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);
- }
- }
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
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