<where></where>標籤的作用就在於，若你拼接的第一條語句前面有and則自動去除and

1，<if title=””></if>

    <select id="queryBlogIF" parameterType="Map" resultType="Blog">

        select * from mybatis.blog

        <where>

            <if test="title != null">

                and title = #{title}

            </if>

            <if test="author != null">

                and author = #{author}

            </if>

        </where>

</select>

if判斷若滿足則拼接

2，

<choose>

<when title=””>

</when>

<otherwise title =””>

</ otherwise >

</choose>

    <select id="queryBlogChoose" parameterType="Map" resultType="Blog">

        select * from mybatis.blog

        <where>

            <choose>

                <when test="title!=null">

                    title = #{title}

                </when>

                <when test="author!=null">

                    and author = #{author}

                </when>

                <otherwise>

                    and views = #{views}

                </otherwise>

            </choose>

        </where>

</select>

When判斷，只會拼接第一條滿足的語句，若均不滿足，則拼接otherwise語句

3，<set></set>標籤為sql語句動態新增set關鍵字，並去除無關的逗號

    <update id="updateBlog" parameterType="Map">

        update mybatis.blog

        <set>

            <if test="title!=null">

                title=#{title}

            </if>

        </set>

        where id=#{id}

</update>