Mybatis if, set, where 动态sql和sql片段的使用

Mybatis 官方文档： https://mybatis.org/mybatis-3/zh/dynamic-sql.html

动态SQL

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

数据表

CREATE TABLE `blog` ( 
    `id` VARCHAR(50) NOT NULL COMMENT '博客id', 
    `title` VARCHAR(100) NOT NULL COMMENT '博客标题', 
    `author` VARCHAR(30) NOT NULL COMMENT '博客作者', 
    `create_time` DATETIME NOT NULL COMMENT '创建时间', 
    `views` INT(30) NOT NULL COMMENT '浏览量'
) ENGINE=INNODB CHARSET=utf8 COLLATE=utf8_general_ci;

实体类

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

开启驼峰命名自动映射

<settings>
    
    <setting name="mapUnderscoreToCamelCase" value="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：在包裹的代码块前面添加一个 xxx
• prefixOverrides：属性会忽略通过管道符分隔的文本序列（注意此例中的空格是必要的）
• suffixOverrides： 忽略最后一个 xxx

## 等价于 where 标签
<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
trim>

## 等价于 set 标签
<trim prefix="SET" suffixOverrides=",">
  ...
trim>

where

若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除。

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 元素会动态地在行首插入 SET 关键字，并会删掉额外的逗号

<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

类似 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

我们可以把一些功能抽取出来，方便复用

• sql：抽取代码片段
• include： 引用sql抽取的代码片段

<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>

注意事项

• 最好基于单表来定义SQL片段
• 不要存在 where 标签

foreach

• collection：遍历对象
• item：每一项
• index：索引
• open：开头
• separator：分隔符
• close：结尾

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>