**SQL RANK() Function**
=======================
The `RANK()` function in SQL is a window function that assigns a ranking to each row within a result set partition, based on a specific column or expression. The ranking is done in descending order by default.
**Syntax**
```sql
RANK() OVER (
[PARTITION BY column1, column2, ...]
ORDER BY column1, column2, ...
)
```
**How it Works**
----------------
1. The `PARTITION BY` clause divides the result set into partitions based on one or more columns.
2. The `ORDER BY` clause sorts the rows within each partition in descending order.
3. The `RANK()` function assigns a ranking to each row within each partition, based on the sorted order.
**Example**
```sql
SELECT
employee_name,
salary,
RANK() OVER (ORDER BY salary DESC) as salary_rank
FROM
employees;
```
This query ranks employees by their salary in descending order.
**Output**
```markdown
+---------------+--------+-------------+
| employee_name | salary | salary_rank |
+---------------+--------+-------------+
| John | 100000 | 1 |
| Jane | 90000 | 2 |
| Bob | 80000 | 3 |
| Alice | 70000 | 4 |
| Mike | 60000 | 5 |
+---------------+--------+-------------+
```
**Use Cases**
-------------
1. **Ranking sales performance**: Rank sales representatives by their sales amount in descending order.
2. **Identifying top performers**: Rank employees by their performance metrics, such as customer satisfaction or productivity.
3. **Analyzing student grades**: Rank students by their grades in descending order, to identify top performers.
**Variations**
--------------
* `DENSE_RANK()`: similar to `RANK()`, but does not skip ranking numbers if there are ties.
* `ROW_NUMBER()`: assigns a unique number to each row within a partition, without considering ties.
**Best Practices**
------------------
* Use `RANK()` and `DENSE_RANK()` judiciously, as they can affect query performance.
* Consider using `ROW_NUMBER()` when you need to assign a unique identifier to each row.
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