目录
1. 批量操作:通过标签支持批量插入
2. 批量操作:通过标签支持批量更新
3. 批量操作：通过标签支持批量删除
4. 动态SQL
3. 多条件分支查询
4. SQL语句优化：使用标签避免多余的AND或OR关键字。
5. 注解方式使用MyBatis
6. 一对多
7. 多对一：每个评论（Comment）都属于一篇文章（Article），并且每篇文章可以有多个评论。
8. MyBatis-Plus集成
1. 批量操作:通过<foreach>标签支持批量插入
<insert id="batchInsert" parameterType="java.util.List">
INSERT INTO user (username, email,phone, create_time) VALUES
<foreach collection="list" item="item" separator=",">
(#{item.username}, #{item.email},#{item.phone}, #{item.createTime})
</foreach>
</insert>2. 批量操作:通过<foreach>标签支持批量更新
<update id="batchUpdate" parameterType="java.util.List">
<foreach collection="list" item="item" separator=";">
UPDATE user
SET username = #{item.username}, email = #{item.email}
WHERE id = #{item.id}
</foreach>
</update>3. 批量操作：通过<foreach>标签支持批量删除
<delete id="batchDelete" parameterType="java.util.List">
DELETE FROM user WHERE id IN
<foreach collection="list" item="id" open="(" separator="," close=")">
#{id}
</foreach>
</delete>4. 动态SQL
<select id="findUsers" resultType="User">
SELECT * FROM user
WHERE 1=1
<if test="username != null and username != ''">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="email != null and email != ''">
AND email = #{email}
</if>
<if test="status != null">
AND status = #{status}
</if>
</select>3. 多条件分支查询
<select id="findUsersByCondition" resultType="User">
SELECT * FROM user
<where>
<choose>
<when test="searchType == 'username'">
username LIKE CONCAT('%', #{keyword}, '%')
</when>
<when test="searchType == 'email'">
email LIKE CONCAT('%', #{keyword}, '%')
</when>
<otherwise>
(username LIKE CONCAT('%', #{keyword}, '%') OR email LIKE CONCAT('%', #{keyword}, '%'))
</otherwise>
</choose>
</where>
</select>4. SQL语句优化：使用<trim>标签避免多余的AND或OR关键字。
<select id="findUsers" resultType="User">
SELECT * FROM user
<trim prefix="WHERE" prefixOverrides="AND |OR ">
<if test="username != null and username != ''">
AND username LIKE CONCAT('%', #{username}, '%')
</if>
<if test="email != null and email != ''">
AND email = #{email}
</if>
<if test="status != null">
AND status = #{status}
</if>
</trim>
</select>5. 注解方式使用MyBatis
public interface UserMapper {
@Select("SELECT * FROM user WHERE id = #{id}")
User getUserById(Long id);
@Insert("INSERT INTO user (username, email, create_time) VALUES (#{username}, #{email}, #{createTime})")
@Options(useGeneratedKeys = true, keyProperty = "id")
int insertUser(User user);
@Update("UPDATE user SET username = #{username}, email = #{email} WHERE id = #{id}")
int updateUser(User user);
@Delete("DELETE FROM user WHERE id = #{id}")
int deleteUser(Long id);
}6. 一对多
<resultMap id="userWithOrdersMap" type="User">
<id property="id" column="user_id"/>
<result property="username" column="username"/>
<collection property="orders" ofType="Order">
<id property="id" column="order_id"/>
<result property="orderNumber" column="order_number"/>
<result property="createTime" column="order_create_time"/>
</collection>
</resultMap>
<select id="getUserWithOrders" resultMap="userWithOrdersMap">
SELECT u.id as user_id, u.username, o.id as order_id, o.order_number, o.create_time as order_create_time
FROM user u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.id = #{userId}
</select>7. 多对一：每个评论（Comment）都属于一篇文章（Article），并且每篇文章可以有多个评论。
<?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.example.mapper.ArticleMapper">
<!-- 定义 Comment 的 resultMap -->
<resultMap id="commentWithArticleMap" type="Comment">
<id property="id" column="comment_id"/>
<result property="content" column="comment_content"/>
<result property="createTime" column="comment_create_time"/>
<association property="article" javaType="Article">
<id property="id" column="article_id"/>
<result property="title" column="article_title"/>
<result property="content" column="article_content"/>
</association>
</resultMap>
<!-- 定义 Article 的 resultMap -->
<resultMap id="articleWithCommentsMap" type="Article">
<id property="id" column="article_id"/>
<result property="title" column="article_title"/>
<result property="content" column="article_content"/>
<collection property="comments" ofType="Comment" resultMap="commentWithArticleMap"/>
</resultMap>
<!-- 查询文章及其评论列表 -->
<select id="getArticleWithComments" resultMap="articleWithCommentsMap">
SELECT
a.id as article_id,
a.title as article_title,
a.content as article_content,
c.id as comment_id,
c.content as comment_content,
c.create_time as comment_create_time
FROM article a
LEFT JOIN comment c ON a.id = c.article_id
WHERE a.id = #{articleId}
</select>
</mapper>8. MyBatis-Plus集成
@Service
public class UserServiceImpl extends ServiceImpl<UserMapper, User> implements UserService {
public List<User> findUsersByCondition(String username, String email) {
return this.list(new QueryWrapper<User>()
.like(StringUtils.isNotBlank(username), "username", username)
.eq(StringUtils.isNotBlank(email), "email", email));
}
}