Mybatis中where 1=1 和＜where＞标签

where 1 = 1 起到一个占位的作用

<select id="listPage3" resultType="com.qcby.shujia.demo.entity.UserBlog">
    select ub.*,u.username,u.head_Img,u.reason as userReason  from user_blog ub

    left join user u on ub.user_id = u.id

    where 1=1 /*如果去掉，下面的条件就直接是where and*/

    <if test="userBlog.title != null">
        and ub.title like CONCAT(%,#{userBlog.title},%)
    </if>
    <if test="userBlog.reason != null">
        and ub.reason like CONCAT(%,#{userBlog.reason},%)
    </if>

    <if test="userBlog.userReason != null">
        and u.reason like CONCAT(%,#{userBlog.userReason},%)
    </if>
</select>

如果将where 1 = 1去掉，则会产生语法错误，所以起到一个占位的作用

用<where>标签可以自动辨别后面的条件

<select id="listPage1" resultType="com.qcby.shujia.demo.entity.UserBlog">
    select ub.*,u.username,u.head_Img,u.reason as userReason  from user_blog ub

    left join user u on ub.user_id = u.id

    <where>
        <if test="userBlog.title != null">
            and ub.title like CONCAT(%,#{userBlog.title},%)
        </if>
        <if test="userBlog.reason != null">
            and ub.reason like CONCAT(%,#{userBlog.reason},%)
        </if>

        <if test="userBlog.userReason != null">
            and u.reason like CONCAT(%,#{userBlog.userReason},%)
        </if>
    </where>
</select>

Mybatis标签解析时会进行优化，把多余的关键词去掉，如果检测到第一个条件的前面有and则会自动把and去掉