[刘阳Java]_MyBatis_动态SQL标签用法_第7讲

1.MyBatis的动态SQL是基于OGNL表达式的，它可以帮助我们方便的在SQL语句中实现某些逻辑。

2.MyBatis中用于实现动态SQL的元素主要有

• if
• choose（when，otherwise）
• trim
• where
• set
• foreach

可以看出MyBatis的动态SQL的标签元素和接近JSP中的JSTL语法，下面我就分别详细的介绍一下

3.动态SQL中if的用法

<?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.mybatis3.mapping.StudentMapper">

    <resultMap type="Student" id="StudentResultMap">
        <id column="id" property="id"/>
        <result column="sname" property="sname"/>
    </resultMap>

    <select id="getStudentMultiple" resultMap="StudentResultMap" parameterType="Student">
        select * from student where 1=1
        <if test="id != 0">
            and id = #{id}
        </if>
        <if test="sname != null">
            and sname = #{sname}
        </if>
    </select>
</mapper>

4.动态SQL中choose用法

<?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.mybatis3.mapping.StudentMapper">

    <resultMap type="Student" id="StudentResultMap">
        <id column="id" property="id"/>
        <result column="sname" property="sname"/>
    </resultMap>

    <select id="getStudentMultipleChoose" resultMap="StudentResultMap" parameterType="Student">
        select * from student where 1=1
        <choose>
            <when test="id != 0">
                and id = #{id}
            </when>
            <when test="sname != null">
                and sname = #{sname}
            </when>
        </choose>
    </select>
</mapper>

5.动态SQL中where语句的作用主要是简化SQL语句中where中的条件判断的

<?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.mybatis3.mapping.StudentMapper">

    <resultMap type="Student" id="StudentResultMap">
        <id column="id" property="id"/>
        <result column="sname" property="sname"/>
    </resultMap>

    <select id="getStudentMultipleWhere" resultMap="StudentResultMap" parameterType="Student">
        select * from student
        <where>
            <if test="id != 0">
                and id = #{id}
            </if>
            <if test="sname != null">
                and sname = #{sname}
            </if>
        </where>
    </select>
</mapper>

注意： where元素的作用是给SQL语句添加一个条件判断. 如果输出后是and 开头的，MyBatis会把第一个and忽略，当然如果是or开头的，MyBatis也会把它忽略；此外，在where元素中你不需要考虑空格的问题，MyBatis会智能的帮你加上

6.动态SQL中set元素主要是用在更新操作的时候，它的主要功能和where元素其实是差不多的

<?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.mybatis3.mapping.StudentMapper">

    <resultMap type="Student" id="StudentResultMap">
        <id column="id" property="id"/>
        <result column="sname" property="sname"/>
    </resultMap>

    <update id="updateStudentMultipleSet" parameterType="Student">
        update student
        <set>
            <if test="sname != null">
                sname = #{sname},
            </if>
        </set>
        where id = #{id}
    </update>
</mapper>

7.动态SQL中foreach

• 主要用在构建in条件中，它可以在SQL语句中进行迭代一个集合。
• foreach元素的属性主要有item，index，collection，open，separator，close

<?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.mybatis3.mapping.StudentMapper">

    <resultMap type="Student" id="StudentResultMap">
        <id column="id" property="id"/>
        <result column="sname" property="sname"/>
    </resultMap>

    <select id="getStudentMultipleForeach" resultMap="StudentResultMap">
        select * from student where id in
        <foreach collection="list" item="item" index="index" open="(" separator="," close=")">
            #{item}
        </foreach>
    </select>
</mapper>

注意：在Java代码中如何使用MyBaits的动态SQL语句

import java.io.IOException;
import java.io.Reader;
import java.util.ArrayList;
import java.util.List;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.mybatis3.entity.Student;

public class Test02 {
    private static SqlSessionFactory sqlSessionFactory;
    private static Reader reader;

    static {
        try {
            reader = Resources.getResourceAsReader("SqlMapConfig.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * 动态SQL中foreach用法
     */
    @Test
    public void m05() {
        SqlSession session = sqlSessionFactory.openSession();
        String statement = "com.mybatis3.mapping.StudentMapper.getStudentMultipleForeach";
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        List<Student> studentList = session.selectList(statement, list);
        session.close();
        System.out.println(studentList);
    }
}

8.动态SQL中trim语句

trim元素的主要功能是可以在自己包含的内容前加上某些前缀，也可以在其后加上某些后缀，与之对应的属性是prefix和suffix；可以把包含内容的首部某些内容覆盖，即忽略，也可以把尾部的某些内容覆盖，对应的属性是prefixOverrides和suffixOverrides

<select id="dynamicTrimTest" parameterType="Blog" resultType="Blog">
        select * from t_blog
        <trim prefix="where" prefixOverrides="and |or">
            <if test="title != null">
                and title = #{title}
            </if>
            <if test="content != null">
                and content = #{content}
            </if>
            <if test="owner != null">
                or owner = #{owner}
            </if>
        </trim>
</select>