Mybatis中and和or的细节处理

当一条SQL中既有条件查又有模糊查的时候，偶尔会遇到这样的and拼接问题。参考如下代码：

<select id="listSelectAllBusiness">
        select * from ***
        where 
        <if test="a!= null">
            a = #{a}
        </if>
        <if test="b!= null">
            and b in
            <foreach collection="list" index="index" item="item" open="(" separator="," close=")">
                #{item}
            </foreach>
        </if>
        <if test="c!= null">
            and name like '%${c}%' or code like '%${c}%'
        </if>
            order by id desc
            limit #{limit} offset #{page}
</select>

这样写的错误是如果a==null那么第二个条件中就会多一个and，语句会变成select * from *** where and b in (...)，而如果条件全都不满足的话SQL会变成select * from *** where order by id desc limit...解决办法：加上<where>标签，如下：

<select id="listSelectAllBusiness">
        select * from ***
        <where> 
        <if test="a!= null">
            a = #{a}
        </if>
        <if test="b!= null">
            and b in
            <foreach collection="list" index="index" item="item" open="(" separator="," close=")">
                #{item}
            </foreach>
        </if>
        <if test="c!= null">
            and name like '%${c}%' or code like '%${c}%'
        </if>
        </where>
            order by id desc
            limit #{limit} offset #{page}
</select>

如上代码所示，加上一个<where>标签即可，where标签会自动识别，如果前面条件不满足的话，会自己去掉and。如果满足的话会自己加上and。但是这句语句还是有问题，就是c条件里的语句里面有一个or，如果前面全部ab条件中有满足的话就会形成这样的SQL，select * from *** where a = ? and name like '%%' or code like '%%'，这条就类似SQL注入了，只要后面or条件满足都能查出来，不满足需求。解决办法：给c条件的语句加上()，如下：

<select id="listSelectAllBusiness">
        select * from ***
        <where> 
        <if test="a!= null">
            a = #{a}
        </if>
        <if test="b!= null">
            and b in
            <foreach collection="list" index="index" item="item" open="(" separator="," close=")">
                #{item}
            </foreach>
        </if>
        <if test="c!= null">
            and (name like '%${c}%' or code like '%${c}%')
        </if>
        </where>
            order by id desc
            limit #{limit} offset #{page}
</select>