Formula (ebean-annotation 6.15 API)

```
@Target({FIELD,TYPE,ANNOTATION_TYPE})
@Retention(RUNTIME)
@Repeatable(List.class)
public @interface Formula
```
Assign to a property to be based on a SQL formula.
This is typically a SQL Literal value, SQL case statement, SQL function or similar.

Any property based on a formula becomes a read only property.

You may also put use the Transient annotation with the Formula annotation. The effect of the Transient annotation in this case is that the formula will NOT be included in queries by default - you have to explicitly include it via Query.select() or Query.fetch(). You may want to do this if the Formula is relatively expensive and only want it included in the query when you explicitly state it.
```
{@code
 // On the Order "master" bean
 // ... a formula using the Order details
 // ... sum(order_qty*unit_price)
```

- Required Element Summary
  
  Required Elements
  Modifier and Type Required Element Description
  
  String select
  The SQL to be used in the SELECT part of the SQL to populate a property.
- Optional Element Summary
  
  Optional Elements
  Modifier and Type Optional Element Description
  
  String join
  OPTIONAL - the SQL to be used in the JOIN part of the SQL to support the formula.
  
  Platform[] platforms

- Element Detail
  - select
```
String select
```
    The SQL to be used in the SELECT part of the SQL to populate a property.
- - join
```
String join
```
    OPTIONAL - the SQL to be used in the JOIN part of the SQL to support the formula.
    This is commonly used to join a 'dynamic view' to support aggregation such as count, sum etc.
    
    The join string should start with either "left join" or "join".
    
    You will almost certainly use the "${ta}" as a place holder for the table alias of the table you are joining back to (the "base table" of the entity bean).
    
    The example below is used to support a total count of topics created by a user.
    
    join (select user_id, count(*) as topic_count from f_topic group by user_id) as _tc on _tc.user_id = ${ta}.id
    Default:
    
    ""
- - platforms
```
Platform[] platforms
```
    Default:
    
    {}