• where if

    <select id="findUsersByNameAndSex" parameterType="com.itheima.pojo.User" resultType="com.itheima.pojo.User">
        select * from user
        <where>
            <if test="name!=null and name!=''">
                and name like '%${name}%'
            </if>
            <if test="sex!=null and sex!=''">
                and sex =#{sex}
            </if>
        </where>
    </select>

• for each

    <select id="findUsersByIds" parameterType="com.itheima.vo.QueryVo" resultType="com.itheima.pojo.User">
        select * from user 
        <where>
            <if test="ids!=null and ids.size>0">
                <foreach collection="ids"  item="id" open="id in (" close=")" separator="," >
                    #{id}
                </foreach>
            </if>
        </where>
    </select>

• sql
  定義sql段

<sql id="query_user_where">
    <if test="id!=null and id!=''">
        and id=#{id}
    </if>
    <if test="username!=null and username!=''">
        and username like '%${username}%'
    </if>
</sql>
<!-- 傳遞pojo綜合查詢用戶信息 引用sql段 -->
    <select id="findUserList" parameterType="user" resultType="user">
        select * from user 
        <where>
        <include refid="query_user_where"/>
        </where>
    </select>