Mybatis中@select注解联合查询

前言

在项目中经常会使用到一些简单的联合查询获取对应的数据信息，我们常规都是会根据对应的mapper接口写对应的mapper.xml的来通过对应的业务方法来调用获取，针对这一点本人感觉有点繁琐，就对@select注解联合查询进行探索和尝试，并将自己总结的分享给大家，有不到之处，敬请大家批评指正！！！

• pom.xml所用到依赖如下

　　　　<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.2.0</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.38</version>
        </dependency>

• application.yml的配置如下：

server:
  port: 8888

spring:
  datasource:
    driver-class-name: com.mysql.jdbc.Driver
    url: jdbc:mysql://localhost:3306/project?useUnicode=true&characterEncoding=UTF8&zeroDateTimeBehavior=convertToNull&useSSL=false
    username: root
    password: 123456
mybatis:
  type-aliases-package: com.songwp.snowflake.entity
  mapper-locations: classpath:mybatis/mapper/*.xml
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

• 数据库测试表

-- ----------------------------
-- Table structure for sys_dept
-- 部门表
-- ----------------------------
DROP TABLE IF EXISTS `sys_dept`;
CREATE TABLE `sys_dept`  (
  `id` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '部门ID',
  `user_id` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '用户ID',
  `dept_name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '部门名称',
  `parent_id` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '父级部门ID',
  `parent_name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '父级部门名称',
  `status` int(5) DEFAULT 0 COMMENT '部门状态：0-正常  1-禁用',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of sys_dept
-- ----------------------------
INSERT INTO `sys_dept` VALUES ('5a8f893eedef4eafbc66feded3541c0f', '4117460f-20f7-47e7-bf8a-507a32880c06', '集团本部', 'GS-001', '集团本部', 0);
INSERT INTO `sys_dept` VALUES ('6271dd03e426400b9fd001bae9074efc', '4117460f-20f7-47e7-bf8a-507a32880c06', '财务部门', 'GS-003', '集团本部', 0);
INSERT INTO `sys_dept` VALUES ('f33503159a084e73b4e1313932cc9629', '4117460f-20f7-47e7-bf8a-507a32880c06', '研发部门', 'GS-002', '集团本部', 0);

-- ----------------------------
-- Table structure for sys_user
-- 用户表
-- ----------------------------
DROP TABLE IF EXISTS `sys_user`;
CREATE TABLE `sys_user`  (
  `id` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '主键ID',
  `username` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '用户名',
  `nickname` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '真实姓名',
  `password` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '密码',
  `gender` int(5) DEFAULT 1 COMMENT '性别：0-女  1-男',
  `age` int(5) DEFAULT NULL COMMENT '年龄',
  `phone` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '联系电话',
  `address` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '住址',
  `status` int(5) DEFAULT 0 COMMENT '用户状态：0-正常  1-冻结  2- 已注销',
  `birthday` date DEFAULT NULL COMMENT '生日',
  `create_time` datetime DEFAULT NULL COMMENT '添加时间',
  `create_user` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '创建人',
  `update_time` datetime DEFAULT NULL COMMENT '修改时间',
  `update_user` varchar(0) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '修改人',
  `remark` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '备注',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of sys_user
-- ----------------------------
INSERT INTO `sys_user` VALUES ('4117460f-20f7-47e7-bf8a-507a32880c06', 'zs', '张三', '123456', 0, 25, '13888888888', '陕西西安', 0, '1996-05-07', '2022-03-28 10:17:54', '张三', NULL, NULL, NULL);
INSERT INTO `sys_user` VALUES ('fb962a7f-3a61-4312-820c-9e67eefaa74a', 'zll', '赵老六', '123456', 1, 28, '13666666666', '陕西西安', 0, '1992-05-07', '2022-03-28 10:14:45', '赵老六', NULL, NULL, NULL);

• @select注解中SQL的写法

假设我想用户名(username)为“zs”和密码(password)为“123456”的条件下用户的信息和对应部门的信息为例：

SELECT
    u.username,
    u.PASSWORD,
    u.nickname,
    u.phone,
    u.gender,
    u.address,
    d.id AS dept_id,
    d.dept_name,
    d.STATUS AS dept_status
FROM
    sys_user u
    INNER JOIN sys_dept d ON u.id = d.user_id
WHERE
    u.username = 'zs'
    AND u.PASSWORD = '123456'

• Navicat中结果执行如下：

• mapper接口

@Select("<script> SELECT " +
            "u.username," +
            "u.password," +
            "u.nickname," +
            "u.phone," +
            "u.gender," +
            "u.address," +
            "d.id as dept_id," +
            "d.dept_name," +
            "d.status as dept_status " +
            "from" +
            " sys_user u" +
            " inner join sys_dept d on u.id = d.user_id " +
            " where u.username = #{username} <when test='password !=null'> " +
            " and u.password = #{password} </when> </script>")
    List<Map> getByParmsMap(String username, String password);

1、@Select注解基本用法
@Select注解的目的是为了取代xml中的select标签，只作用于方法上面。下面看一下@Select注解的源码介绍：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Select
{
    String[] value();
}

从上述可以看到两点信息：

（1）@Select注解只能修饰方法
（2）@Select注解的值是字符数组。

所以，@Select注解的用法是这样的：

@Select({ "select * from xxx", "select * from yyy" })
Person selectPersonById(Integer id);

虽然@Select注解的值是字符数组，但是真正生效的应该是最后那条SQL语句。这一点请大家要留意一下。

2、@Select注解动态SQL拼写
普通的字符串值，只能实现变量的替换功能，如下所示，

@Select("select * from t_person where id = #{id}")
Person selectPersonById(Integer id);

如果要想实现复杂的逻辑判断，则需要使用标签，如下所示：

@Select("<script> select * from t_person where id = #{id}
<when test='address !=null'> and address = #{address}
</when> </script>")
Person selectPersonById(Integer id);

其实，标签并非是@Select注解专用的，其他的注解，例如@Insert，@Update等等，都可以使用的。

• 业务层service接口

List<Map> getByParmsMap(String username, String password);

• 业务实现类的方法

public List<Map> getByParmsMap(String username, String password) {
        return userMapper.getByParmsMap(username,password);
 }

• 控制器controller方法

@RequestMapping(value = {"/getByParmsMap"}, method = RequestMethod.GET)
@ResponseBody
public List<Map> getByParmsMap(@RequestParam("username")String username,@RequestParam("password")String password){
    return userService.getByParmsMap(username,password);
}

• Postman接口调用如下

[
    {
        "password": "123456",
        "address": "陕西西安",
        "gender": 0,
        "phone": "13888888888",
        "nickname": "张三",
        "dept_name": "集团本部",
        "dept_status": 0,
        "dept_id": "5a8f893eedef4eafbc66feded3541c0f",
        "username": "zs"
    },
    {
        "password": "123456",
        "address": "陕西西安",
        "gender": 0,
        "phone": "13888888888",
        "nickname": "张三",
        "dept_name": "财务部门",
        "dept_status": 0,
        "dept_id": "6271dd03e426400b9fd001bae9074efc",
        "username": "zs"
    },
    {
        "password": "123456",
        "address": "陕西西安",
        "gender": 0,
        "phone": "13888888888",
        "nickname": "张三",
        "dept_name": "研发部门",
        "dept_status": 0,
        "dept_id": "f33503159a084e73b4e1313932cc9629",
        "username": "zs"
    }
]

• 如下图所示：