1 基于OGNL表达式(类似jstl表达式)
2 完成多条件查询等逻辑实现
3 实现动态SQL的元素
<if>
<where>
<set>
<trim>
<foreach>
<choose>
<when>
<otherwise>
4 if
语法：
<if test="条件"></if>
注意:如果判断变量的类型是字符串时，要注意是否需要判断空串.
例:<if test="userName!=null and userName !=''"></if>
5 where
智能处理and或者or.(会去掉where后面的第一个and或者or)
例:
select * from smbms_user
<where>
<if test="uName!=null and uName !=''">
and username like CONCAT('%',#{uName},'%')
</if>
<if test="uRole != null">
and userRole = #{uRole}
</if>
</where>
6 set
智能处理update时的逗号.(会去掉set后面最后个set值的逗号)
例:
update smbms_user
<set>
<if test="userCode!=null">
userCode = #{userCode},
</if>
<if test="userName!=null">
userName = #{userName},
</if>
<if test="userPassword!=null">
userPassword = #{userPassword}
</if>
</set>
where id = #{id}
7 trim
a.更灵活地去除多余关键字
b.属性:
prefix(前缀)
suffix(后缀)
prefixOverrides(前缀覆盖)
suffixOverrides(后缀覆盖)
c.替换where
<trim prefix="where" prefixOverrides="or|and">
<if test="uName!=null and uName !=''">
and username like CONCAT('%',#{uName},'%')
</if>
<if test="uRole != null">
and userRole = #{uRole}
</if>
</trim>
d.替换set
update smbms_user
<trim prefix="set" suffixOverrides=","
suffix="where id = #{id}">
<if test="userCode!=null">
userCode = #{userCode},
</if>
<if test="userName!=null">
userName = #{userName},
</if>
<if test="userPassword!=null">
userPassword = #{userPassword}
</if>
</trim>
8 foreach
a)迭代一个集合，通常用于in条件
b)属性
item :集合参数
index :下标
collection:必须指定
list (集合)
array (数组)
map-key(map)
open :开始"("
separator: 分割符：,
close: 结尾:")"
例:
<select id="getUserByRoles" resultType="User">
select * from smbms_user
where gender= #{gender} and userRole in
<foreach collection="uRole" item="roles"
open="(" close=")" separator=",">
#{roles}
</foreach>
</select>
9 choose
a.相当于Java中switch语句
b.当when有条件满足的时候，就跳出choose
<choose>
<when test ="条件1"> …</when>
<when test ="条件2"> …</when>
<when test ="条件3"> …</when>
…
<otherwise>…</otherwise>
</choose>