• 動態SQL
• if
• trim (where, set)
• choose (when, otherwise)
• SQL片段
• foreach

Mybatis 官方檔案： https://mybatis.org/mybatis-3/zh/dynamic-sql.html

動態SQL

• if
• choose (when, otherwise)
• trim (where, set)
• foreach

資料表

CREATE TABLE `blog` ( 
    `id` VARCHAR(50) NOT NULL COMMENT '博客id', 
    `title` VARCHAR(100) NOT NULL COMMENT '博客標題', 
    `author` VARCHAR(30) NOT NULL COMMENT '博客作者', 
    `create_time` DATETIME NOT NULL COMMENT '創建時間', 
    `views` INT(30) NOT NULL COMMENT '瀏覽量'
) ENGINE=INNODB CHARSET=utf8 COLLATE=utf8_general_ci;

物體類

public class Blog {
    private String id;
    private String title;
    private String auther;
    private java.util.Date createTime;
    private int views;
}

開啟駝峰命名自動映射

<settings>
    <!--開啟駝峰命名自動映射-->
    <setting name="mapUnderscoreToCamelCase" value="https://www.cnblogs.com/pojo/p/true"/>
</settings>

if

<select id="queryBlogIf" parameterType="map" resultType="Blog">
    select * from blog where 1=1
    <if test="title != null">
        and title = #{title}
    </if>
</select>

trim (where, set)

trim

• prefix：在包裹的代碼塊前面添加一個 xxx
• prefixOverrides：屬性會忽略通過管道符分隔的文本序列（注意此例中的空格是必要的）
• suffixOverrides： 忽略最后一個 xxx

## 等價于 where 標簽
<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
</trim>

## 等價于 set 標簽
<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

where

若子句的開頭為 “AND” 或 “OR”，where 元素也會將它們去除，

HashMap hashMap = new HashMap();
hashMap.put("title","java");
hashMap.put("author","自己");

<select id="queryBlogIf" parameterType="map" resultType="Blog">
    select * from blog
    <where>
        <if test="title != null">
            title = #{title}
        </if>
        <if test="author != null">
            and author = #{author}
        </if>
    </where>
</select>

set

set 元素會動態地在行首插入 SET 關鍵字，并會刪掉額外的逗號

<update id="updateBlog" parameterType="map">
    update blog
    <set>
        <if test="title != null">
            title = #{title},
        </if>
        <if test="author != null">
            author = #{author},
        </if>
    </set>
    where id = #{id}
</update>

choose (when, otherwise)

choose

類似 Java 中的 switch

HashMap hashMap = new HashMap();
hashMap.put("title","java");
//        hashMap.put("author","自己");
hashMap.put("views", 1000);

<select id="queryBlogChoose" parameterType="map" resultType="Blog">
    select * from blog
    <where>
        <choose>
            <when test="title != null">
                title = #{title}
            </when>
            <when test="author != null">
                author = #{author}
            </when>
            <otherwise>
                views = #{views}
            </otherwise>
        </choose>
    </where>
</select>

SQL片段

我們可以把一些功能抽取出來，方便復用

• sql：抽取代碼片段
• include： 參考sql抽取的代碼片段

<sql id="if-title-author">
    <if test="title != null">
        title = #{title}
    </if>
    <if test="author != null">
        and author = #{author}
    </if>
</sql>

<select id="queryBlogIf" parameterType="map" resultType="Blog">
    select * from blog
    <where>
        <include refid="if-title-author"/>
    </where>
</select>

注意事項

• 最好基于單表來定義SQL片段
• 不要存在 where 標簽

foreach

• collection：遍歷物件
• item：每一項
• index：索引
• open：開頭
• separator：分隔符
• close：結尾

int[] array = new int[]{10, 5000, 9999};
List<Integer> list = new ArrayList<>();
for (int i : array) {
    list.add(i);
}

<select id="getBlogIn" parameterType="list" resultType="Blog">
    select * from blog
    <where>
        <if test="list != null and list.size() > 0">
            views in
            <foreach collection="list" item="id" index="index" open="(" separator="," close=")">
                #{id}
            </foreach>
        </if>
    </where>
</select>

標籤：Java

上一篇：在IDEA中通過Module管理多個專案

下一篇：java開發學習路線~