文章目录
- 动态 sql 语句
- 1. 动态 sql 语句概述
- 2. 动态 SQL 之
- 3. 动态 SQL 之
- SQL片段抽取
- 知识小结
动态 sql 语句
1. 动态 sql 语句概述
Mybatis 的映射文件中,前面我们的 SQL 都是比较简单的,有些时候业务逻辑复杂时,我们的 SQL是动态变化的,此时在前面的学习中我们的 SQL 就不能满足要求了。参考的官方文档,描述如下:
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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。
<select id="findByCondition" parameterType="user" resultType="user">
-- select * from user2
<include refid="selectUser"></include>
<where>
<if test="id!=0">
and id=#{id}
</if>
<if test="username!=null">
and username=#{username}
</if>
<if test="password!=null">
and password=#{password}
</if>
</where>
</select>
当查询条件 id 和 username 都存在时,控制台打印的sql语句如下:
当查询条件只有id存在时,控制台打印的sql语句如下:
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循环执行 sql 的拼接操作,例如:SELECT * FROM USER WHERE id IN (1,2,5)。
<select id="findByIds" parameterType="list" resultType="user">
-- select * from user2
<include refid="selectUser"></include>
<where>
<foreach collection="list" open="id in(" close=")" item="id" separator=",">
#{id}
</foreach>
</where>
</select>
测试代码片段如下:
测试程序
public class MapperTest {
public void test1() throws IOException {
InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
// // 模拟条件user
// User condition = new User();
// condition.setId(1);
// condition.setUsername("zhangsan");
// condition.setPassword("123");
//
// List<User> userList = mapper.findByCondition(condition);
// System.out.println(userList);
List<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
List<User> userList = mapper.findByIds(ids);
System.out.println(userList);
}
}
foreach 标签的属性含义如下:
<foreach>标签用于遍历集合,它的属性:
• collection:代表要遍历的集合元素,注意编写时不要写#{}
• open:代表语句的开始部分
• close:代表结束部分
• item:代表遍历集合的每个元素,生成的变量名
• sperator:代表分隔符
SQL片段抽取
Sql 中可将重复的 sql 提取出来,使用时用 include 引用即可,最终达到 sql 重用的目的
<!-- sql语句的抽取-->
<sql id="selectUser">select * from user2</sql>
<select id="findByCondition" parameterType="user" resultType="user">
-- select * from user2
<include refid="selectUser"></include>
<!-- 省略 -->
</select>
知识小结
MyBatis映射文件配置:
<select>:查询
<insert>:插入
<update>:修改
<delete>:删除
<where>:where条件
<if>:if判断
<foreach>:循环
<sql>:sql 片段抽取