例子、
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.atguigu.dao.TeacherDao">
<resultMap type="com.atguigu.bean.Teacher" id="teacherMap">
<id property="id" column="id" />
<result property="address" column="address" />
<result property="birth" column="birth_date" />
<result property="course" column="class_name" />
<result property="name" column="teacherName" />
</resultMap>
<!--public Teacher getTeacherById(Integer id); -->
<select id="getTeacherById" resultMap="teacherMap">
select * from t_teacher
where id=#{id}
</select>
<!-- if：判断 -->
<!--public List<Teacher> getTeacherByCondition(Teacher teacher); -->
<select id="getTeacherByCondition" resultMap="teacherMap">
select * from t_teacher
<!-- test=""：编写判断条件 id!=null：取出传入的javaBean属性中的id的值，判断其是否为空 -->
<!-- where可以帮我们去除掉前面的and; -->
<!-- trim：截取字符串
prefix=""：前缀；为我们下面的sql整体添加一个前缀
prefixOverrides=""： 取出整体字符串前面多余的字符
suffix=""：为整体添加一个后缀
suffixOverrides=""：后面哪个多了可以去掉; -->
<!-- 我们的查询条件就放在where标签中；每个and写在前面，
where帮我们自动取出前面多余的and -->
<trim prefix="where" prefixOverrides="and" suffixOverrides="and">
<if test="id!=null">
id > #{id} and
</if>
<!-- 空串 "" and； && or: ||； if()：传入非常强大的判断条件；
OGNL表达式；对象导航图
Person
===lastName
===email
===Address
===city
===province
===Street
===adminName
===info
===perCount
方法、静态方法、构造器。xxx
在mybatis中，传入的参数可以用来做判断；
额外还有两个东西；
_parameter：代表传入来的参数；
1）、传入了单个参数：_parameter就代表这个参数
2）、传入了多个参数：_parameter就代表多个参数集合起来的map
_databaseId：代表当前环境
如果配置了databaseIdProvider：_databaseId就有值
-->
<if test="name!=null && !name.equals("")">
teacherName like #{name} and
</if>
<if test="birth!=null">
birth_date < #{birth} and
</if>
</trim>
</select>
<!-- public List<Teacher> getTeacherByIdIn(List<Integer> ids); -->
<select id="getTeacherByIdIn" resultMap="teacherMap">
SELECT * FROM t_teacher WHERE id IN
<!-- 帮我们遍历集合的； collection=""：指定要遍历的集合的key
close=""：以什么结束
index="i"：索引；
如果遍历的是一个list；
index：指定的变量保存了当前索引
item：保存当前遍历的元素的值
如果遍历的是一个map：
index：指定的变量就是保存了当前遍历的元素的key
item：就是保存当前遍历的元素的值
item="变量名"：每次遍历出的元素起一个变量名方便引用
open=""：以什么开始
separator=""：每次遍历的元素的分隔符
(#{id_item},#{id_item},#{id_item} -->
<if test="ids.size >0">
<foreach collection="ids" item="id_item" separator="," open="("
close=")">
#{id_item}
</foreach>
</if>
</select>
<!--public List<Teacher> getTeacherByConditionChoose(Teacher teacher); -->
<select id="getTeacherByConditionChoose" resultMap="teacherMap">
select * from t_teacher
<where>
<choose>
<when test="id!=null">
id=#{id}
</when>
<when test="name!=null and !name.equals("")">
teacherName=#{name}
</when>
<when test="birth!=null">
birth_date = #{birth}
</when>
<otherwise>
1=1
</otherwise>
</choose>
实例、
<mapper namespace="com.apcstudy.mybatis.dao.TeacherDao">
<resultMap id="teacherMap" type="com.apcstudy.mybatis.bean.Teacher">
<id property="id" column="id" />
<result property="name" column="name"/>
<result property="course" column="course" />
<result property="address" column="address" />
<result property="birth" column="birth" />
</resultMap>
<select id="getTeacherById" parameterType="integer" resultMap="teacherMap">
select * from t_teacher
where id=#{id}
</select>
<select id="getTeacherByCondition01" resultMap="teacherMap">
select * from t_teacher
where 1=1
<if test="name!=null and name!='' ">
and name=#{name}
</if>
<if test="course!=null and course!='' ">
and course=#{course}
</if>
</select>
<select id="getTeacherByCondition02" resultMap="teacherMap">
select * from t_teacher
<where>
<if test="name!=null and name!='' ">
name=#{name}
</if>
<if test="course!=null and course!='' ">
and course=#{course}
</if>
</where>
</select>
<select id="getTeacherByCondition03" resultMap="teacherMap">
select * from t_teacher
<trim prefix="where" prefixOverrides="and" suffixOverrides="and">
<if test="name!=null and name!='' ">
name=#{name} and
</if>
<if test="course!=null and course!='' ">
course=#{course} and
</if>
</trim>
</select>
<update id="updateTeacher">
update t_teacher
<set>
<if test="name!=null and name!='' ">
name=#{name},
</if>
<if test="course!=null and course!='' ">
course=#{course},
</if>
<if test="address!=null and address!='' ">
address=#{address},
</if>
<if test="birth!=null and birth!='' ">
birth=#{birth}
</if>
</set>
<where>
id=#{id}
</where>
</update>
<select id="getTeacherByIdIn" resultMap="teacherMap">
select * from t_teacher where id in
<if test="ids!=null">
<foreach collection="ids" item="id" separator="," open="(" close=")">
#{id}
</foreach>
</if>
</select>
<select id="getTeacherByConditionChoose" resultMap="teacherMap">
select * from t_teacher
<where>
<choose>
<when test="name!=null and name!='' ">
name=#{name}
</when>
<when test="course!=null and course!=''">
course=#{course}
</when>
<when test="address!=null and address!=''">
address=#{address}
</when>
<otherwise>
1=1
</otherwise>
</choose>
</where>
</select>
