通过XML、注解配置MyBatis SQL映射器
SpringBoot配置MyBatis在 SpringBoot整合MyBatis开发 这篇文章中已经描述了，这篇文章主要讲解一些SQL映射器中的写法及常用标签。
定义xml映射文件
"com.test.springboot.mybatis.dao.UserMapper">
id="AddressResultMap" type="com.test.springboot.mybatis.bean.AddressPo">
<id column="address_id" jdbcType="INTEGER" property="id" />
<result column="address" jdbcType="VARCHAR" property="address" />
<result column="user_id" jdbcType="VARCHAR" property="user_id" />
id="UserResultMap" type="com.test.springboot.mybatis.bean.UserPo">
<id column="id" jdbcType="VARCHAR" property="id" />
<result column="username" jdbcType="VARCHAR" property="username" />
<result column="password" jdbcType="VARCHAR" property="password" />
<result column="realname" jdbcType="VARCHAR" property="realname" />
<result column="company" jdbcType="VARCHAR" property="company" />
<result column="job" jdbcType="VARCHAR" property="job" />
<result column="salt" jdbcType="VARCHAR" property="salt" />
<result column="status" jdbcType="TINYINT" property="status" />
<result column="time" jdbcType="BIGINT" property="time" />
-- 一对多映射 -->
property="addressList" resultMap="AddressResultMap"/>
id="Base_Column_List" >
id, username, password, realname, company, job, salt, status, time
namespace：该标签用于指定映射器对应的接口文件，需要写接口文件全路径。
resultMap：用户设置SQL返回数据保存字段信息。
resultMap.type：用于指定返回信息对应的实体类。
sql：可编写一个SQL片段，用于复用到具体的SQL操作中。
select：用作查询操作的SQL标签。
MyBatis 不同映射语句实例
INSERT
通过自增长设置主键
"insertStudent" parameterType="Student" useGeneratedKeys="true" keyProperty="studId">
INSERT INTO STUDENTS(NAME, EMAIL, PHONE) VALUES(#{name},#{email},#{phone})
针对支持序列生成主键值
<insert id="insertStudent" parameterType="Student">
<selectKey keyProperty="studId" resultType="int" order="BEFORE">
SELECT ELEARNING.STUD_ID_SEQ.NEXTVAL FROM DUAL
selectKey>
INSERT INTO STUDENTS(STUD_ID,NAME,EMAIL, PHONE) VALUES(#{studId},#{name},#{email},#{phone})
insert>
//使用触发器
<insert id="insertStudent" parameterType="Student">
INSERT INTO STUDENTS(NAME,EMAIL, PHONE) VALUES(#{name},#{email},#{phone})
<selectKey keyProperty="studId" resultType="int" order="AFTER">
SELECT ELEARNING.STUD_ID_SEQ.CURRVAL FROM DUAL
selectKey>
insert>SELECT
<select id="queryList" resultMap="UserResultMap">
SELECT
"Base_Column_List"/>
FROM auth_user
<where>
<if test="username != null">
username = #{username}
if>
<if test="job != null">
AND job = #{job}
if>
where>
select> 条件不定查询，通过where标签及if、choose等标签可对字段进行判断，看是否需要添加到where条件中进行查询。
多表联合查询，需要在resultMap配置好一对多、一对一等关系。SQL返回结果会直接注入到resultMap配置的实体类中。
MyBatis结果集映射规则
- 对于 List, Collection, Iterable类型，返回 java.util.ArrayList
- 对于Map 类型，返回 java.util.HashMap
- 对于Set 类型： 返回 java.util.HashSet
- 对于SortedSet类型： 返回java.util.TreeSet
MyBatis 映射关系
一对一映射
方式1： 使用点语法进行映射
public class Student {
private Integer studId;
private String name;
private String email;
private Address address; //一个地址对象， 每个学生对应一个地址
}
"Student" id="StudentWithAddressResult">
"studId" column="stud_id" />
"name" column="name" />
"email" column="email" />
"phone" column="phone" />
"address.addrId" column="addr_id" />
"address.street" column="street" />
"address.city" column="city" />
"address.state" column="state" />
"address.zip" column="zip" />
"address.country" column="country" />
<select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">
SELECT
STUD_ID, NAME, EMAIL, A.ADDR_ID, STREET, CITY, STATE, ZIP, COUNTRY
FROM
STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
WHERE
STUD_ID=#{studId}
select>方式2： 使用嵌套结果 ResultMap进行映射
"Address" id="AddressResult">
<id property="addrId" column="addr_id" />
<result property="street" column="street" />
<result property="city" column="city" />
"Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
property="address" resultMap="AddressResult" />
方式3：本身内部嵌套
"Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
property="address" javaType="Address">
<id property="addrId" column="addr_id" />
<result property="street" column="street" />
<result property="city" column="city" />
<result property="state" column="state" />
<result property="zip" column="zip" />
<result property="country" column="country" />
一对多映射
使用
使用嵌套对象形式显示
public class Tutor {
private Integer tutorId;
private String name;
private String email;
private Address address;
private List courses;
/ setters & getters
}
"Tutor" id="TutorResult">
"tutor_id" property="tutorId" />
"tutor_name" property="name" />
"email" property="email" />
property="courses" resultMap="CourseResult" />
<select id="findTutorById" parameterType="int" resultMap="TutorResult">
SELECT
T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL, C.COURSE_ID, C.NAME, DESCRIPTION, START_DATE, END_DATE
FROM
TUTORS T LEFT OUTER JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID
LEFT OUTER JOIN
COURSES C ON T.TUTOR_ID=C.TUTOR_ID
WHERE
T.TUTOR_ID=#{tutorId}
select> 使用嵌套语句实现
"Tutor" id="TutorResult">
"tutor_id" property="tutorId" />
"tutor_name" property="name" />
"email" property="email" />
property="address" resultMap="AddressResult" />
property="courses" column="tutor_id" select="findCoursesByTutor" />
<select id="findTutorById" parameterType="int" resultMap="TutorResult">
SELECT
T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL
FROM TUTORS T WHERE T.TUTOR_ID=#{tutorId}
select>
<select id="findCoursesByTutor" parameterType="int" resultMap="CourseResult">
SELECT * FROM COURSES WHERE TUTOR_ID=#{tutorId}
select>- 注意：嵌套 Select 语句查询会导致 N+1 选择问题。 首先，主查询将会执行(1 次),对于主 查询返回的每一行，另外一个查询将会被执行(主查询 N 行,则此查询 N 次)。对于 大型数据库而言,这会导致很差的性能问题。
动态SQL
mybatis 提供:
if
<select id="searchCourses" parameterType="hashmap" resultMap="CourseResult">select>
SELECT * FROM COURSES WHERE TUTOR_ID= #{tutorId}
<if test="courseName != null">
AND NAME LIKE #{courseName}
if>
select>当if中test条件成立时， 才会添加if中的内容到SQL语句中。
choose, when, otherwise
<select id="searchCourses" parameterType="hashmap" resultMap="CourseResult">
SELECT * FROM COURSES
<when test="searchBy == 'Tutor'">
WHERE TUTOR_ID= #{tutorId}
when>
<when test="searchBy == 'CourseName'">
WHERE name like #{courseName}
when>
WHERE TUTOR start_date >= now()
select>mybatis计算true的子句， 如果没有条件为true，则使用
where
<select id="searchCourses" parameterType="hashmap" resultMap="CourseResult">
SELECT * FROM COURSES
<where>
<if test=" tutorId != null ">
TUTOR_ID= #{tutorId}
if>
<if test="courseName != null">
AND name like #{courseName}
if>
<if test="startDate != null">
AND start_date >= #{startDate}
if>
<if test="endDate != null">
AND end_date <= #{endDate}
if>
where>
select>trim
<select id="searchCourses" parameterType="hashmap" resultMap="CourseResult">
SELECT * FROM COURSES
"WHERE" prefixOverrides="AND | OR">
<if test=" tutorId != null ">
TUTOR_ID= #{tutorId}
if>
<if test="courseName != null">
AND name like #{courseName}
if>
select>trim标记是一个格式化的标记，可以完成set或者是where标记的功能。
prefix：前缀。
prefixoverride：去掉第一个and或者是or。
"set" suffixoverride="," suffix=" where id = #{id} ">
<if test="name != null and name.length()>0"> name=#{name} , if>
<if test="gender != null and gender.length()>0"> gender=#{gender} , if>
假如说name和gender的值都不为null的话打印的SQL为：update user set name='xx' , gender='xx' where id='x'。gender后面跟的逗号不会显示。
suffixoverride：去掉最后一个逗号（也可以是其他的标记，就像是上面前缀中的and一样）
suffix：后缀
foreach
<select id="searchCoursesByTutors" parameterType="map" resultMap="CourseResult">
SELECT * FROM COURSES
<if test="tutorIds != null">
<where>
"tutorId" collection="tutorIds">
OR tutor_id=#{tutorId}
where>
if>
select>多个参数操作
使用map形式引入
<select id="findAllStudentsByNameEmail" resultMap="StudentResult" parameterType="map">
select
stud_id, name,email, phone from Students
where
name=#{name} and email=#{email}
select>使用参数替代
<select id="findAllStudentsByNameEmail" resultMap="StudentResult">
select
stud_id, name,email, phone from Students
where
name=#{param1} and email=#{param2}
select>注解配置SQL映射器
基于注解的一些内容
INSERTUPDATESELECTDELETE
基本使用
数据插入操作
@Insert("INSERT INTO STUDENTS(NAME,EMAIL,ADDR_ID, PHONE)
VALUES(#{name},#{email},#{address.addrId},#{phone})")通过以下注解实现主键
@Options(useGeneratedKeys = true, keyProperty = "studId")通过此注解为任意SQL语句指定主键值(使用此注解生成主键)
@SelectKey(statement="SELECT STUD_ID_SEQ.NEXTVAL FROM DUAL",
keyProperty="studId", resultType=int.class, before=true)使用触发器生成主键
@SelectKey(statement="SELECT STUD_ID_SEQ.CURRVAL FROM DUAL",
keyProperty="studId", resultType=int.class, before=false)结果集映射
@Results(
{
@Result(id = true, column = "stud_id", property = "studId"),
@Result(column = "name", property = "name"),
@Result(column = "email", property = "email"),
@Result(column = "addr_id", property = "address.addrId")
})一对一映射
嵌套sql语句形式
@Results(
{
@Result(id = true, column = "stud_id", property = "studId"),
@Result(column = "name", property = "name"),
@Result(column = "email", property = "email"),
@Result(property = "address", column = "addr_id", one = @One(select = "com.mybatis3.mappers.StudentMapper.findAddressById"))
})嵌套对象
此种方式并没有注解形式的实现 ，可以通过在XML中定义映射resultMap集，然后通过@ResultMap进行映射
@ResultMap("com.mybatis3.mappers.StudentMapper.StudentWithAddressResult")一对多映射
同样的只有嵌套SQL的形式
@Result(property = "courses", column = "tutor_id", many = @Many(select = "com.mybatis3.mappers.TutorMapper.findCoursesByTutorId"))