案例一：if-else

在使用mybatis mapper 动态sql时，不免会出现if-else的使用，但是好像又没有这种语法，提供的是choose标签代替if-else

例如：

select * from t_stu t
<where>
    <choose>
        <when test="query == 0">
            and t.status = 1 
        </when>
        <otherwise>
                and t.status  NOT IN (9,5)
        </otherwise>
    </choose>
    and t.delete_status = 1
</where>

也可以用多个if判断实现：

select * from t_stu t
<where>
    <if test="query == 0">
        and t.status = 1 
    </if>
    <if test="query != 0">
        and t.status  NOT IN (9,5)
    </if>
    and t.delete_status = 1
</where>

案例二：if嵌套

在实际编码过程中会有一些判断条件会一直重复使用，一直写在if标签中写的代码会特长，而且臃肿

select * from t_stu t
<where>
    <if test="query == 0 and type = 1">
        and t.type = 'we' and t.delete = 1
    </if>
    <if test="query == 0 and type = 2">
        and t.type = 'wq' and t.delete = 1
    </if>
    <if test="query == 0 and type = 3">
        and t.type = 'wr' and t.delete = 1
    </if>
</where>

变现后：

select * from t_stu t
<where>
    <if test="query == 0">
        <if test="type == 1">
            and t.type = 'we'
        </if>
         <if test="type == 2">
            and t.type = 'wq'
        </if>
        <if test="type == 3">
            and t.type = 'wr'
        </if>
    </if>
    and t.delete = 1
</where>