Shardingsphere JDBC ＜ bind table ＞ avoid join Cartesian product

During normal project development , It is inevitable to use cascading query to realize some functions , The same is true after dividing the database and tables . But when operating two tables , We found that our ten pieces of data appeared 20 strip , That is, the Cartesian product phenomenon . So how to avoid this situation ？
In the first part, we sorted out some The core concept Binding tables can avoid this situation .

One 、 What is Cartesian product ？

In short, it is the result of multiplication of two sets . At the database level , When the Cartesian phenomenon occurs , In fact, it is not the result of the program . So in the process of development , See what the requirements are , Avoid unnecessary results returning .

Two 、demo Scenario implementation

We have two tables , One course, One user,user Table has status,course Watch, too status, according to status The same will course Of value The value assigned to user Of status Field and then query the results , And then determine whether the results are qualified .
Tips ：course and user The entity is not displayed this time

1） The key codes are as follows ：

/** * @Auther: huoyajing * @Date: 2021/12/4 05:55 * @Description:courseMapper, use mybatis-plus It's directly packaged CRUD */
@Repository
public interface CourseMapper extends BaseMapper<Course> {
    

}

@Repository
public interface UserMapper extends BaseMapper<User> {
    

	/** *  Query the user status * @return */
	@Select("select u.id,u.name,u.type,c.value as status from user u left join course c on u.status=c.status")
	List<User> queryUserStatus();
}

@Test
	public void addCourse1() {
    
		// Course insertion 
		Course course1=new Course();
		course1.setName("huohuo0");
		course1.setType(0L);
		course1.setStatus(0);
		course1.setValue(" fail, ");
		courseMapper.insert(course1);

		Course course2=new Course();
		course2.setName("huohuo1");
		course2.setType(1L);
		course2.setStatus(1);
		course2.setValue(" pass ");
		courseMapper.insert(course2);

		// Student information insert 
		for (int i=0;i<10;i++){
    
			User user=new User();
			user.setName("user No:"+i);
			user.setStatus(""+i%2);
			user.setType(i);
			userMapper.insert(user);
		}

	}

2） Query results

The test result of itself is ten users , The results of the plan are also ten . But there was 20 strip , The Cartesian product phenomenon appears

3） Cartesian product emergence analysis

Through analysis, it is found that the query result is A1B1,A1B2,A2B1,A2B2. But this is not the result we want , How to solve it ？

3、 ... and 、 What is a bound table ？

Main table and sub table with consistent fragmentation rules

Four 、 Binding table configuration

spring.shardingsphere.sharding.binding-tables[0]=user,course

1） What does the binding table solve ？

take table-strategy.inline.algorithm-expression Of value The same calculated values are bound together to query , Just like ours course,user Two status Equality query , if user Tabular status=0 stay user_1, that course Of status=0 Will only be in course_1. Then other intersection queries are meaningless .

2） Why? binding-tables[0]?

We found that binding-tables yes Collection type , It means that during your normal project development process , There is not necessarily only one bound table , There must be multiple , We have just given an example here .

5、 ... and 、 result

6、 ... and 、 summary

The database and table must be designed for most scenarios from the beginning , Otherwise, later maintenance is not easy