In the first and the second part of window
function series, I talked about the `OVER`

clause, with/without partitions and with/without ordering. Till then, no new function had been
introduced, so it has about the already existing aggregate functions.

Now, new window-only functions has been introduced. I’ll separate them in two groups: ranking and navigational. Both set of functions can be used with/without partition/ordering, but them does not make much sense without ordering.

### Ranking functions (DENSE_RANK, RANK and ROW_NUMBER)

With these functions, one can create different type of incremental counters. Think about `SUM(1) OVER (ORDER BY SALARY)`

, these functions
does this type of thing, but all of them in different ways. Lets see an example query, also comparing with the `SUM`

behavior.

```
select
id,
salary,
dense_rank() over (order by salary),
rank() over (order by salary),
row_number() over (order by salary),
sum(1) over (order by salary)
from employee
order by salary;
```

And the result set:

id | salary | dense_rank | rank | row_number | sum |
---|---|---|---|---|---|

3 | 8.00 | 1 | 1 | 1 | 1 |

4 | 9.00 | 2 | 2 | 2 | 2 |

1 | 10.00 | 3 | 3 | 3 | 4 |

5 | 10.00 | 3 | 3 | 4 | 4 |

2 | 12.00 | 4 | 5 | 5 | 5 |

As you see, the functions differs when repeated values are found in the order key (the salary of 10.00). With `DENSE_RANK`

, no gaps are
created and all repeated values receive the same counter. With `RANK`

, the initial counter is used for all repeated values, but it make
gaps so the next non-repeating value (the salary of 12.00) does not consider the previously repeated values as in different positions.
With `ROW_NUMBER`

, each line receives an incremental value. And `SUM(1)`

is very like `RANK`

, but the value is computed after all repeated
values are summed, so the gap is before the repeated values.

### Navigational functions (LAG and LEAD)

With these functions, an expression could get the value of a previously (`LAG`

) or a posterior (`LEAD`

) row of the query. Follow the
demonstrating query.

```
select
id,
salary,
lag(salary) over (order by salary),
lead(salary) over (order by salary)
from employee
order by salary;
```

And the result set:

id | salary | lag | lead |
---|---|---|---|

3 | 8.00 | `<null>` |
9.00 |

4 | 9.00 | 8.00 | 10.00 |

1 | 10.00 | 9.00 | 10.00 |

5 | 10.00 | 10.00 | 12.00 |

2 | 12.00 | 10.00 | `<null>` |

The functions have a second argument, that is implicitly 1, which means the number of previously/posterior rows will be queried. If this
row does not exist, `<null>`

is returned.

And of course, you can use these functions as expressions and compute, for example, the difference of an employee salary from the one greater and/or lesser.