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);
} }
com.jmu.test.MyTest
public interface IStudentDao {
// 根据条件查询问题
List<Student> selectStudentsByCondition(Student student);
}
com.jmu.dao.IStudentDao
<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>
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
@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);
} }
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
<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>
/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。
<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>
/mybatis7-dynamicSql/src/com/jmu/dao/mapper.xml
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); }
com.jmu.dao.IStudentDao
}
@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);
} }
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。
List<Student> selectStudentsByForeach(int[] ids);
com.jmu.dao.IStudentDao
@Test
public void test04() {
int[] ids={197,198,199};
List<Student> students = dao.selectStudentsByForeach(ids);
for (Student student : students) {
System.out.println(student);
} }
MyTest
<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>
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
@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);
} }
MyTest
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);
}
com.jmu.dao.IStudentDao
<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>
/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
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);
}
com.jmu.dao.IStudentDao
@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);
} }
MyTest
<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>
mapper.xml
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]
output
九、SQL片段
