4.3、trim标签

（1）意义

trim标记是一个格式化的标记，主要用于拼接sql的条件语句（前缀、后缀的添加或忽略），可以完成set或者是where标记的功能。

（2）属性介绍

• prefix：增加prefix前缀内容
• suffix：增加suffix后缀内容
• prefixOverrides：忽略前缀的内容
• suffixOverrides：忽略后缀的内容

（3）实例

update语句

<update id="updateByPrimaryKey" parameterType="Object">
        update student set 
  <trim  suffixOverrides="," > 
    <if test="name != null  ">
        NAME=#{name},
    </if>
    <if test="hobby != null  ">
        HOBBY=#{hobby},
    </if>
  </trim> where id=#{id}
</update>


如果name和hobby的值都不为空的话，会执行如下语句
update student set NAME='XX',HOBBY='XX' where id='XX'

select语句

<select id="selectByNameOrHobby" resultMap="BaseResultMap">
    select * from student 
    <trim prefix="WHERE" prefixOverrides="AND | OR">
        <if test="name != null and name.length()>0"> AND name=#{name}
        </if>
        <if test="hobby != null and hobby.length()>0"> AND hobby=#{hobby}
        </if>
    </trim>
</select>


如果name和hobby的值都不为空的话，会执行如下语句
select * from user WHERE /*and*/ name = ‘xx’ and hobby= ‘xx’

insert语句

<insert id="insert" parameterType="Object">
    insert into student    
    <trim     prefix="("    suffix=")"    suffixOverrides="," >
        <if test="name != null  ">
            NAME,
        </if>
        <if test="hobby != null  ">
            HOBBY,
        </if>    
    </trim>    

    <trim     prefix="values("    suffix=")"    suffixOverrides="," >  
        <if test="name != null  ">
            #{name},
        </if>
        <if test="hobby != null  ">
            #{hobby},
        </if>
    </trim>
</insert>