, so filtering with WHERE reaction != 0 allows us to get only active or significant reactions.
• How GROUP BY works: groups rows by specified columns so that aggregate functions (like COUNT, SUM) are applied to each group separately.
• How ORDER BY works: sorts the final dataset by specified columns, for example, by the number of reactions or date.

Types of JOINs and their differences:

• INNER JOIN: returns only rows that match the condition in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): returns all rows from the left table and matching rows from the right; if there's no match, the right side will have NULL.
• RIGHT JOIN (or RIGHT OUTER JOIN): conversely, all rows from the right table and matching from the left; if no match, the left side will have NULL.

Example:

SELECT users.id, COUNT(reactions.id) AS reaction_count
FROM users
LEFT JOIN reactions ON users.id = reactions.user_id AND reactions.type != 0
GROUP BY users.id
ORDER BY reaction_count DESC;

Here, we take all users and count their reactions, ignoring reactions with type 0, group by users, and sort by the number of reactions in descending order.