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landing/docs/SQL-101/007-order-and-group-by.md

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+# ORDER By, GROUP By, HAVING
+
+Applying structure and performance considerations when querying data is paramount. `ORDER BY` provides sorted result sets, essential for presentation and predictable processing. `GROUP BY` aggregates data, enabling summary reporting and analytical operations, often combined with functions like `COUNT`, `MIN `, or `MAX`. The `HAVING` clause refines these aggregations, filtering grouped data based on conditions applied after aggregation.
+
+## ORDER BY
+
+Use `ORDER BY` to enforce a specific order on your query results. Specify one or more columns ; multiple columns are comma-separated.
+
+Consider a simple selection from a `users` table without ordering:
+
+```sql
+SELECT id, username FROM users;
+```
+
+Typical output without `ORDER BY` relies on internal database storage  order (often clustered by primary key), not a guaranteed sequence:
+
+| id  | username      |
+| :-- | :------------ |
+| 2   | shinobi       |
+| 3   | Java Team Six |
+| 4   | tony          |
+| 5   | shinobi       |
+| 6   | Java Team Six |
+| 7   | tony          |
+
+To ensure results are sorted by username alphabetically (ascending):
+
+```sql
+SELECT id, username FROM users ORDER BY username;
+```
+
+Output ordered by `username`:
+
+| id  | username      |
+| :-- | :------------ |
+| 2   | shinobi       |
+|  5   | shinobi       |
+| 3   | Java Team Six |
+| 6   | Java Team Six |
+| 4   | tony          |
+| 7   | tony          |
+
+`ORDER BY` defaults to ascending  (`ASC`). Explicitly specify `DESC` for descending order.
+
+Sort usernames in reverse alphabetical order:
+
+```sql
+SELECT id, username FROM users ORDER BY username DESC;
+```
+
+Output in descending username order:
+
+| id   | username      |
+| :-- | :------------ |
+| 4   | tony          |
+| 7   | tony          |
+| 3   | Java Team Six |
+| 6   | Java Team  Six |
+| 2   | shinobi       |
+| 5   | shinobi       |
+
+Place `ORDER BY` after the `WHERE` clause when both are used.
+
+## GROUP BY
+
+`GROUP BY` aggregates rows  with identical values in specified columns into summary rows. This clause is typically used with aggregate functions (`COUNT`, `SUM`, `AVG`, `MIN`, `MAX`) to perform calculations on each group.
+
+To count users sharing the same username:
+
+```sql
+SELECT COUNT(username), username FROM users GROUP BY username;
+```
+
+This groups users by identical usernames and counts entries within each group:
+
+| COUNT(username) | username      |
+|:-------------- | :------------ |
+| 2               | shinobi       |
+| 2               | Java Team Six |
+| 2               | tony          |
+
+`GROUP BY` follows `FROM` and `WHERE` clauses.
+
+## HAVING Clause
+
+`HAVING` filters  groups created by `GROUP BY`. Unlike `WHERE`, which filters individual rows before grouping, `HAVING` filters grouped rows based on conditions, often involving aggregate functions.
+
+Identify usernames that appear more than once:
+
+```sql
+SELECT COUNT (username), username
+FROM users
+GROUP BY username
+HAVING COUNT(username) > 1;
+```
+
+This groups users by username, counts occurrences, and then filters to show only groups where the count is greater than  1:
+
+| COUNT(username) | username      |
+| :-------------- | :------------ |
+| 2               | shinobi       |
+| 2               | Java Team Six |
+| 2               | tony          |
+
+`WHERE` filters rows before aggregation; `HAVING` filters groups after aggregation.