2.3.20. @Formula

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Sometimes, you want the Database to do some computation for you rather than in the JVM, you might also create some kind of virtual column.
You can use a SQL fragment (aka formula) instead of mapping a property into a column. This kind of property is read only (its value is calculated by your formula fragment)

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You should be aware that the `@Formula` annotation takes a native SQL clause which can affect database portability.

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<div class="title">Example 62. `@Formula` mapping usage</div>
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<pre class="prettyprint highlight">`@Entity(name = "Account")
public static class Account {

    @Id
    private Long id;

    private Double credit;

    private Double rate;

    @Formula(value = "credit * rate")
    private Double interest;

    //Getters and setters omitted for brevity

}`</pre>
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When loading the `Account` entity, Hibernate is going to calculate the `interest` property using the configured `@Formula`:

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<div class="title">Example 63. Persisting an entity with a `@Formula` mapping</div>
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<pre class="prettyprint highlight">`doInJPA( this::entityManagerFactory, entityManager -&gt; {
    Account account = new Account( );
    account.setId( 1L );
    account.setCredit( 5000d );
    account.setRate( 1.25 / 100 );
    entityManager.persist( account );
} );

doInJPA( this::entityManagerFactory, entityManager -&gt; {
    Account account = entityManager.find( Account.class, 1L );
    assertEquals( Double.valueOf( 62.5d ), account.getInterest());
} );`</pre>
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<pre class="prettyprint highlight">`INSERT INTO Account (credit, rate, id)
VALUES (5000.0, 0.0125, 1)

SELECT
    a.id as id1_0_0_,
    a.credit as credit2_0_0_,
    a.rate as rate3_0_0_,
    a.credit * a.rate as formula0_0_
FROM
    Account a
WHERE
    a.id = 1`</pre>
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The SQL fragment can be as complex as you want and even include subselects.

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