Which SQL clause filters groups AFTER aggregation, and what is the equivalent clause that filters rows BEFORE aggregation?

Which JOIN type returns all rows from the left table and matched rows from the right table, filling NULLs where there is no match?

What is the difference between RANK() and DENSE_RANK() window functions when there are ties?

What type of index should you use for a column that is only ever queried with exact equality checks (e.g., WHERE user_id = 42)?

Which ACID property guarantees that a committed transaction's data survives a system crash?

At which isolation level can 'phantom reads' occur — where a transaction re-runs a query and finds new rows inserted by another committed transaction?