> For the complete documentation index, see [llms.txt](https://fistkim101.gitbook.io/tech-blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://fistkim101.gitbook.io/tech-blog/undefined/querydsl/+-alias.md).

# (+) 별칭(alias)

queryDsl 로 join 을 할 때, alias 를 걸어줘도 되고 안걸어줘도 무관하다.

```java
    /**
     * Create a left join with the given target.
     * Use fetchJoin() to add the fetchJoin parameter to this join.
     *
     * @param <P>
     * @param target target
     * @return the current object
     */
    <P> JPQLQuery<T> leftJoin(CollectionExpression<?,P> target);

    /**
     * Create a left join with the given target and alias.
     *
     * @param <P>
     * @param target target
     * @param alias alias
     * @return the current object
     */
    <P> JPQLQuery<T> leftJoin(CollectionExpression<?,P> target, Path<P> alias);
```

첫번째 파라미터가 target 으로 이름이 지어져 있는데, 말 그대로 from 에 걸어준 테이블과 해당 target 을 join 한다는 것이다.

만약 target 으로 정한 테이블의 컬럼과 추가적인 join 설정을 해야 할 경우 아래와 같이 alias 를 지정해주고, 지정해준 alias 의 path 를 이용해서 join 을 걸어주면 된다. 아래는 예시 코드이다.

```java
    public Meal findMealByMealId(Long mealId) {
        QMealFeedback mealFeedbackAlias = new QMealFeedback("mealFeedback");
        return from(meal)
                .distinct()
                .innerJoin(meal.user).fetchJoin()
                .leftJoin(meal.mealFeedbacks, mealFeedbackAlias).fetchJoin()
                .leftJoin(mealFeedbackAlias.user).fetchJoin()
                .where(meal.id.eq(mealId))
                .fetchOne();
    }
```

```sql
    select
        distinct m1_0.id,
        m1_0.created_at,
        m1_0.image_url,
        m1_0.intake_time,
        m2_0.meal_id,
        m2_0.id,
        m2_0.created_at,
        m2_0.feedback_contents,
        m2_0.feedback_type,
        m2_0.updated_at,
        u2_0.id,
        u2_0.created_at,
        u2_0.is_social_login_user,
        u2_0.phone_number,
        u2_0.updated_at,
        u2_0.active_score,
        u2_0.meal_feedback_total_count,
        u2_0.meal_total_count,
        u2_0.grade_expired_at,
        u2_0.user_grade,
        u2_0.description,
        u2_0.gender_type,
        u2_0.image_url,
        u2_0.nick_name,
        u2_0.user_resource_id,
        m1_0.memo,
        m1_0.updated_at,
        u1_0.id,
        u1_0.created_at,
        u1_0.is_social_login_user,
        u1_0.phone_number,
        u1_0.updated_at,
        u1_0.active_score,
        u1_0.meal_feedback_total_count,
        u1_0.meal_total_count,
        u1_0.grade_expired_at,
        u1_0.user_grade,
        u1_0.description,
        u1_0.gender_type,
        u1_0.image_url,
        u1_0.nick_name,
        u1_0.user_resource_id 
    from
        meal m1_0 
    join
        user u1_0 
            on u1_0.id=m1_0.user_id 
    left join
        meal_feedback m2_0 
            on m1_0.id=m2_0.meal_id 
    left join
        user u2_0 
            on u2_0.id=m2_0.user_id 
    where
        m1_0.id=?
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://fistkim101.gitbook.io/tech-blog/undefined/querydsl/+-alias.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.