MyBatis_动态SQL

一、动态SQL

 动态SQL，主要用于解决查询条件不确定的情况：在程序运行期间，根据提交的查询条件进行查询。

 动态SQL，即通过MyBatis提供的各种标签对条件作出判断以实现动态拼接SQL语句。

二、使用动态SQL原因

 提供的查询条件不同，执行的SQL语句不同。若将每种可能的情况均逐一列出，就将出现大量的SQL语句。

三、<if/>标签

注意事项：

（1）

@Test
    public void test08() {
        Student stu = new Student("明", 20, 0);
        List<Student> students = dao.selectStudentsByCondition(stu);
        for (Student student : students) {
            System.out.println(student);
        }

    }

public interface IStudentDao {
    // 根据条件查询问题
    List<Student> selectStudentsByCondition(Student student);
}

<mapper namespace="com.jmu.dao.IStudentDao">    
    <select id="selectStudentsByCondition" resultType="Student">
    select id,name,age,score 
    from student 
    where  
    <if test="name !=null and name !=''">
        name like '%' #{name} '%'
    </if>
     <if test="age>0">
         and age >#{age}
     </if>
    </select>
</mapper>

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - ==>  Preparing: select id,name,age,score from student where name like '%' ? '%' and age >? 
48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - ==> Parameters: 明(String), 20(Integer)
96 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - <==      Total: 15
Student [id=159, name=明明, score=87.9, age=23]
Student [id=173, name=小明明, score=99.5, age=23]
Student [id=175, name=明明, score=99.5, age=23]
Student [id=177, name=明明, score=99.5, age=23]
Student [id=179, name=明明, score=99.5, age=23]
Student [id=181, name=明明, score=99.5, age=23]
Student [id=183, name=明明, score=99.5, age=23]
Student [id=185, name=明明, score=99.5, age=23]
Student [id=187, name=明明, score=99.5, age=23]
Student [id=189, name=明明, score=99.5, age=23]
Student [id=191, name=明明, score=99.5, age=23]
Student [id=193, name=明明, score=99.5, age=23]
Student [id=195, name=明明, score=99.5, age=23]
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]

（2） 针对第一个值为空，sql语句“where and”出错的情况

	
//		Student stu = new Student("明", 20, 0);
		Student stu = new Student("", 20, 0);
	

　　

解决：

四、<where/>标签

当数据量特别大，做“where 1= 1”的判断，就降低了整个系统的执行效率

@Test
    public void test02() {
//        Student stu = new Student("明", 20, 0);
        Student stu = new Student("", 20, 0);
        List<Student> students = dao.selectStudentsByWhere(stu);
        for (Student student : students) {
            System.out.println(student);
        }

    }

import java.util.List;
import java.util.Map;

import com.jmu.bean.Student;

public interface IStudentDao {
    // 根据条件查询问题
    List<Student> selectStudentsByIf(Student student);
    List<Student> selectStudentsByWhere(Student student);
}

<select id="selectStudentsByWhere" resultType="Student">
        select id,name,age,score
        from student
        <where>
            <if test="name !=null and name !=''">
                and name like '%' #{name} '%'
            </if>
            <if test="age>0">
                and age >#{age}
            </if>
        </where>
    </select>

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==>  Preparing: select id,name,age,score from student where 1=1 and age >? 
48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==> Parameters: 20(Integer)
86 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]
111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >? 
111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >? 
111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
115 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
115 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]

五、<choose/>标签

该标签可以包含多个<when/>和一个<otherwise/>,它们联合使用来完成JAVA的开关语句switch...case功能。

<select id="selectStudentsByChoose" resultType="Student">
        select id,name,age,score
        from student
        <where>
            <choose>
                <when test="name !=null and name !=''">
                    and name like '%' #{name} '%'
                </when>
                <when test="age>0">
                    and age>#{age}
                </when>
                <otherwise>
                     1=2 <!-- false,使得查询不到结果 -->
                </otherwise>
            </choose>
        </where>
    </select>

import java.util.List;
import java.util.Map;

import com.jmu.bean.Student;

public interface IStudentDao {
    // 根据条件查询问题
    List<Student> selectStudentsByIf(Student student);
    List<Student> selectStudentsByWhere(Student student);
    List<Student> selectStudentsByChoose(Student student);
    
}

}
    @Test
    public void test03() {
//        Student stu = new Student("明", 20, 0);
        Student stu = new Student("", 20, 0);
//        Student stu = new Student("", 0, 0);
        List<Student> students = dao.selectStudentsByChoose(stu);
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==>  Preparing: select id,name,age,score from student where 1=1 and age >? 
50 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==> Parameters: 20(Integer)
86 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]
104 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >? 
104 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >? 
105 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
105 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
109 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
109 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]
121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>? 
121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>? 
121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>? 
122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=200, name=明明, score=99.5, age=23]

当查询的条件均不满足时：

Student stu = new Student("", 0, 0);

输出：

六、<foreach/>标签--遍历数组

该标签用于实现对数组与集合的遍历，对其使用需要注意：

• collection表示要遍历的集合类型，这里是数组，即array。
• open、close、separate为遍历内容的SQL拼接。

List<Student> selectStudentsByForeach(int[] ids);

@Test
    public void test04() {
       int[] ids={197,198,199};
        List<Student> students = dao.selectStudentsByForeach(ids);
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

<select id="selectStudentsByForeach" resultType="Student">
        <!-- select id,name,age,score from student where id in (1,3,5) -->
        select id,name,age,score
        from student
        <if test="array.length>0">
            where id in
            <foreach collection="array" item="myid" open="(" close=")"
                separator=",">
                #{myid}
            </foreach>
        </if>
    </select>

输出：

176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? ) 
176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? ) 
176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? ) 
176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? ) 
177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
Student [id=197, name=明明, score=87.9, age=19]
Student [id=198, name=明明, score=99.5, age=23]
Student [id=199, name=明明, score=87.9, age=19]

七、<foreach/>标签--遍历泛型为基本类型的List

@Test
    public void test05() {
        List<Integer> ids =new ArrayList<>();
        ids.add(198);
        ids.add(199);
        List<Student> students = dao.selectStudentsByForeach2(ids);
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

import java.util.List;
import com.jmu.bean.Student;

public interface IStudentDao {
    // 根据条件查询问题
    List<Student> selectStudentsByIf(Student student);
    List<Student> selectStudentsByWhere(Student student);
    List<Student> selectStudentsByChoose(Student student);
    List<Student> selectStudentsByForeach(int[] ids);
    List<Student> selectStudentsByForeach2(List<Integer> ids);
}

<select id="selectStudentsByForeach2" resultType="Student">
        <!-- select id,name,age,score from student where id in (1,3,5) -->
        select id,name,age,score
        from student
        <if test="list.size>0">
            where id in
            <foreach collection="list" item="myid" open="(" close=")"
                separator=",">
                #{myid}
            </foreach>
        </if>
    </select>

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? ) 
48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - ==> Parameters: 198(Integer), 199(Integer)
78 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=199, name=明明, score=87.9, age=19]

八、<foreach/>标签--遍历泛型为自定义类型的List

import java.util.List;
import com.jmu.bean.Student;

public interface IStudentDao {
    // 根据条件查询问题
    List<Student> selectStudentsByIf(Student student);
    List<Student> selectStudentsByWhere(Student student);
    List<Student> selectStudentsByChoose(Student student);
    List<Student> selectStudentsByForeach(int[] ids);
    List<Student> selectStudentsByForeach2(List<Integer> ids);
    List<Student> selectStudentsByForeach3(List<Student> ids);
}

@Test
    public void test06() {
        Student stu1 = new Student();
        stu1.setId(198);
        Student stu2 = new Student();
        stu2.setId(199);
        List<Student> stus =new ArrayList<>();
        stus.add(stu1);
        stus.add(stu2);
        List<Student> students = dao.selectStudentsByForeach3(stus);
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

<select id="selectStudentsByForeach3" resultType="Student">
        select id,name,age,score
        from student
        <if test="list.size>0">
            where id in
            <foreach collection="list" item="stu" open="(" close=")"
                separator=",">
                #{stu.id}
            </foreach>
        </if>
    </select>

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? ) 
49 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - ==> Parameters: 198(Integer), 199(Integer)
90 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - <==      Total: 2
Student [id=198, name=明明, score=99.5, age=23]
Student [id=199, name=明明, score=87.9, age=19]

九、SQL片段