A detailed explanation of the specific use of MySql external connection query in SpringBoot

When using MySQL's outer connection query in Spring Boot, it is usually implemented through persistence layer frameworks such as JPA, MyBatis, or JDBC. Outer join queries are mainly used to get data from multiple tables, even if there are no matching records in some tables. External connections are divided into left outer connection (LEFT JOIN), right outer connection (RIGHT JOIN) and full outer connection (FULL JOIN). MySQL does not support full outer connection.

1. Left Outer Join (LEFT JOIN)

The left outer join returns all records in the left table, even if there are no matching records in the right table. If there is no matching record in the right table, the field in the right table in the result is NULL.

Sample SQL:

SELECT 
    , 
    , 
    b.order_id, 
    b.order_date
FROM 
    customers a
LEFT JOIN 
    orders b 
ON 
     = b.customer_id;

Use in Spring Boot:

Using JPA's @Query annotation:

public interface CustomerRepository extends JpaRepository<Customer, Long> {
    @Query("SELECT new (, , , ) " +
           "FROM Customer c LEFT JOIN  o")
    List<CustomerOrderDTO> findCustomerOrders();
}

Using MyBatis:

<select  resultType="">
    SELECT 
        , 
        , 
        b.order_id AS orderId, 
        b.order_date AS orderDate
    FROM 
        customers a
    LEFT JOIN 
        orders b 
    ON 
         = b.customer_id
</select>

2. Right outer connection (RIGHT JOIN)

The right outer join returns all records in the right table, even if there are no matching records in the left table. If there is no matching record in the left table, the field of the left table in the result is NULL.

Sample SQL:

SELECT 
    , 
    , 
    b.order_id, 
    b.order_date
FROM 
    customers a
RIGHT JOIN 
    orders b 
ON 
     = b.customer_id;

Use in Spring Boot:

Using JPA's @Query annotation:

public interface OrderRepository extends JpaRepository<Order, Long> {
    @Query("SELECT new (, , , ) " +
           "FROM Customer c RIGHT JOIN  o")
    List<CustomerOrderDTO> findOrderCustomers();
}

Using MyBatis:

<select  resultType="">
    SELECT 
        , 
        , 
        b.order_id AS orderId, 
        b.order_date AS orderDate
    FROM 
        customers a
    RIGHT JOIN 
        orders b 
    ON 
         = b.customer_id
</select>

3. Full external connection (FULL JOIN)

MySQL does not support full external connections, but can be simulated through UNION operations.

Sample SQL:

SELECT 
    , 
    , 
    b.order_id, 
    b.order_date
FROM 
    customers a
LEFT JOIN 
    orders b 
ON 
     = b.customer_id
UNION
SELECT 
    , 
    , 
    b.order_id, 
    b.order_date
FROM 
    customers a
RIGHT JOIN 
    orders b 
ON 
     = b.customer_id;

Use in Spring Boot:

Using JPA's @Query annotation:

public interface CustomerOrderRepository extends JpaRepository<Customer, Long> {
    @Query("SELECT new (, , , ) " +
           "FROM Customer c LEFT JOIN  o " +
           "UNION " +
           "SELECT new (, , , ) " +
           "FROM Customer c RIGHT JOIN  o")
    List<CustomerOrderDTO> findAllCustomerOrders();
}

Using MyBatis:

<select  resultType="">
    SELECT 
        , 
        , 
        b.order_id AS orderId, 
        b.order_date AS orderDate
    FROM 
        customers a
    LEFT JOIN 
        orders b 
    ON 
         = b.customer_id
    UNION
    SELECT 
        , 
        , 
        b.order_id AS orderId, 
        b.order_date AS orderDate
    FROM 
        customers a
    RIGHT JOIN 
        orders b 
    ON 
         = b.customer_id
</select>

Summarize

When using MySQL's outer connection query in Spring Boot, it can be implemented through persistence layer frameworks such as JPA and MyBatis. Left outer connection and right outer connection are the most commonly used types of external connections, while full outer connections can be simulated through UNION operations. According to the specific business needs, select the appropriate connection type and map the query results through DTO or entity classes.

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