Learn to use MyBatis annotations to handle complex dynamic SQL queries

Master MyBatis annotation dynamic SQL and easily handle complex query logic
With the rapid development of the Internet, data processing and storage have become increasingly large and complex. In order to cope with this change, we need a query method that is flexible, easy to maintain and expand, so that it can handle various complex query logic. As an excellent ORM framework, MyBatis provides a way to annotate dynamic SQL to meet this need.

MyBatis annotation dynamic SQL allows us to use annotations on the methods of the SQL mapping interface to write SQL statements instead of through XML configuration files. This method brings us a lot of convenience, and we can flexibly write different SQL statements according to different needs.

Below, we will use several specific code examples to illustrate how to use MyBatis annotation dynamic SQL to handle complex query logic.

1. Single table query

Suppose we have a user table user, which contains fields such as id, name, age, etc. We need to query qualified users based on different conditions. When using MyBatis to annotate dynamic SQL, we can use the @Select annotation to define the query statement, and then specify the parameters through the @Param annotation.

@Select("SELECT * FROM user WHERE id = #{id}")
User getUserById(@Param("id") Long id);

@Select("SELECT * FROM user WHERE name = #{name}")
User getUserByName(@Param("name") String name);

2. Multiple table query

In actual use, we may encounter situations where we need to query between multiple tables. MyBatis annotated dynamic SQL can also support this requirement well.

Suppose we have two tables user and order, where the user table contains user information and the order table contains order information, and the two tables are related through userId. We need to query the order information of the specified user.

@Select("SELECT * FROM `order` o LEFT JOIN user u ON o.userId = u.id WHERE u.name = #{name}")
List<Order> getOrdersByUserName(@Param("name") String name);

3. Dynamic Conditional Query

Sometimes, we need to query data based on different conditions, which requires using dynamic SQL to build query statements. MyBatis annotation dynamic SQL provides the @if tag to implement this function.

@Select({
    "<script>",
    "SELECT * FROM user",
    "WHERE 1=1",
    "<if test='name != null and name != ""'>",
    "AND name = #{name}",
    "</if>",
    "<if test='age != null and age != 0'>",
    "AND age = #{age}",
    "</if>",
    "</script>"
})
List<User> getUsersByCondition(@Param("name") String name, @Param("age") Integer age);

In the above code, we use the @if tag to determine whether name and age are empty. If they are not empty, the corresponding conditions are spliced ​​into the query statement.

Summary:
Through the above example code, we can see the powerful function of MyBatis annotation dynamic SQL, which can flexibly build complex query logic. There is no need to write cumbersome XML configuration files. You can directly use annotations on methods to write SQL statements, which is simple and clear.

Of course, the above is just to illustrate the use of MyBatis annotation dynamic SQL. The actual situation may be more complicated and needs to be adjusted according to specific business needs. However, compared with the traditional XML configuration file method, using annotated dynamic SQL can be more intuitive and flexible, making the development process more efficient.

Therefore, mastering MyBatis annotation dynamic SQL is an essential skill for every MyBatis developer. It can help us handle various complex query logic and improve development efficiency and code readability. Come learn and use it!

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