1. 程式人生 > >MyBatis初級實戰之五:一對一關聯查詢

MyBatis初級實戰之五:一對一關聯查詢

### 歡迎訪問我的GitHub [https://github.com/zq2599/blog_demos](https://github.com/zq2599/blog_demos) 內容:所有原創文章分類彙總及配套原始碼,涉及Java、Docker、Kubernetes、DevOPS等; ### 本篇概覽 - 本文是《MyBatis初級實戰》系列的第五篇,從多表獲取資料是個常見的場景,一般有以下兩種方式: - 聯表查詢:join操作,一次查詢完成 - 多次查詢:用第一次查詢的結果作為條件,再做查詢(MyBatis中叫做巢狀查詢) - 本篇的內容就是學習MyBatis對上述兩種查詢的支援,全文由以下章節組成: 1. 準備資料; 2. 本次實戰的java工程 3. 最簡單的聯表(兩個表的資料儲存在一個實體類的不同欄位); 4. 一對一聯表查詢(兩個表的資料分別儲存在不同實體類,假設是A和B,A是B的成員變數) 5. 一對一巢狀查詢(兩個表的資料分別儲存在不同實體類,假設是A和B,A是B的成員變數) ### 原始碼下載 1. 如果您不想編碼,可以在GitHub下載所有原始碼,地址和連結資訊如下表所示(https://github.com/zq2599/blog_demos): | 名稱 | 連結 | 備註| | :-------- | :----| :----| | 專案主頁| https://github.com/zq2599/blog_demos | 該專案在GitHub上的主頁 | | git倉庫地址(https)| https://github.com/zq2599/blog_demos.git | 該專案原始碼的倉庫地址,https協議 | | git倉庫地址(ssh)| [email protected]:zq2599/blog_demos.git | 該專案原始碼的倉庫地址,ssh協議 | 2. 這個git專案中有多個資料夾,本章的應用在mybatis資料夾下,如下圖紅框所示: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082516339-304561694.png) 3. mybatis是個父工程,裡面有數個子工程,本篇的原始碼在relatedoperation子工程中,如下圖紅框所示: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082519889-1247316723.jpg) ### 準備資料 1. 本次實戰,在名為mybatis的資料庫中建立兩個表(和前面幾篇文章中的表結構一模一樣):user和log表; 2. user表記錄用戶資訊,非常簡單,只有三個欄位:主鍵、名稱、年齡 3. log表記錄用戶行為,四個欄位:主鍵、使用者id、行為描述、行為時間 4. user和log的關係如下圖: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082520138-74316177.jpg) 5. 建表和新增資料的語句如下: ```sql use mybatis; DROP TABLE IF EXISTS `user`; CREATE TABLE `user` ( `id` int(32) NOT NULL AUTO_INCREMENT, `name` varchar(32) NOT NULL, `age` int(32) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8; DROP TABLE IF EXISTS `log`; CREATE TABLE `log` ( `id` int(32) NOT NULL AUTO_INCREMENT, `user_id` int(32), `action` varchar(255) NOT NULL, `create_time` datetime not null, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8; INSERT INTO mybatis.user (id, name, age) VALUES (3, 'tom', 11); INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (3, 3, 'read book', '2020-08-07 08:18:16'); INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (4, 3, 'go to the cinema', '2020-09-02 20:00:00'); INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (5, 3, 'have a meal', '2020-10-05 12:03:36'); INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (6, 3, 'have a sleep', '2020-10-06 13:00:12'); INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (7, 3, 'write', '2020-10-08 09:21:11'); ``` ### 本次實戰的java工程 1. 在父工程mybatis下新建子工程relatedoperation,pom.xml如下: ```xml ``` 2. 基本配置檔案application.yml: ```yml server: port: 8080 spring: #1.JDBC資料來源 datasource: username: root password: 123456 url: jdbc:mysql://192.168.50.43:3306/mybatis?useUnicode=true&characterEncoding=utf-8&useSSL=true&serverTimezone=UTC driver-class-name: com.mysql.cj.jdbc.Driver #2.連線池配置 druid: #初始化連線池的連線數量 大小,最小,最大 initial-size: 5 min-idle: 5 max-active: 20 #配置獲取連線等待超時的時間 max-wait: 60000 #配置間隔多久才進行一次檢測,檢測需要關閉的空閒連線,單位是毫秒 time-between-eviction-runs-millis: 60000 # 配置一個連線在池中最小生存的時間,單位是毫秒 min-evictable-idle-time-millis: 30000 # 配置一個連線在池中最大生存的時間,單位是毫秒 max-evictable-idle-time-millis: 300000 validation-query: SELECT 1 FROM user test-while-idle: true test-on-borrow: true test-on-return: false # 是否快取preparedStatement,也就是PSCache 官方建議MySQL下建議關閉 個人建議如果想用SQL防火牆 建議開啟 pool-prepared-statements: true max-pool-prepared-statement-per-connection-size: 20 # 配置監控統計攔截的filters,去掉後監控介面sql無法統計,'wall'用於防火牆 filters: stat,wall,slf4j filter: stat: merge-sql: true slow-sql-millis: 5000 #3.基礎監控配置 web-stat-filter: enabled: true url-pattern: /* #設定不統計哪些URL exclusions: "*.js,*.gif,*.jpg,*.png,*.css,*.ico,/druid/*" session-stat-enable: true session-stat-max-count: 100 stat-view-servlet: enabled: true url-pattern: /druid/* reset-enable: true #設定監控頁面的登入名和密碼 login-username: admin login-password: admin allow: 127.0.0.1 #deny: 192.168.1.100 # mybatis配置 mybatis: # 配置檔案所在位置 config-location: classpath:mybatis-config.xml # 對映檔案所在位置 mapper-locations: classpath:mappers/*Mapper.xml # 日誌配置 logging: level: root: INFO com: bolingcavalry: relatedoperation: mapper: debug ``` 3. 再準備名為application-test.yml的配置檔案,這是執行單元測試時用到的,和application.yml的不同之處是spring.datasource.druid.web-stat-filter.enabled配置設定成false; 4. mybatis的配置檔案mybatis-config.xml如下: ```xml ``` 5. 資料來源配置類DruidConfig.java: ```java package com.bolingcavalry.relatedoperation; import com.alibaba.druid.pool.DruidDataSource; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.beans.factory.annotation.Value; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; @Configuration public class DruidConfig { private static final Logger logger = LoggerFactory.getLogger(DruidConfig.class); @Value("${spring.datasource.url}") private String dbUrl; @Value("${spring.datasource.username}") private String username; @Value("${spring.datasource.password}") private String password; @Value("${spring.datasource.driver-class-name}") private String driverClassName; @Value("${spring.datasource.druid.initial-size}") private int initialSize; @Value("${spring.datasource.druid.max-active}") private int maxActive; @Value("${spring.datasource.druid.min-idle}") private int minIdle; @Value("${spring.datasource.druid.max-wait}") private int maxWait; @Value("${spring.datasource.druid.pool-prepared-statements}") private boolean poolPreparedStatements; @Value("${spring.datasource.druid.max-pool-prepared-statement-per-connection-size}") private int maxPoolPreparedStatementPerConnectionSize; @Value("${spring.datasource.druid.time-between-eviction-runs-millis}") private int timeBetweenEvictionRunsMillis; @Value("${spring.datasource.druid.min-evictable-idle-time-millis}") private int minEvictableIdleTimeMillis; @Value("${spring.datasource.druid.max-evictable-idle-time-millis}") private int maxEvictableIdleTimeMillis; @Value("${spring.datasource.druid.validation-query}") private String validationQuery; @Value("${spring.datasource.druid.test-while-idle}") private boolean testWhileIdle; @Value("${spring.datasource.druid.test-on-borrow}") private boolean testOnBorrow; @Value("${spring.datasource.druid.test-on-return}") private boolean testOnReturn; @Value("${spring.datasource.druid.filters}") private String filters; @Value("{spring.datasource.druid.connection-properties}") private String connectionProperties; /** * Druid 連線池配置 */ @Bean public DruidDataSource dataSource() { DruidDataSource datasource = new DruidDataSource(); datasource.setUrl(dbUrl); datasource.setUsername(username); datasource.setPassword(password); datasource.setDriverClassName(driverClassName); datasource.setInitialSize(initialSize); datasource.setMinIdle(minIdle); datasource.setMaxActive(maxActive); datasource.setMaxWait(maxWait); datasource.setTimeBetweenEvictionRunsMillis(timeBetweenEvictionRunsMillis); datasource.setMinEvictableIdleTimeMillis(minEvictableIdleTimeMillis); datasource.setMaxEvictableIdleTimeMillis(minEvictableIdleTimeMillis); datasource.setValidationQuery(validationQuery); datasource.setTestWhileIdle(testWhileIdle); datasource.setTestOnBorrow(testOnBorrow); datasource.setTestOnReturn(testOnReturn); datasource.setPoolPreparedStatements(poolPreparedStatements); datasource.setMaxPoolPreparedStatementPerConnectionSize(maxPoolPreparedStatementPerConnectionSize); try { datasource.setFilters(filters); } catch (Exception e) { logger.error("druid configuration initialization filter", e); } datasource.setConnectionProperties(connectionProperties); return datasource; } } ``` 6. swagger配置類: ```java package com.bolingcavalry.relatedoperation; import springfox.documentation.service.Contact; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import springfox.documentation.builders.ApiInfoBuilder; import springfox.documentation.builders.PathSelectors; import springfox.documentation.builders.RequestHandlerSelectors; import springfox.documentation.service.ApiInfo; import springfox.documentation.service.Tag; import springfox.documentation.spi.DocumentationType; import springfox.documentation.spring.web.plugins.Docket; import springfox.documentation.swagger2.annotations.EnableSwagger2; @Configuration @EnableSwagger2 public class SwaggerConfig { @Bean public Docket createRestApi() { return new Docket(DocumentationType.SWAGGER_2) .apiInfo(apiInfo()) .tags(new Tag("UserController", "使用者服務"), new Tag("LogController", "日誌服務")) .select() // 當前包路徑 .apis(RequestHandlerSelectors.basePackage("com.bolingcavalry.relatedoperation.controller")) .paths(PathSelectors.any()) .build(); } //構建 api文件的詳細資訊函式,注意這裡的註解引用的是哪個 private ApiInfo apiInfo() { return new ApiInfoBuilder() //頁面標題 .title("MyBatis CURD操作") //建立人 .contact(new Contact("程式設計師欣宸", "https://github.com/zq2599/blog_demos", "[email protected]")) //版本號 .version("1.0") //描述 .description("API 描述") .build(); } } ``` 7. springboot引導類: ```java package com.bolingcavalry.relatedoperation; import org.mybatis.spring.annotation.MapperScan; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication @MapperScan("com.bolingcavalry.relatedoperation.mapper") public class RelatedOperationApplication { public static void main(String[] args) { SpringApplication.run(RelatedOperationApplication.class, args); } } ``` 8. 使用者表的實體類: ```java package com.bolingcavalry.relatedoperation.entity; import io.swagger.annotations.ApiModel; import io.swagger.annotations.ApiModelProperty; import lombok.Data; import lombok.NoArgsConstructor; @Data @NoArgsConstructor @ApiModel(description = "使用者實體類") public class User { @ApiModelProperty(value = "使用者ID") private Integer id; @ApiModelProperty(value = "使用者名稱", required = true) private String name; @ApiModelProperty(value = "使用者地址", required = false) private Integer age; } ``` 9. 日誌表的實體類: ```java package com.bolingcavalry.relatedoperation.entity; import io.swagger.annotations.ApiModel; import io.swagger.annotations.ApiModelProperty; import lombok.Data; import lombok.NoArgsConstructor; import java.sql.Date; @Data @NoArgsConstructor @ApiModel(description = "日誌實體類") public class Log { @ApiModelProperty(value = "日誌ID") private Integer id; @ApiModelProperty(value = "使用者ID") private Integer userId; @ApiModelProperty(value = "日誌內容") private String action; @ApiModelProperty(value = "建立時間") private Date createTime; } ``` - 以上就是本篇的準備程式碼,接下來在此基礎上實現各種多表關聯查詢 ### 最簡單的聯表 - 先實戰的是最普通的聯表,如下圖所示,查詢結果是名為LogExtend的實體類,這個類有5個欄位,其中四個來自日誌表log,一個來自使用者表user: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082520332-1934786433.jpg) - 下圖是開發步驟: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082520518-1632391366.jpg) 1. 實體類LogExtend的原始碼如下,可見和Log相比多了個userName欄位: ```java package com.bolingcavalry.relatedoperation.entity; import io.swagger.annotations.ApiModel; import io.swagger.annotations.ApiModelProperty; import lombok.Data; import lombok.NoArgsConstructor; @Data @NoArgsConstructor @ApiModel(description = "日誌實體類(含使用者表的欄位)") public class LogExtend extends Log { @ApiModelProperty(value = "使用者名稱") private String userName; } ``` 2. 新建log表對應的對映檔案LogMapper.xml,如下所示,裡面是通過left join語法執行的簡單的聯表查詢,以及查詢結果對應的resultMap定義: ```xml ``` 3. mapper介面程式碼: ```java package com.bolingcavalry.relatedoperation.mapper; import com.bolingcavalry.relatedoperation.entity.LogAssociateUser; import com.bolingcavalry.relatedoperation.entity.LogExtend; import org.springframework.stereotype.Repository; @Repository public interface LogMapper { LogExtend oneObjectSel(int id); } ``` 4. service層的程式碼在LogService.java檔案中: ```java package com.bolingcavalry.relatedoperation.service; import com.bolingcavalry.relatedoperation.entity.LogAssociateUser; import com.bolingcavalry.relatedoperation.entity.LogExtend; import com.bolingcavalry.relatedoperation.mapper.LogMapper; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Service; @Service public class LogService { @Autowired LogMapper logMapper; public LogExtend oneObjectSel(int id){ return logMapper.oneObjectSel(id); } } ``` 5. controller層的程式碼在LogController.java檔案中: ```java @RestController @RequestMapping("/log") @Api(tags = {"LogController"}) public class LogController { @Autowired private LogService logService; @ApiOperation(value = "根據ID查詢日誌記錄,帶userName欄位,該欄位通過聯表查詢實現", notes="根據ID查詢日誌記錄,帶userName欄位,該欄位通過聯表查詢實現") @ApiImplicitParam(name = "id", value = "日誌ID", paramType = "path", required = true, dataType = "Integer") @RequestMapping(value = "/aggregate/{id}", method = RequestMethod.GET) public LogExtend oneObjectSel(@PathVariable int id){ return logService.oneObjectSel(id); } ``` 6. 編寫單元測試的程式碼ControllerTest.java,由於今天的測試涉及到user和log兩個表,因此在測試類ControllerTest的內部準備了兩個內部類,分別用於測試user和log表: ```java package com.bolingcavalry.relatedoperation.controller; import lombok.extern.slf4j.Slf4j; import org.junit.jupiter.api.*; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc; import org.springframework.boot.test.context.SpringBootTest; import org.springframework.http.MediaType; import org.springframework.test.context.ActiveProfiles; import org.springframework.test.web.servlet.MockMvc; import org.springframework.test.web.servlet.request.MockMvcRequestBuilders; import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status; @SpringBootTest @DisplayName("Web介面的單元測試") @AutoConfigureMockMvc @ActiveProfiles("test") @Slf4j public class ControllerTest { /** * 查詢方式:聯表 */ final static String SEARCH_TYPE_LEFT_JOIN = "leftjoin"; /** * 查詢方式:巢狀 */ final static String SEARCH_TYPE_NESTED = "nested"; final static int TEST_USER_ID = 3; final static String TEST_USER_NAME = "tom"; @Autowired MockMvc mvc; @Nested @TestMethodOrder(MethodOrderer.OrderAnnotation.class) @DisplayName("使用者服務") class User { } @Nested @TestMethodOrder(MethodOrderer.OrderAnnotation.class) @DisplayName("日誌服務") class Log { final static int TEST_LOG_ID = 5; @Test @DisplayName("通過日誌ID獲取日誌資訊,帶userName欄位,該欄位通過聯表查詢實現") @Order(1) void oneObjectSel() throws Exception { mvc.perform(MockMvcRequestBuilders.get("/log/aggregate/" + TEST_LOG_ID) .accept(MediaType.APPLICATION_JSON)) .andExpect(status().isOk()) .andExpect(jsonPath("$.id").value(TEST_LOG_ID)) .andExpect(jsonPath("$.userName").value(TEST_USER_NAME)) .andDo(print()); } } } ``` 7. 執行上述單元測試方法,結果如下圖,紅框中就是controller層返回的資料,可見已通過Mybatis成功取得LogExtend例項: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082520874-337907740.jpg) - 下一站是一對一聯表查詢; ### 關於一對一關聯的兩種方式 - 前面的查詢有個特點:儘管查詢了兩個表,但結果都在同一實體類的不同欄位,而更符合業務邏輯的關係應該是log類中有個user類的成員變數,即如下形式: ```java @Data @NoArgsConstructor @ApiModel(description = "日誌實體類") public class LogAssociateUser { @ApiModelProperty(value = "日誌ID") private Integer id; @ApiModelProperty(value = "使用者物件") private User user; @ApiModelProperty(value = "日誌內容") private String action; @ApiModelProperty(value = "建立時間") private Date createTime; } ``` - 接下來的實戰就是如何用MyBatis查詢得到上述LogAssociateUser 型別的結果; - 一對一關聯的實現有聯表和巢狀查詢兩種,它們的差異在Mybatis中體現在association的子節點上: 1. 聯表時,association內使用result子節點,將聯表查詢的結果對映到關聯物件; 2. 巢狀時,association內使用select子節點,觸發一次新的查詢; ### 一對一(聯表) 所謂一對一,就是一個物件關聯了另一個物件,例如一條log記錄中,帶有對應的user資訊; 1. 下面是新的實體類LogAssociateUser,該類對應的是log表記錄,有個user欄位,型別是User物件: ```java @Data @NoArgsConstructor @ApiModel(description = "日誌實體類") public class LogAssociateUser { @ApiModelProperty(value = "日誌ID") private Integer id; @ApiModelProperty(value = "使用者物件") private User user; @ApiModelProperty(value = "日誌內容") private String action; @ApiModelProperty(value = "建立時間") private Date createTime; } ``` 2. 對映檔案LogMapper.xml中,sql和resultMap如下,可見查詢的時候將user表的欄位都查出來了,然後在resultMap中用association節點去處理sql中查出的user表的資料,通過javaType屬性轉為User類的例項: ```xml ``` 3. 以上就是一對一(聯表)的關鍵點,接下來按部就班的在LogMapper、LogService、LogController中新增方法即可,下面是LogController中對應的web介面,稍後會在單元測試中呼叫這個介面進行驗證: ```java @ApiOperation(value = "根據ID查詢日誌記錄,帶使用者物件,聯表查詢實現", notes="根據ID查詢日誌記錄,帶使用者物件,聯表查詢實現") @ApiImplicitParam(name = "id", value = "日誌ID", paramType = "path", required = true, dataType = "Integer") @RequestMapping(value = "/leftjoin/{id}", method = RequestMethod.GET) public LogAssociateUser leftJoinSel(@PathVariable int id){ return logService.leftJoinSel(id); } ``` 4. 最後是單元測試的程式碼(ControllerTest.java檔案),用來測試上述程式碼是否有效,注意下面的queryAndCheck私有方法,該方法中發起請求並驗證結果: ```java /** * 通過日誌ID獲取日誌資訊有兩種方式:聯表和巢狀查詢, * 從客戶端來看,僅一部分path不同,因此將請求和檢查封裝到一個通用方法中, * 呼叫方法只需要指定不同的那一段path * @param subPath * @throws Exception */ private void queryAndCheck(String subPath) throws Exception { String queryPath = "/log/" + subPath + "/" + TEST_LOG_ID; log.info("query path [{}]", queryPath); mvc.perform(MockMvcRequestBuilders.get(queryPath) .accept(MediaType.APPLICATION_JSON)) .andExpect(status().isOk()) .andExpect(jsonPath("$.id").value(TEST_LOG_ID)) .andExpect(jsonPath("$.user.id").value(TEST_USER_ID)) .andDo(print()); } @Test @DisplayName("通過日誌ID獲取日誌資訊(關聯了使用者),聯表查詢") @Order(2) void leftJoinSel() throws Exception { queryAndCheck(SEARCH_TYPE_LEFT_JOIN); } ``` 5. 執行單元測試結果如下,可見:內部嵌套了一個json物件,就是user表的資料: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082522389-99862093.jpg) ### 一對一(巢狀) 1. 接下來試試巢狀的方式; 2. LogMapper.xml中對應的sql: ```xml ``` 3. 上述sql對應的resultMap如下,可見association節點中有個select屬性,這就是MyBatis支援巢狀查詢的關鍵,該屬性的值是個select節點: ```java ``` 4. 上述節點中select屬性的值,對應一個select節點,如下: ```xml ``` 5. 以上就是一對一(巢狀)的關鍵點,接下來按部就班的在LogMapper、LogService、LogController中新增方法即可,下面是LogController中對應的web介面,稍後會在單元測試中呼叫這個介面進行驗證: ```java @ApiOperation(value = "根據ID查詢日誌記錄,帶使用者物件,巢狀查詢實現", notes="根據ID查詢日誌記錄,帶使用者物件,巢狀查詢實現") @ApiImplicitParam(name = "id", value = "日誌ID", paramType = "path", required = true, dataType = "Integer") @RequestMapping(value = "/nested/{id}", method = RequestMethod.GET) public LogAssociateUser nestedSel(@PathVariable int id){ return logService.nestedSel(id); } ``` 6. 最後是單元測試的程式碼(ControllerTest.java檔案),用來測試上述程式碼是否有效,如下可見,直接呼叫了前面的queryAndCheck來驗證: ```java @Test @DisplayName("通過日誌ID獲取日誌資訊(關聯了使用者),巢狀查詢") @Order(3) void nestedSel() throws Exception { queryAndCheck(SEARCH_TYPE_NESTED); } ``` 7. 執行上述單元測試程式碼,結果如下,可見巢狀查詢的方式也能將user表的資料成功獲取,放入log例項的成員變數中: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082522846-909187286.jpg) 8. 最後是對比聯表和巢狀查詢的差異,先看聯表查詢的MyBatis日誌,如下圖紅框所示,只有一次sql查詢: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082523338-2029184207.jpg) 9. 再看巢狀查詢的日誌,如下圖,紅框是第一次查詢,結果中的userid作為綠框中的第二次查詢的條件: ![在這裡插入圖片描述](https://img2020.cnblogs.com/other/485422/202101/485422-20210121082523873-399309500.jpg) - 至此,一對一的多表查詢實戰就完成了,本篇的邏輯是一條log記錄關聯一條user記錄,下一篇文章,咱們學習一對多關聯,即一個user有多條log記錄; ### 你不孤單,欣宸原創一路相伴 1. 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