# Functions

## Built-In Functions

SQL Server built-in functions are either deterministic or nondeterministic.

|                                                       Deterministic Functions                                                       |                                                             Non-Deterministic Functions                                                             |
| :---------------------------------------------------------------------------------------------------------------------------------: | :-------------------------------------------------------------------------------------------------------------------------------------------------: |
| Functions are deterministic when they always return the same result anytime they're called by using a specific set of input values. | Functions are nondeterministic when they could return different results every time they're called, even with the same specific set of input values. |

* **Aggregate Functions** - An aggregate function performs a calculation on a set of values, and returns a single value.

Examples: `AVG(), COUNT(), MAX(), MIN(), SUM()`

* **Ranking Functions** - Ranking functions return a ranking value for each row in a partition.

Examples: `DENSERANK(), NTILE(), RANK(), ROW_NUMBER()`

* **Scaler Functions** - Operate on a single value and then return a single value. Scalar functions can be used wherever an expression is valid.
  * Date and Time Functions
* **Error Functions**

Examples: `ERROR_LINE()`, `ERROR_NUMBER()`, `ERROR_STATE()`, `ERROR_SEVERITY()`, `ERROR_PROCEDURE()`, `ERROR_MESSAGE()`

## User-Defined Functions <a href="#date-and-time-data-types-and-functions-transact-sql" id="date-and-time-data-types-and-functions-transact-sql"></a>

```csharp
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions{
    [SqlFunction]
    public static float InchtoMillimeters(float inches){
        return 25.4f * inches;
    }
}
```