1.动态SQL：在SQL语句中加入流程控制。比如加入if，foreach等。
重点掌握if语句：
案例1：
<update id="updateItem" parameterType="com.gxa.bj.model.UserInfo">
update userinfo set
<if test="userName!=null">
userName = #{userName},
</if>
<if test="userPwd!=null">
userPwd = #{userPwd},
</if>
<if test= "userEmail!=null">
userEmail = #{userEmail},
</if>
<if test= "userAddress!=null">
userAddress = #{userAddress},
</if>
<if test= "regReason!=null">
regReason = #{regReason},
</if>
userId=#{userId} Where userId=#{userId}
</update>
案例2：
<select id="getList" parameterType="com.gxa.bj.model.UserInfo" resultType="com.gxa.bj.model.UserInfo">
Select * From userInfo Where 1=1
<if test="userName!=null">
And userName like #{userName}
</if>
<if test="userId>0">
And userId =#{userId}
</if>
<if test="userPwd!=null And userPwd!=''">
And userPwd like #{userPwd}
</if>
</select>