Mybatis使用XML实现动态sql的示例代码

<!-- 在 Mapper XML 文件中编写动态 SQL 查询 -->
<select id="selectUsers" resultType="User">
  SELECT * FROM users
  <where>
    <if test="id != null">
      AND id = #{id}
    </if>
    <if test="username != null and username != ''">
      AND username = #{username}
    </if>
    <if test="email != null and email != ''">
      AND email = #{email}
    </if>
  </where>
</select>

<!-- 在 Mapper XML 文件中编写动态 SQL 更新 -->
<update id="updateUser" parameterType="User">
  UPDATE users
  <set>
    <if test="username != null">
      username = #{username},
    </if>
    <if test="email != null">
      email = #{email},
    </if>
  </set>
  WHERE id = #{id}
</update>

<!-- 在 Mapper XML 文件中编写动态 SQL 插入 -->
<insert id="insertUser" parameterType="User">
  INSERT INTO users
  <trim prefix="(" suffix=")" suffixOverrides=",">
    <if test="id != null">
      id,
    </if>
    <if test="username != null and username != ''">
      username,
    </if>
    <if test="email != null and email != ''">
      email,
    </if>
  </trim>
  <trim prefix="VALUES (" suffix=")" suffixOverrides=",">
    <if test="id != null">
      #{id},
    </if>
    <if test="username != null and username != ''">
      #{username},
    </if>
    <if test="email != null and email != ''">
      #{email},
    </if>
  </trim>
</insert>

<!-- 使用 choose、when、otherwise 标签实现条件选择 -->
<select id="getUserByIdOrUsername" resultType="User">
    SELECT * FROM users
    <where>
        <choose>
            <when test="id != null">
                AND id = #{id}
            </when>
            <when test="username != null and username != ''">
                AND username = #{username}
            </when>
            <otherwise>
                AND 1=1
            </otherwise>
        </choose>
    </where>
</select>
​​​​​​​
<!-- 使用 foreach 标签进行遍历操作 -->
<select id="getUsersByIdList" resultType="User">
    SELECT * FROM users
    WHERE id IN
    <foreach collection="ids" item="id" open="(" separator="," close=")">
        #{id}
    </foreach>
</select>