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）

  1.  @Test
  2.      public void test08() {
  3.          Student stu = new Student("明", 20, 0);
  4.          List<Student> students = dao.selectStudentsByCondition(stu);
  5.          for (Student student : students) {
  6.              System.out.println(student);
  7.          }
  8.  
  9.      }

com.jmu.test.MyTest

  1.  public interface IStudentDao {
  2.      // 根据条件查询问题
  3.      List<Student> selectStudentsByCondition(Student student);
  4.  }

com.jmu.dao.IStudentDao

  1.  <mapper namespace="com.jmu.dao.IStudentDao">
  2.      <select id="selectStudentsByCondition" resultType="Student">
  3.      select id,name,age,score
  4.      from student
  5.      where
  6.      <if test="name !=null and name !=''">
  7.          name like '%' #{name} '%'
  8.      </if>
  9.       <if test="age>0">
  10.           and age >#{age}
  11.       </if>
  12.      </select>
  13.  </mapper>

mapper.xml

输出：

  1. 0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - ==>  Preparing: select id,name,age,score from student where name like '%' ? '%' and age >?
  2. 48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - ==> Parameters: 明(String), 20(Integer)
  3. 96 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByCondition  - <==      Total: 15
  4. Student [id=159, name=明明, score=87.9, age=23]
  5. Student [id=173, name=小明明, score=99.5, age=23]
  6. Student [id=175, name=明明, score=99.5, age=23]
  7. Student [id=177, name=明明, score=99.5, age=23]
  8. Student [id=179, name=明明, score=99.5, age=23]
  9. Student [id=181, name=明明, score=99.5, age=23]
  10. Student [id=183, name=明明, score=99.5, age=23]
  11. Student [id=185, name=明明, score=99.5, age=23]
  12. Student [id=187, name=明明, score=99.5, age=23]
  13. Student [id=189, name=明明, score=99.5, age=23]
  14. Student [id=191, name=明明, score=99.5, age=23]
  15. Student [id=193, name=明明, score=99.5, age=23]
  16. Student [id=195, name=明明, score=99.5, age=23]
  17. Student [id=198, name=明明, score=99.5, age=23]
  18. Student [id=200, name=明明, score=99.5, age=23]

output

（2） 针对第一个值为空，sql语句“where and”出错的情况

  1. //		Student stu = new Student("明", 20, 0);
  2. 		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

  1.  @Test
  2.      public void test02() {
  3.  //        Student stu = new Student("明", 20, 0);
  4.          Student stu = new Student("", 20, 0);
  5.          List<Student> students = dao.selectStudentsByWhere(stu);
  6.          for (Student student : students) {
  7.              System.out.println(student);
  8.          }
  9.  
  10.      }

com.jmu.test.MyTest

  1. import java.util.List;
  2. import java.util.Map;
  3.  
  4. import com.jmu.bean.Student;
  5.  
  6. public interface IStudentDao {
  7.     // 根据条件查询问题
  8.     List<Student> selectStudentsByIf(Student student);
  9.     List<Student> selectStudentsByWhere(Student student);
  10. }

com.jmu.dao.IStudentDao

  1.  <select id="selectStudentsByWhere" resultType="Student">
  2.          select id,name,age,score
  3.          from student
  4.          <where>
  5.              <if test="name !=null and name !=''">
  6.                  and name like '%' #{name} '%'
  7.              </if>
  8.              <if test="age>0">
  9.                  and age >#{age}
  10.              </if>
  11.          </where>
  12.      </select>

/mybatis7-dynamicSql/src/com/jmu/dao/mapper.xml

输出：

  1. 0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==>  Preparing: select id,name,age,score from student where 1=1 and age >?
  2. 48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==> Parameters: 20(Integer)
  3. 86 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - <==      Total: 2
  4. Student [id=198, name=明明, score=99.5, age=23]
  5. Student [id=200, name=明明, score=99.5, age=23]
  6. 111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >?
  7. 111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >?
  8. 111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
  9. 111 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
  10. 115 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
  11. 115 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
  12. Student [id=198, name=明明, score=99.5, age=23]
  13. Student [id=200, name=明明, score=99.5, age=23]

output

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。

  1.  <select id="selectStudentsByChoose" resultType="Student">
  2.          select id,name,age,score
  3.          from student
  4.          <where>
  5.              <choose>
  6.                  <when test="name !=null and name !=''">
  7.                      and name like '%' #{name} '%'
  8.                  </when>
  9.                  <when test="age>0">
  10.                      and age>#{age}
  11.                  </when>
  12.                  <otherwise>
  13.                       1=2 <!-- false,使得查询不到结果 -->
  14.                  </otherwise>
  15.              </choose>
  16.          </where>
  17.      </select>

/mybatis7-dynamicSql/src/com/jmu/dao/mapper.xml

  1.  import java.util.List;
  2.  import java.util.Map;
  3.  
  4.  import com.jmu.bean.Student;
  5.  
  6.  public interface IStudentDao {
  7.      // 根据条件查询问题
  8.      List<Student> selectStudentsByIf(Student student);
  9.      List<Student> selectStudentsByWhere(Student student);
  10.      List<Student> selectStudentsByChoose(Student student);
  11.  
  12.  }

com.jmu.dao.IStudentDao

  1.  }
  2.      @Test
  3.      public void test03() {
  4.  //        Student stu = new Student("明", 20, 0);
  5.          Student stu = new Student("", 20, 0);
  6.  //        Student stu = new Student("", 0, 0);
  7.          List<Student> students = dao.selectStudentsByChoose(stu);
  8.          for (Student student : students) {
  9.              System.out.println(student);
  10.          }
  11.  
  12.      }

MyTest

输出：

  1. 0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==>  Preparing: select id,name,age,score from student where 1=1 and age >?
  2. 50 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - ==> Parameters: 20(Integer)
  3. 86 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByIf  - <==      Total: 2
  4. Student [id=198, name=明明, score=99.5, age=23]
  5. Student [id=200, name=明明, score=99.5, age=23]
  6. 104 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >?
  7. 104 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==>  Preparing: select id,name,age,score from student WHERE age >?
  8. 105 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
  9. 105 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - ==> Parameters: 20(Integer)
  10. 109 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
  11. 109 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByWhere  - <==      Total: 2
  12. Student [id=198, name=明明, score=99.5, age=23]
  13. Student [id=200, name=明明, score=99.5, age=23]
  14. 121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>?
  15. 121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>?
  16. 121 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==>  Preparing: select id,name,age,score from student WHERE age>?
  17. 122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
  18. 122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
  19. 122 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - ==> Parameters: 20(Integer)
  20. 124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
  21. 124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
  22. 124 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByChoose  - <==      Total: 2
  23. Student [id=198, name=明明, score=99.5, age=23]
  24. Student [id=200, name=明明, score=99.5, age=23]

output

当查询的条件均不满足时：

  1. 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。
  1.  List<Student> selectStudentsByForeach(int[] ids);

com.jmu.dao.IStudentDao

  1.  @Test
  2.      public void test04() {
  3.         int[] ids={197,198,199};
  4.          List<Student> students = dao.selectStudentsByForeach(ids);
  5.          for (Student student : students) {
  6.              System.out.println(student);
  7.          }
  8.  
  9.      }

MyTest

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

mapper.xml

输出：

  1. 176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? )
  2. 176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? )
  3. 176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? )
  4. 176 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? , ? )
  5. 177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
  6. 177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
  7. 177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
  8. 177 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - ==> Parameters: 197(Integer), 198(Integer), 199(Integer)
  9. 179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
  10. 179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
  11. 179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
  12. 179 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach  - <==      Total: 3
  13. Student [id=197, name=明明, score=87.9, age=19]
  14. Student [id=198, name=明明, score=99.5, age=23]
  15. Student [id=199, name=明明, score=87.9, age=19]

output

七、<foreach/>标签--遍历泛型为基本类型的List

  1.  @Test
  2.      public void test05() {
  3.          List<Integer> ids =new ArrayList<>();
  4.          ids.add(198);
  5.          ids.add(199);
  6.          List<Student> students = dao.selectStudentsByForeach2(ids);
  7.          for (Student student : students) {
  8.              System.out.println(student);
  9.          }
  10.  
  11.      }

MyTest

  1.  import java.util.List;
  2.  import com.jmu.bean.Student;
  3.  
  4.  public interface IStudentDao {
  5.      // 根据条件查询问题
  6.      List<Student> selectStudentsByIf(Student student);
  7.      List<Student> selectStudentsByWhere(Student student);
  8.      List<Student> selectStudentsByChoose(Student student);
  9.      List<Student> selectStudentsByForeach(int[] ids);
  10.      List<Student> selectStudentsByForeach2(List<Integer> ids);
  11.  }

com.jmu.dao.IStudentDao

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

/mybatis7-dynamicSql/src/com/jmu/dao/mapper.xml

输出：

  1. 0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? )
  2. 48 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - ==> Parameters: 198(Integer), 199(Integer)
  3. 78 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach2  - <==      Total: 2
  4. Student [id=198, name=明明, score=99.5, age=23]
  5. Student [id=199, name=明明, score=87.9, age=19]

output

八、<foreach/>标签--遍历泛型为自定义类型的List

  1.  import java.util.List;
  2.  import com.jmu.bean.Student;
  3.  
  4.  public interface IStudentDao {
  5.      // 根据条件查询问题
  6.      List<Student> selectStudentsByIf(Student student);
  7.      List<Student> selectStudentsByWhere(Student student);
  8.      List<Student> selectStudentsByChoose(Student student);
  9.      List<Student> selectStudentsByForeach(int[] ids);
  10.      List<Student> selectStudentsByForeach2(List<Integer> ids);
  11.      List<Student> selectStudentsByForeach3(List<Student> ids);
  12.  }

com.jmu.dao.IStudentDao

  1.  @Test
  2.      public void test06() {
  3.          Student stu1 = new Student();
  4.          stu1.setId(198);
  5.          Student stu2 = new Student();
  6.          stu2.setId(199);
  7.          List<Student> stus =new ArrayList<>();
  8.          stus.add(stu1);
  9.          stus.add(stu2);
  10.          List<Student> students = dao.selectStudentsByForeach3(stus);
  11.          for (Student student : students) {
  12.              System.out.println(student);
  13.          }
  14.  
  15.      }

MyTest

  1.  <select id="selectStudentsByForeach3" resultType="Student">
  2.          select id,name,age,score
  3.          from student
  4.          <if test="list.size>0">
  5.              where id in
  6.              <foreach collection="list" item="stu" open="(" close=")"
  7.                  separator=",">
  8.                  #{stu.id}
  9.              </foreach>
  10.          </if>
  11.      </select>

mapper.xml

输出：

  1.  0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - ==>  Preparing: select id,name,age,score from student where id in ( ? , ? )
  2.  49 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - ==> Parameters: 198(Integer), 199(Integer)
  3.  90 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentsByForeach3  - <==      Total: 2
  4.  Student [id=198, name=明明, score=99.5, age=23]
  5.  Student [id=199, name=明明, score=87.9, age=19]

output

九、SQL片段

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