前面已經(jīng)介紹了簡(jiǎn)單的MyBatis單表操作，這里來(lái)用一個(gè)簡(jiǎn)單的工程來(lái)演示下多表關(guān)聯(lián)。
工程創(chuàng)建參考MyBatis(一)單表操作
1.新建三張表（用戶表author, 文章表article, 評(píng)論表comment）
CREATE TABLE author (
id INT AUTO_INCREMENT PRIMARY KEY,
username VARCHAR(255) NOT NULL UNIQUE,
nickname VARCHAR(255),
birthday DATE,
register_time DATETIME NOT NULL
) default charset=utf8;
CREATE TABLE article (
id INT AUTO_INCREMENT PRIMARY KEY,
title VARCHAR(255) NOT NULL,
content TEXT NOT NULL,
author INT NOT NULL,
create_time DATETIME NOT NULL,
modify_time DATETIME NOT NULL,
FOREIGN KEY (author) REFERENCES author (id)
) default charset=utf8;
CREATE TABLE comment (
id INT AUTO_INCREMENT PRIMARY KEY,
author INT NOT NULL,
article INT NOT NULL,
content TINYTEXT NOT NULL,
create_time DATETIME NOT NULL,
FOREIGN KEY (author) REFERENCES author (id),
FOREIGN KEY (article) REFERENCES article (id)
) default charset=utf8;
2.新建三個(gè)實(shí)體類(Author, Article, Comment)--get,set, 構(gòu)造方法省略
public class Author {
private int id;
private String username;
private String nickname;
private Date birthday;
private Timestamp registerTime;
}
public class Article {
private int id;
private String title;
private String content;
private Author author;
private List<Comment> comments;
private Timestamp createTime;
private Timestamp modifyTime;
}
public class Comment {
private int id;
private String content;
private Author author;
private Article article;
private Timestamp createTime;
}
3.先看第一種查詢操作，第一章我們的例子中表中的列名和實(shí)體類中的屬性名是一一對(duì)應(yīng)的，所以，查詢結(jié)果我們就直接用resultType來(lái)映射了，但是當(dāng)列名和屬性名不一致的時(shí)候，resultType就不行了，所以才有了resultMap來(lái)指定每個(gè)列名和哪個(gè)屬性名對(duì)應(yīng)，如下：
<select id="selectAuthor" resultMap="authorResult">
SELECT
id,
username,
nickname,
birthday,
register_time
FROM author
WHERE id = #{id}
</select>
<resultMap id="authorResult" type="Author">
<id property="id" column="id"/>
<result property="registerTime" column="register_time"/>
</resultMap>
上面的例子中：<id>用來(lái)映射主鍵，<result>用來(lái)映射一邊的屬性，如果列名和屬性名一樣，還可以省略映射關(guān)系。比如這里只指定registerTime屬性和register_time列名映射。
4. 第二種，多表查詢的第一種方式。比如我們要查詢某篇文章和對(duì)應(yīng)的作者。先看例子：
<!-- 查詢作者 -->
<select id="selectAuthor" resultMap="authorResult">
SELECT
id,
username,
nickname,
birthday,
register_time
FROM author
WHERE id = #{id}
</select>
<resultMap id="authorResult" type="Author">
<id property="id" column="id"/>
<result property="registerTime" column="register_time"/>
</resultMap>
<!-- 查詢文章 -->
<select id="selectArticle" resultMap="articleMap">
SELECT
id,
title,
content,
author,
create_time,
modify_time
FROM article
WHERE id = #{id}
</select>
<resultMap id="articleMap" type="Article">
<id property="id" column="id"/>
<result property="createTime" column="create_time"/>
<result property="modifyTime" column="modify_time"/>
<!-- 嵌套查詢作者 -->
<association property="author" column="author" select="selectAuthor"/>
</resultMap>
上面的例子中，第一個(gè)select查詢只是一個(gè)簡(jiǎn)單的單表查詢，第二個(gè)select查詢中也是一個(gè)簡(jiǎn)單的單表查詢（看起來(lái)是），但是在resultMap中指定了一個(gè)association標(biāo)簽，其中就指定著另一個(gè)select查詢的ID。MyBatis會(huì)在每一條查詢記錄上再執(zhí)行一次association指定的查詢。以此來(lái)達(dá)到多表關(guān)聯(lián)查詢的目的。但是，很顯然，當(dāng)數(shù)據(jù)量大的時(shí)候，這種方式的效率堪憂，因?yàn)槊繄?zhí)行一次sql語(yǔ)句，我們還要執(zhí)行一次association指定的查詢。所以一般使用下面的查詢方式：
5.第三種查詢方式。下面的例子中，我們查詢某篇文章對(duì)應(yīng)的作者，以及對(duì)應(yīng)的所有評(píng)論。
<select id="selectArticleDetails" resultMap="detailedArticleResultMap">
select
A.id as article_id,
A.title as article_title,
A.content as article_content,
A.create_time as article_createTime,
A.modify_time as article_modifyTime,
B.id as author_id,
B.username as author_username,
B.nickname as author_nickname,
B.birthday as author_birthday,
B.register_time as author_registerTime,
C.id as comment_id,
C.content as comment_content,
C.create_time as comment_createTime
from Article A
left outer join Author B on B.id = A.author
left outer join Comment C on A.id = C.article
where A.id = #{id}
</select>
<resultMap id="detailedArticleResultMap" type="Article">
<id property="id" column="article_id"/>
<result property="title" column="article_title"/>
<result property="content" column="article_content"/>
<result property="createTime" column="article_createTime"/>
<result property="modifyTime" column="article_modifyTime"/>
<!-- 指定某些實(shí)體類屬性對(duì)應(yīng)的類型 -->
<association property="author" javaType="Author">
<id property="id" column="author_id"/>
<result property="username" column="author_username"/>
<result property="nickname" column="author_nickname"/>
<result property="birthday" column="author_birthday"/>
<result property="registerTime" column="author_registerTime"/>
</association>
<!-- 指定集合屬性對(duì)應(yīng)的類型 -->
<collection property="comments" ofType="Comment" columnPrefix="comment_">
<id property="id" column="id"/>
<result property="content" column="content"/>
<result property="createTime" column="createTime"/>
</collection>
</resultMap>
單看查詢語(yǔ)句來(lái)說(shuō)，這種查詢方式和我們平時(shí)使用的多表關(guān)聯(lián)查詢幾乎一模一樣，只是在resultMap中需要用association關(guān)鍵字來(lái)指定對(duì)應(yīng)的映射，這里association指定的不再是一個(gè)select查詢的ID,而是resultMap屬性。同時(shí)用collection關(guān)鍵字指定集合屬性對(duì)應(yīng)的類型和resultMap屬性。這里columnPrefix用來(lái)指定列名的前綴，result中的column就可以直接寫成content，而不用寫成comment_content了。
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