动态sql
    利用动态 SQL，可以彻底摆脱动态拼接 sql 的痛苦
    ●if
    ●choose (when, otherwise)
    ●trim (where, set)
    ●foreach
    if + where 条件

    1. <!--姓名、密码动态查询，为空就是全查询-->
    2. <select id="find1" resultMap="usermap">
    3.   select * from user where 1=1
    4.   <if test="name!=null">
    5.     and username = #{name}
    6.   </if>
    7.   <if test="password!=null">
    8.     and password = #{password}
    9.   </if>
    10. </select>

    改进写法，使用 where 标签，会智能的给我们添加 where 关键字，并且能智能的帮我们去掉 and

    1. <select id="find1" resultMap="usermap">
    2.     select * from user
    3.   <where>
    4.     <if test="name!=null">
    5.       and username = #{name}
    6.     </if>
    7.     <if test="password!=null">
    8.       and password = #{password}
    9.     </if>
    10.   </where>
    11. </select>

    if + set 条件
    set 标签同样很智能，会帮我们智能的去掉最后的逗号

    1. <!--传什么参数就修改什么字段-->
    2.     <update id="update">
    3.         update user
    4.         <set>
    5.             <if test="name!=null">
    6.                 username = #{name},
    7.             </if>
    8.             <if test="age!=null">
    9.                 age = #{age},
    10.             </if>
    11.             <if test="address!=null">
    12.                 address = #{address},
    13.             </if>
    14.             <if test="password!=null">
    15.                 password = #{password},
    16.             </if>
    17.         </set>
    18.             where id = #{id}
    19.     </update>

    trim（基本上用不到了） ；
    属性
    ●prefix：加前缀
    ●prefixOverrides：去掉前缀
    ●suffix：加后缀
    ●suffixOverrides：去掉后缀

    1.  <select id="find2" resultMap="usermap">
    2.         select * from user
    3.         <trim prefix="where" prefixOverrides="and">
    4.             <if test="name!=null">
    5.                 and username = #{name}
    6.             </if>
    7.             <if test="password!=null">
    8.                 and password = #{password}
    9.             </if>
    10.         </trim>
    11.     </select>
    1. <update id="update2">
    2.         update user
    3.         <trim prefix="set" suffixOverrides=",">
    4.             <if test="name!=null">
    5.                 username = #{name},
    6.             </if>
    7.             <if test="age!=null">
    8.                 age = #{age},
    9.             </if>
    10.             <if test="address!=null">
    11.                 address = #{address},
    12.             </if>
    13.             <if test="password!=null">
    14.                 password = #{password},
    15.             </if>
    16.         </trim>
    17.         where id = #{id}
    18.     </update>