Return distinct values

Use DISTINCT when the result needs unique values instead of every matching row:

SELECT DISTINCT country
FROM users
ORDER BY country;

If 50 users live in Italy and 30 live in Denmark, the result contains two country rows.

With several selected columns, uniqueness applies to the complete combination:

SELECT DISTINCT country, city
FROM users;

Italy, Rome and Italy, Milan remain separate. Adding a unique column such as id would make every row distinct and remove the benefit.

Several NULL values normally appear as one distinct result value. This is result-set behavior; it does not mean ordinary comparisons consider NULL equal to NULL.

Be suspicious when DISTINCT is added only to hide duplicates after a join. The join may be matching more related rows than expected, or the schema may be missing a uniqueness rule. Removing visible duplicates does not fix an incorrect total or an accidental many-to-many relationship.

DISTINCT can require sorting or hashing a large result. Use it because the question asks for unique combinations, not as a default decoration.

Your action is to run DISTINCT on one column, two columns, and two columns including a unique ID. Predict the number of result rows before each query.