via: http://www.cnblogs.com/qiankun-site/p/5758924.html
The trim tag is a formatted tag that can complete the function of set or where tag, as follows:
1、
select * from user
<trim prefix="WHERE" prefixoverride="AND |OR">
<if test="name != null and name.length()>0"> AND name=#{name}</if>
<if test="gender != null and gender.length()>0"> AND gender=#{gender}</if>
</trim>
If the values of name and gender are not null, the printed SQL is: select * from user where name = 'xx' and gender = 'xx'
The first and does not exist in the place marked in red. The meanings of the above two properties are as follows:
prefix: prefix
prefixoverride: remove the first and or or
2、
update user
<trim prefix="set" suffixoverride="," suffix=" where id = #{id} ">
<if test="name != null and name.length()>0"> name=#{name} , </if>
<if test="gender != null and gender.length()>0"> AND gender=#{gender} , </if>
</trim>
If the values of name and gender are not null, the printed SQL is: update user set name='xx' , gender='xx' where id='x'
There is no comma in the red marked place, and a set prefix and a where suffix are automatically added. The meanings of the above three attributes are as follows, and the meaning of prefix is as above:
suffixoverride: remove the last comma (it can also be other tokens, like the and in the prefix above)
suffix: suffix