MyBatis 动态sql

1.什么是动态SQL?

MyBatis的映射文件中支持在基础SQL上添加一些逻辑操作，并动态拼接成完整的SQL之后再执行，以达到SQL复用、简化编程的效果。

2.if标签

我们根据实体类的不同取值，使用不同的SQL语句来进行查询。比如在id如果不为空时可以根据 id查询，如果username不同空时还要加入用户名作为条件。这种情况在我们的多条件组合查询中经常会碰到。

• mapper接口

  public interface UserDao {
  
      //复杂条件查询
      public List<User> findByUser(User user);
  }
  

• mapper映射文件

  <?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  <mapper namespace="com.by.dao.UserDao">
      <!--使用别名-->
      <select id="findByUser" resultType="User">
          select * from user where 1=1
          <if test="username!=null and username != ''">
              and username=#{username}
          </if>
          <if test="birthday!=null">
              and birthday=#{birthday}
          </if>
          <if test="sex!=null and sex != ''">
              and sex=#{sex}
          </if>
          <if test="address!=null and address != ''">
              and address=#{address}
          </if>
      </select>
  </mapper>
  

• 测试

      @Test
      public void testFindAll(){
          UserDao userDao = sqlSession.getMapper(UserDao.class);
          User user = new User();
          user.setSex("男");
          user.setAddress("香港");
          List<User> userList = userDao.findByUser(user);
          for(User u : userList){
              System.out.println(u);
          }
      }
  

3.where标签

为了简化上面where 1=1的条件拼装，我们可以使用where标签将if标签代码块包起来，将1=1条件去掉。

若查询条件的开头为 “AND” 或 “OR”，where 标签会将它们去除。

• mapper映射文件

  <?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  <mapper namespace="com.by.dao.UserDao">
      <select id="findByUser" resultType="User">
          select * from user
          <!--where标签将if标签代码块包起来去掉开头 “AND” 或 “OR”-->
          <where>
              <if test="username!=null and username != ''">
                  and username=#{username}
              </if>
              <if test="birthday!=null">
                  and birthday=#{birthday}
              </if>
              <if test="sex!=null and sex != ''">
                  and sex=#{sex}
              </if>
              <if test="address!=null and address != ''">
                  and address=#{address}
              </if>
          </where>
      </select>
  </mapper>
  

4.set标签

set标签用于动态包含需要更新的列，并会删掉额外的逗号

• mapper

  public void updateByUser(User user);
  

      <update id="updateByUser" parameterType="user">
          update user
          <set>
              <if test="username!=null and username != '' ">
                  username=#{username},
              </if>
              <if test="birthday!=null">
                  birthday=#{birthday},
              </if>
              <if test="sex!=null and sex != '' ">
                  sex=#{sex},
              </if>
              <if test="address!=null and address != '' ">
                  address=#{address},
              </if>
          </set>
          where id=#{id}
      </update>
  

• 测试

      @Test
      public void testUpdateByUser(){
          UserDao userDao = sqlSession.getMapper(UserDao.class);
          User user = new User();
          user.setId(50);
          user.setBirthday(new Date());
          user.setAddress("加拿大");
          userDao.updateByUser(user);
      }
  

5.trim标签

trim标签可以代替where标签、set标签

• mapper

      //修改
      public void updateByUser2(User user);
  

  <update id="updateByUser2" parameterType="User">
          update user
          <!-- 增加SET前缀，忽略，后缀 -->
          <trim prefix="SET" suffixOverrides=",">
              <if test="birthday!=null">
                  birthday=#{birthday},
              </if>
              <if test="sex!=null and username != '' ">
                  sex=#{sex},
              </if>
              <if test="address!=null and username != '' ">
                  address=#{address},
              </if>
          </trim>
          where id=#{id}
      </update>
  

6.foreach标签

foreach标签的常见使用场景是集合进行遍历

• mapper

      //批量删除
      public void deleteUserByIds(@Param("ids") List<Integer> ids);
  	//批量添加
      public void insertUsers(@Param("userList") List<User> userList);
  

      <delete id="deleteUserByIds" parameterType="list">
          delete from user where id in
          <!--
              collection：取值list、array、map、@Param("keyName")、对象的属性名
              item：循环取出的具体对象
              open：起始符
              separator：分隔符
              close：结束符
          -->
          <foreach collection="ids" item="id" open="(" close=")" separator=",">
              #{id}
          </foreach>
      </delete>
  
  	<insert id="insertUsers" parameterType="list">
          INSERT INTO user(username,password,birthday,sex,address)
          VALUES
          <foreach collection ="userList" item="user" separator =",">
              (#{user.username},#{user.password},#{user.birthday},
              							#{user.sex},#{user.address})
          </foreach>
      </insert>
  

• 测试

      @Test
      public void testDeleteUserByIds(){
          UserDao userDao = sqlSession.getMapper(UserDao.class);
          List<Integer> ids = new ArrayList();
          ids.add(50);
          ids.add(64);
          ids.add(67);
          userDao.deleteUserByIds(ids);
      }
  
  	@Test
      public void testInsertUsers(){
          UserDao userDao = sqlSession.getMapper(UserDao.class);
          long start = System.currentTimeMillis();
          List<User> userList = new ArrayList<>();
          for(int i = 0 ;i < 10000; i++) {
              User user = new User();
              user.setUsername("刘德华");
              user.setPassword("111");
              user.setBirthday(new Date());
              user.setSex("男");
              user.setAddress("香港");
              //userDao.insertUser(user);
              userList.add(user);
          }
          userDao.insertUsers(userList);
          long end = System.currentTimeMillis();
          System.out.println("一万条数据总耗时：" + (end-start) + "ms" );
          sqlSession.commit();
      }
  

7.sql标签

sql元素标签用来定义可重复使用的SQL代码片段，使用时只需要用include元素标签引用即可

• mapper

      //复杂条件查询
      public List<User> findByUser3(User user);
  

      <!-- 定义SQL片段 -->
      <sql id="query_user_where">
          <if test="username!=null and username != ''">
              and username=#{username}
          </if>
          <if test="birthday!=null">
              and birthday=#{birthday}
          </if>
          <if test="sex!=null and sex != ''">
              and sex=#{sex}
          </if>
          <if test="address!=null and address != ''">
              and address=#{address}
          </if>
      </sql>
      <select id="findByUser3" resultType="User">
          select * from user
          <where>
              <include refid="query_user_where"></include>
          </where>
      </select>
  

• 测试

      @Test
      public void testFindAll3(){
          UserDao userDao = sqlSession.getMapper(UserDao.class);
          User user = new User();
          user.setAddress("香港");
          user.setUsername("刘德华");
          List<User> userList = userDao.findByUser3(user);
          for(User u : userList){
              System.out.println(u);
          }
      }