1）sql where 条件
select id="find" parameterType="User" resultType="User">
select id,name, age,address from user_c where 1=1
<if test="id!=null">
and id=#{id}
</if>
<if test="name!=null">
and name like "%"#{name}"%"
</if>
<if test="age!=null">
and age=#{age}
</if>
<if test="address!=null">
and address=#{address}
</if>
</select>
2）<where>…</where> 会自动将sql中的 and 去掉，就无需写 where 1=1
select <include refid="cols"/> from user_c
<where>
<if test="name!=null">
and name like "%"#{name}"%"
</if>
<if test="age!=null">
and age=#{age}
</if>
<if test="address!=null">
and address like #{address}
</if>
</where>
3）<set>…</set> 标签
会将update sql中的 set 最后一个条件多余的逗号去掉
<update id="update" parameterType="User">
update user_c
<set>
<if test="name!=null">
and name like "%"#{name}"%“,
</if>
<if test="age!=null">
age=#{age},
</if>
<if test="address!=null">
address=#{address},
</if>
</set>
where id=#{id}
</update>
