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@@ -306,3 +306,193 @@ sql标签定义的sql片段可以被include标签进行引用，并且include标
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</include>
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</select>
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```
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## parameter
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- 简单的参数映射
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```xml
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<!-- 在该简单映射中，对于基本数据类型或者如String、Integer等没有属性的简单数据类型，会用该类数据的值完全替换参数的值 -->
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<select id="selectUsers" resultType="User">
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select id, username, password
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from users
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where id = #{id}
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</select>
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```
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- 向参数传递复杂对象，例如User类型对象，那么将会从对象中获取id、username、password等属性并传递给参数
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```xml
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<insert id="insertUser" parameterType="User">
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insert into users (id, username, password)
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values (#{id}, #{username}, #{password})
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</insert>
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```
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## mybatis ${}
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对于mybatis，可以通过#{}来设置PreparedStatement的占位参数，但是，当想要动态设置sql statement中的元数据（如表名称、字段名称）时，可以通过${}来插入一个未修改的字符串进行拼串。
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```java
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// 未使用${}
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@Select("select * from user where id = #{id}")
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User findById(@Param("id") long id);
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@Select("select * from user where name = #{name}")
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User findByName(@Param("name") String name);
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@Select("select * from user where email = #{email}")
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User findByEmail(@Param("email") String email);
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// 使用${}之后
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@Select("select * from user where ${column} = #{value}")
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User findByColumn(@Param("column") String column, @Param("value") String value);
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```
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## resultMap
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- 对于简单的statement，其映射不需要用到resultMap
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```xml
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<!-- 返回集中所有的column会自动映射到HashMap中的key -->
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<select id="selectUsers" resultType="map">
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select id, username, hashedPassword
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from some_table
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where id = #{id}
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</select>
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```
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- 对于POJO，当select的返回列名和POJO类的属性名相同时，会自动生成resultMap来将column和POJO对象属性关联到一起
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```xml
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<select id="selectUsers" resultType="com.someapp.model.User">
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select
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user_id as "id",
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user_name as "userName",
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hashed_password as "hashedPassword"
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from some_table
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where id = #{id}
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</select>
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```
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- 自定义外部的resultMap来映射到User类
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```xml
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<resultMap id="userResultMap" type="com.someapp.model.User">
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<id property="id" column="user_id" />
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<result property="username" column="user_name"/>
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<result property="password" column="hashed_password"/>
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</resultMap>
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<select id="selectUsers" resultMap="userResultMap">
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select user_id, user_name, hashed_password
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from some_table
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where id = #{id}
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</select>
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```
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- 高级结果集映射
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```xml
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<!-- Very Complex Statement -->
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<select id="selectBlogDetails" resultMap="detailedBlogResultMap">
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select
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B.id as blog_id,
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B.title as blog_title,
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B.author_id as blog_author_id,
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A.id as author_id,
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A.username as author_username,
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A.password as author_password,
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A.email as author_email,
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A.bio as author_bio,
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A.favourite_section as author_favourite_section,
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P.id as post_id,
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P.blog_id as post_blog_id,
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P.author_id as post_author_id,
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P.created_on as post_created_on,
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P.section as post_section,
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P.subject as post_subject,
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P.draft as draft,
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P.body as post_body,
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C.id as comment_id,
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C.post_id as comment_post_id,
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C.name as comment_name,
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C.comment as comment_text,
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T.id as tag_id,
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T.name as tag_name
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from Blog B
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left outer join Author A on B.author_id = A.id
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left outer join Post P on B.id = P.blog_id
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left outer join Comment C on P.id = C.post_id
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left outer join Post_Tag PT on PT.post_id = P.id
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left outer join Tag T on PT.tag_id = T.id
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where B.id = #{id}
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</select>
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<!-- Very Complex Result Map -->
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<resultMap id="detailedBlogResultMap" type="Blog">
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<constructor>
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<idArg column="blog_id" javaType="int"/>
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</constructor>
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<result property="title" column="blog_title"/>
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<association property="author" javaType="Author">
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<id property="id" column="author_id"/>
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<result property="username" column="author_username"/>
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<result property="password" column="author_password"/>
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<result property="email" column="author_email"/>
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<result property="bio" column="author_bio"/>
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<result property="favouriteSection" column="author_favourite_section"/>
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</association>
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<collection property="posts" ofType="Post">
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<id property="id" column="post_id"/>
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<result property="subject" column="post_subject"/>
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<association property="author" javaType="Author"/>
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<collection property="comments" ofType="Comment">
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<id property="id" column="comment_id"/>
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</collection>
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<collection property="tags" ofType="Tag" >
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<id property="id" column="tag_id"/>
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</collection>
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<discriminator javaType="int" column="draft">
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<case value="1" resultType="DraftPost"/>
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</discriminator>
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</collection>
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</resultMap>
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```
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### resultMap的元素和属性
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- resultMap中可嵌套如下元素：
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- constructor：在通过构造方法实例化类时，将select语句返回的result注入到构造方法中
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- idArg：标识为id的参数，将result标识为id可以提高整体的性能
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- arg：将result注入到constructor中
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- id：标识为id的result，将result标识为id可以提高整体性能
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- result：select语句返回的result，将被注入到返回POJO类型的field
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- association：复杂关联，返回results中的部分results将会被包装到这种关联中
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- 关联被嵌套在resultMap中，关联本身可以是resultMap，或者可以引用一个外部resultMap
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- collection：复杂类型的集合
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- 集合被嵌套在resultMap中，其本身可以是一个resultMap，或者其可以引用要给外部的resultMap
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- discrimination：鉴别器，可以通过结果值来判断使用哪个resultMap
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- case：条件选项，当值为xx时使用特定的resultMap
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- case同样是一个resultMap，case可以是resultMap本身，也可以引用其他的resultMap
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- resultMap元素可包含如下属性：
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- id：在命名空间内，可以通过该id引用该resultMap
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- type：java类的全限定类名或typeAlias
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- autoMapping：为resultMap开启或关闭自动映射
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### id和result元素
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id和result元素是resultMap的基础，id和result都将一个column字段值映射到java类型的field域。
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> id和result标签的区别是，当result被标识为id时，该result将会被看作对象的标识符，在比较对象时id result将会被使用。这样可以提高整体性能，尤其是在缓存或是嵌套resultMap时。
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- id和result元素的属性：
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- property：column映射到的java对象的field
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- column：数据库表的列名
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- javaType：java类型的全限定类名或者typeAlias，通常映射到java bean时mybatis可以推断java类型，但是当映射到HashMap时，需要显式指定javaType
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- jdbcType
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- typeHandler
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### constructor元素
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当想要构建不可变对象时，可以通过constructor属性来向构造方法中注入result构建对象。
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```java
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public class User {
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//...
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public User(Integer id, String username, int age) {
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//...
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}
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//...
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}
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```
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```xml
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<constructor>
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<!-- 该constructor会自动匹配构造方法参数为(Integer,String,int)的构造函数 -->
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<idArg column="id" javaType="int"/>
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<arg column="username" javaType="String"/>
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<arg column="age" javaType="_int"/>
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</constructor>
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```
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- constructor元素的属性：
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- column：数据库表列名
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- javaType
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- jdbcType
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- typeHandler
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- resultMap
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- select
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