Summary of the application usage of MyBatis-Plus

MyBatis-Plusis a very popular MyBatis enhancement tool that provides many ways to simplify queries and operations.applyis one of the most useful methods, which allows you to add native SQL snippets directly into the query criteria. This feature is usually used in scenarios where complex queries are required.

Apply method overview

applyMethods are mainly used inQueryWrapperorLambdaQueryWrapper, custom add a native SQL fragment to the query condition and can be parameterized through placeholders.

Sample code

Below are some common usage examples, showingapplyVarious application scenarios of the method:

Basic usage

Basic usage: Add directlySQLFragment, without parameterized placeholders.

// Assume this variable is your Mapper objectUserMapper userMapper = ...;

QueryWrapper<User> queryWrapper = new QueryWrapper<>();
("date(created_at) = '2023-10-01'");

List<User> users = (queryWrapper);

With parameterized placeholders: Pass parameters using placeholders to prevent SQL injection.

QueryWrapper<User> queryWrapper = new QueryWrapper<>();
String dateParam = "2023-10-01";
("date(created_at) = {0}", dateParam);

List<User> users = (queryWrapper);

Combined with other conditions: Can be used in combination with other query conditions.

QueryWrapper<User> queryWrapper = new QueryWrapper<>();
String startDate = "2023-10-01";
String endDate = "2023-10-31";
queryWrapper
    .ge("age", 18)  // Age is greater than or equal to 18    .apply("date(created_at) BETWEEN {0} AND {1}", startDate, endDate);

List<User> users = (queryWrapper);

LambdaQueryWrapper Usage

Lambda expressions:useLambdaQueryWrappercan provide a more type-safe way to pass fields.

LambdaQueryWrapper<User> lambdaQuery = new LambdaQueryWrapper<>();
("date(created_at) = {0}", dateParam);

List<User> users = (lambdaQuery);

Entity class and Mapper interface in the example

Suppose there is an entity classUserand the corresponding Mapper interfaceUserMapper：

User Entity Class

public class User {
    private Long id;
    private String name;
    private Integer age;
    private Date createdAt;

    // Getters and Setters
}

UserMapper Interface

import ;

public interface UserMapper extends BaseMapper<User> {
}

Complex query example

In practical applications,applyMethods can be used to build more complex query conditions, such as multi-table joint query, custom complex filter conditions, etc. The following example shows how to use it in multi-table joint searchapply：

Multi-table joint search example

Suppose we have another tableOrder, you need to query the user and his latest order information:

public class Order {
    private Long id;
    private Long userId;
    private Date orderDate;
    private BigDecimal amount;

    // Getters and Setters
}

import ;

public interface OrderMapper extends BaseMapper<Order> {
}

Complex query example

import ;
import ;
import ;

public class UserService {

    private final UserMapper userMapper;
    private final OrderMapper orderMapper;

    public UserService(UserMapper userMapper, OrderMapper orderMapper) {
         = userMapper;
         = orderMapper;
    }

    public List<User> getUsersWithRecentOrders() {
        QueryWrapper<User> queryWrapper = ();
        ("EXISTS (SELECT 1 FROM orders o WHERE o.user_id =  AND o.order_date = " +
                           "(SELECT MAX(order_date) FROM orders WHERE user_id = o.user_id))");

        return (queryWrapper);
    }
}

Summarize

From the above example, we can see thatapplyThe method isMyBatis-PlusVery powerful, providing the ability to directly insert native SQL fragments, which makes developers more flexible when building complex query conditions. At the same time, using parameterized queries can effectively prevent SQL injection problems and ensure the security of queries. In actual development, according to specific needs, it can be flexibly used in combination with other conditional construction methods.applyMethods, construct various complex queries.

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