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：
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]
output
（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
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
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);
}
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：
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]
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：
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]
output
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。
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：
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]
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：
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]
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
时间： 2024-08-04 00:33:37