106 lines
3.3 KiB
Markdown
106 lines
3.3 KiB
Markdown
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# ORDER By, GROUP By, HAVING
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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.
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## ORDER BY
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Use `ORDER BY` to enforce a specific order on your query results. Specify one or more columns ; multiple columns are comma-separated.
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Consider a simple selection from a `users` table without ordering:
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```sql
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SELECT id, username FROM users;
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```
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Typical output without `ORDER BY` relies on internal database storage order (often clustered by primary key), not a guaranteed sequence:
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| id | username |
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| :-- | :------------ |
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| 2 | shinobi |
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| 3 | Java Team Six |
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| 4 | tony |
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| 5 | shinobi |
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| 6 | Java Team Six |
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| 7 | tony |
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To ensure results are sorted by username alphabetically (ascending):
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```sql
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SELECT id, username FROM users ORDER BY username;
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```
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Output ordered by `username`:
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| id | username |
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| :-- | :------------ |
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| 2 | shinobi |
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| 5 | shinobi |
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| 3 | Java Team Six |
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| 6 | Java Team Six |
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| 4 | tony |
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| 7 | tony |
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`ORDER BY` defaults to ascending (`ASC`). Explicitly specify `DESC` for descending order.
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Sort usernames in reverse alphabetical order:
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```sql
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SELECT id, username FROM users ORDER BY username DESC;
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```
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Output in descending username order:
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| id | username |
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| :-- | :------------ |
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| 4 | tony |
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| 7 | tony |
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| 3 | Java Team Six |
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| 6 | Java Team Six |
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| 2 | shinobi |
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| 5 | shinobi |
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Place `ORDER BY` after the `WHERE` clause when both are used.
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## GROUP BY
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`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.
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To count users sharing the same username:
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```sql
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SELECT COUNT(username), username FROM users GROUP BY username;
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```
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This groups users by identical usernames and counts entries within each group:
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| COUNT(username) | username |
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|:-------------- | :------------ |
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| 2 | shinobi |
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| 2 | Java Team Six |
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| 2 | tony |
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`GROUP BY` follows `FROM` and `WHERE` clauses.
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## HAVING Clause
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`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.
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Identify usernames that appear more than once:
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```sql
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SELECT COUNT (username), username
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FROM users
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GROUP BY username
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HAVING COUNT(username) > 1;
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```
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This groups users by username, counts occurrences, and then filters to show only groups where the count is greater than 1:
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| COUNT(username) | username |
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| :-------------- | :------------ |
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| 2 | shinobi |
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| 2 | Java Team Six |
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| 2 | tony |
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`WHERE` filters rows before aggregation; `HAVING` filters groups after aggregation.
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