Mybatis mapping configuration file
1, obtaining the value of increment primary key
<!-- public void addEmp(Employee employee); -->
<!-- parameterType：参数类型，可以省略，
获取自增主键的值：
mysql支持自增主键，自增主键值的获取，mybatis也是利用statement.getGenreatedKeys()；
useGeneratedKeys="true"；使用自增主键获取主键值策略
keyProperty；指定对应的主键属性，也就是mybatis获取到主键值以后，将这个值封装给javaBean的哪个属性
-->
<insert id="addEmp" parameterType="com.atguigu.mybatis.bean.Employee"
useGeneratedKeys="true" keyProperty="id" databaseId="mysql">
insert into tbl_employee(last_name,email,gender)
values(#{
lastName},#{
email},#{
gender})
</insert>
Expand: Oracle does not support auto-increment, Oracle uses sequence to simulate auto-increment; the primary key of each inserted data is the value obtained from the sequence, as shown below:
<!--
获取非自增主键的值：
Oracle不支持自增；Oracle使用序列来模拟自增；
每次插入的数据的主键是从序列中拿到的值；如何获取到这个值；
-->
<insert id="addEmp" databaseId="oracle">
<!--
keyProperty:查出的主键值封装给javaBean的哪个属性
order="BEFORE":当前sql在插入sql之前运行
AFTER：当前sql在插入sql之后运行
resultType:查出的数据的返回值类型
BEFORE运行顺序：
先运行selectKey查询id的sql；查出id值封装给javaBean的id属性
在运行插入的sql；就可以取出id属性对应的值
AFTER运行顺序：
先运行插入的sql（从序列中取出新值作为id）；
再运行selectKey查询id的sql；
-->
<selectKey keyProperty="id" order="BEFORE" resultType="Integer">
<!-- 编写查询主键的sql语句 -->
<!-- BEFORE-->
select EMPLOYEES_SEQ.nextval from dual
<!-- AFTER：
select EMPLOYEES_SEQ.currval from dual -->
</selectKey>
<!-- 插入时的主键是从序列中拿到的 -->
<!-- BEFORE:-->
insert into employees(EMPLOYEE_ID,LAST_NAME,EMAIL)
values(#{id},#{lastName},#{email<!-- ,jdbcType=NULL -->})
<!-- AFTER：
insert into employees(EMPLOYEE_ID,LAST_NAME,EMAIL)
values(employees_seq.nextval,#{lastName},#{email}) -->
</insert>
2, mybatis processing parameters
a) Single parameter: mybatis will not do special processing; #{parameter name/arbitrary name}: Take out the parameter value. example:#{id}
b) Multiple parameters: Mybatis will do special processing, multiple parameters will be encapsulated into a map
key：param1...paramN,或者参数的索引也可以 value：传入的参数值
If you still follow the writing of a single parameter, #{id},#{lastName}an exception will be thrown (the following code). Here, multiple parameter values need to use named parameters to solve the problem. The example is as follows.
异常：
org.apache.ibatis.binding.BindingException:
Parameter 'id' not found.
Available parameters are [1, 0, param1, param2]
操作：
方法：public Employee getEmpByIdAndLastName(Integer id,String lastName);
取值：#{id},#{lastName}
[Named Parameters]: Clearly specify the key of the map when encapsulating parameters:@Param(“id”)
Multiple parameters will be encapsulated into a map:key：使用@Param注解指定的值 value：参数值 #{指定的key}取出对应的参数值
public Employee getEmpByIdAndLastName(@Param("id")Integer id,@Param("lastName")String lastName);
C） POJO
If multiple parameters happen to be the data model of our business logic, we can directly pass in pojo;#{属性名}：取出传入的pojo的属性值
d）Map
If multiple data parameters are not in the business model, there is no corresponding pojo, not often used, for convenience, we can also pass map#{key}：取出map中对应的值
Summary: parameters for a long time will be packaged map, in order not to chaos, we can use @Param to specify the key used for packaging;
3, the parameter value obtaining (The difference between #{} and ${} in mybatis）
#{}: You can get the value in the map or the value of the pojo object attribute;
${}: You can get the value in the map or the value of the pojo object attribute;
select * from tbl_employee where id=${id} and last_name=#{lastName}
Preparing: select * from tbl_employee where id=2 and last_name=?
区别：
#{}:是以预编译的形式，将参数设置到sql语句中；PreparedStatement；防止sql注入
${}:取出的值直接拼装在sql语句中；会有安全问题；
大多情况下，我们去参数的值都应该去使用#{}；
原生jdbc不支持占位符的地方我们就可以使用${}进行取值
比如分表、排序。。。；按照年份分表拆分
select * from ${
year}_salary where xxx;
select * from tbl_employee order by ${f_name} ${
order}
4, when the value of the specified parameters relevant rules
#{}: Richer usage:
Some rules for specifying parameters:
a) javaType, jdbcType, mode (stored procedure), numericScale, resultMap, typeHandler, jdbcTypeName, expression (functions to be supported in the future);
b) jdbcType usually needs to be set under certain conditions:
When our data is null, some databases may not recognize mybatis's default handling of null. For example, Oracle (error report);
c) JdbcType OTHER: invalid type; because mybatis maps all nulls to the OTHER type of native Jdbc, oracle cannot handle it correctly; due to the global configuration: jdbcTypeForNull=OTHER; oracle does not support; two methods
1、#{email,jdbcType=NULL};
2、jdbcTypeForNull=NULL
<setting name="jdbcTypeForNull" value="NULL"/> （这里是在mybatis全局配置文件下配置）
5, package records map mapping file -select_
//多条记录封装一个map：Map<Integer,Employee>:键是这条记录的主键，值是记录封装后的javaBean
//@MapKey:告诉mybatis封装这个map的时候使用哪个属性作为map的key！！
@MapKey("lastName")
public Map<String, Employee> getEmpByLastNameLikeReturnMap(String lastName);
<!--resultType：如果返回的是一个集合，要写集合中元素的类型 -->
<!--public Map<Integer, Employee> getEmpByLastNameLikeReturnMap(String lastName); -->
<select id="getEmpByLastNameLikeReturnMap" resultType="com.atguigu.mybatis.bean.Employee">
select * from tbl_employee where last_name like #{lastName}
</select>
6, the results of the mapping file -select_resultMap custom mapping rules
<!--自定义某个javaBean的封装规则
type：自定义规则的Java类型
id:唯一id方便引用
-->
<resultMap type="com.atguigu.mybatis.bean.Employee" id="MySimpleEmp">
<!--指定主键列的封装规则
id定义主键会底层有优化；
column：指定哪一列
property：指定对应的javaBean属性
-->
<id column="id" property="id"/>
<!-- 定义普通列封装规则 -->
<result column="last_name" property="lastName"/>
<!-- 其他不指定的列会自动封装：我们只要写resultMap就把全部的映射规则都写上。 -->
<result column="email" property="email"/>
<result column="gender" property="gender"/>
</resultMap>
<!-- resultMap:自定义结果集映射规则； -->
<!-- public Employee getEmpById(Integer id); -->
<select id="getEmpById" resultMap="MySimpleEmp">
select * from tbl_employee where id=#{id}
</select>
7, the mapping file _resultMap_ property associated with the query package cascade results _
Scenario 1:
Query the Employee while querying the department corresponding to the employee
Employee===Department
An employee has department information corresponding to it;
id last_name gender d_id did dept_name (private Department dept;)
Method 1 : joint inquiry:Cascading attributesPackage result set
<resultMap type="com.atguigu.mybatis.bean.Employee" id="MyDifEmp">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="gender" property="gender"/>
<result column="did" property="dept.id"/>
<result column="dept_name" property="dept.departmentName"/>
</resultMap>
Second way : UsingassociationDefine encapsulation rules for associated single objects
<resultMap type="com.atguigu.mybatis.bean.Employee" id="MyDifEmp2">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="gender" property="gender"/>
<!-- association可以指定联合的javaBean对象
property="dept"：指定哪个属性是联合的对象
javaType:指定这个属性对象的类型[不能省略]
-->
<association property="dept" javaType="com.atguigu.mybatis.bean.Department">
<id column="did" property="id"/>
<result column="dept_name" property="departmentName"/>
</association>
</resultMap>
8. Mapping file_select_resultMap_associated query_distributed query & lazy loading
Use association to perform step-by-step query:
1. First query the employee information according to the employee id
2. Go to the department table to find out the department information according to the d_id value in the query employee information
3. Set the department to the employee;
<resultMap type="com.atguigu.mybatis.bean.Employee" id="MyEmpByStep">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<!-- association定义关联对象的封装规则
select:表明当前属性是调用select指定的方法查出的结果
column:指定将哪一列的值传给这个方法
流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性
-->
<association property="dept"
select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"
column="d_id">
</association>
</resultMap>
<!-- public Employee getEmpByIdStep(Integer id);-->
<select id="getEmpByIdStep" resultMap="MyEmpByStep">
select * from tbl_employee where id=#{id}
</select>
<!-- 可以使用延迟加载（懒加载）；(按需加载)
Employee==>Dept：
我们每次查询Employee对象的时候，都将一起查询出来。
部门信息在我们使用的时候再去查询；
分段查询的基础之上加上两个配置：
-->
<!--在mybatis的全局配置文件中添加懒加载配置-->
<settings>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
Scenario 2: When
querying the department, all the employee information corresponding to the department is also queried: the annotation is in the DepartmentMapper.xml
<!-- public List<Employee> getEmpsByDeptId(Integer deptId); -->
<select id="getEmpsByDeptId" resultType="com.atguigu.mybatis.bean.Employee">
select * from tbl_employee where d_id=#{deptId}
</select>
<!--嵌套结果集的方式，使用collection标签定义关联的集合类型的属性封装规则 -->
<!--
public class Department {
private Integer id;
private String departmentName;
private List<Employee> emps;//集合类型得使用collection
-->
<resultMap type="com.atguigu.mybatis.bean.Department" id="MyDept">
<id column="did" property="id"/>
<result column="dept_name" property="departmentName"/>
<!--
collection定义关联集合类型的属性的封装规则
ofType:指定集合里面元素的类型
-->
<collection property="emps" ofType="com.atguigu.mybatis.bean.Employee">
<!-- 定义这个集合中元素的封装规则 -->
<id column="eid" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
</collection>
</resultMap>
<!-- public Department getDeptByIdPlus(Integer id); -->
<select id="getDeptByIdPlus" resultMap="MyDept">
SELECT d.id did,d.dept_name dept_name,
e.id eid,e.last_name last_name,e.email email,e.gender gender
FROM tbl_dept d
LEFT JOIN tbl_employee e
ON d.id=e.d_id
WHERE d.id=#{id}
</select>
9, map file _select_resultMap_discriminator discriminator
<!-- <discriminator javaType=""></discriminator>
鉴别器：mybatis可以使用discriminator判断某列的值，然后根据某列的值改变封装行为
封装Employee：
如果查出的是女生：就把部门信息查询出来，否则不查询；
如果是男生，把last_name这一列的值赋值给email;
-->
<resultMap type="com.atguigu.mybatis.bean.Employee" id="MyEmpDis">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<!--
column：指定判定的列名
javaType：列值对应的java类型 -->
<discriminator javaType="string" column="gender">
<!--女生 resultType:指定封装的结果类型；不能缺少。/resultMap-->
<case value="0" resultType="com.atguigu.mybatis.bean.Employee">
<association property="dept"
<!--这里是从接口DepartmentMapper中拿到getDeptById方法-->
select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"
column="d_id">
</association>
</case>
<!--男生 ;如果是男生，把last_name这一列的值赋值给email; -->
<case value="1" resultType="com.atguigu.mybatis.bean.Employee">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="last_name" property="email"/>
<result column="gender" property="gender"/>
</case>
</discriminator>
</resultMap>
<select id="getEmpByIdStep" resultMap="MyEmpDis">
select * from tbl_employee where id=#{id}
</select>