MyBatis 实践 -动态SQL/关联查询
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MyBatis 实践
标签: Java与存储
动态SQL
动态SQL提供了对SQL语句的灵活操作,通过表达式进行判断,对SQL进行拼接/组装.
if
对查询条件进行判断,如果输入参数不为空才进行查询条件的拼接.
- mapper
<select id="selectUser" resultType="com.fq.domain.User" parameterType="com.fq.domain.User"> SELECT * FROM user <where> <if test="id != null"> AND id = #{id} </if> <if test="name != null"> AND name = #{name} </if> <if test="password != null"> AND password = #{password} </if> </where></select>
<where/>
会自动处理第一个AND
(MyBatis还提供了自定义<where/>
行为的<trim/>
元素, 详细可参考MyBatis文档).
- UserDAO
List<User> selectUser(User user) throws Exception;
- Client
@Testpublic void selectUserClient() throws Exception { UserDAO dao = session.getMapper(UserDAO.class); User user = dao.selectUser(new User(null, null, "new_password")); System.out.println(user);}
由于id与name为
null
, 因此这两个条件不会拼接在SQL中,这一点可以调试时日志中看出.
choose/when/otherwise
有些时候,我们并不想用到所有的条件语句,而只想从中选择一二.针对这种情况,MyBatis提供了<choose/>
元素,他有点像Java中的switch
.
<select id="selectUser" resultType="com.fq.domain.User" parameterType="com.fq.domain.User"> SELECT * FROM user <where> <choose> <when test="id != null"> AND id = #{id} </when> <when test="name != null"> AND name = #{name} </when> <otherwise> AND password = #{password} </otherwise> </choose> </where></select>
set
用于动态更新语句的解决方案为<set/>
,set元素可以被用于动态包含需要更新的列, 而舍去其他的.
<update id="updateUserById" parameterType="com.fq.domain.User"> UPDATE user <set> <if test="name != null"> name = #{name} , </if> <if test="password != null"> password = #{password} , </if> </set> WHERE id = #{id};</update>
foreach
使用foreach可以实现向SQL中传递数组或List
:
传入List
查询多个id的用户信息, 可以由下面两种SQL实现:
SELECT * FROM user WHERE (id = ? OR id = ? OR id = ?);SELECT * FROM user WHERE id IN (?, ?, ?, ?);
因此其foreach的定义也有如下两种方案:
<select id="selectUser" parameterType="java.util.List" resultType="com.fq.domain.User"> SELECT * FROM user <where> <if test="list != null and list.size >= 1"> <foreach collection="list" item="id" open="(" separator="or" close=")"> id = #{id} </foreach> </if> </where></select>
<select id="selectUser" parameterType="java.util.List" resultType="com.fq.domain.User"> SELECT * FROM user <where> <if test="list != null and list.size >= 1"> <foreach collection="list" item="id" open="id IN (" separator="," close=")"> #{id} </foreach> </if> </where></select>
collection
SQL解析的参数名 index
循环下标 item
单个元素的名 open
循环开始输出 close
循环结束输出 separator
中间分隔输出传递
List
作为parameterType
时,SQL解析参数名固定为list
.
- UserDAO
List<User> selectUser(List<Integer> ids) throws Exception;
批量插入用户案例
- mapper
<insert id="insertUserList" parameterType="java.util.List"> INSERT INTO user(name, password) VALUES <if test="list != null and list.size != 0"> <foreach collection="list" item="user" separator=","> (#{user.name}, #{user.password}) </foreach> </if></insert>
- UserDAO
void insertUserList(List<User> users) throws Exception;
- Client
@Testpublic void insertUserListClient() throws Exception { UserDAO dao = session.getMapper(UserDAO.class); dao.insertUserList(Lists.newArrayList(new User(null, "mojia5", "mojia5"), new User(null, "mojia6", "mojia6"), new User(null, "mojia7", "mojia7")));}
传入数组
- mapper
<select id="selectUser" parameterType="Object[]" resultType="com.fq.domain.User"> SELECT * FROM user <where> <if test="array != null and array.length >= 1"> <foreach collection="array" item="id" open="id IN (" separator="," close=")"> #{id} </foreach> </if> </where></select>
与
List
类似,传递数组作为parameterType
时,SQL解析参数名固定为array
.
- UserDAO
List<User> selectUser(Integer[] ids) throws Exception;
SQL片段
可以将一段公共的SQL语句抽取出来, 作为一个SQL片段, 供其他SQL调用:
<sql id="user_where"> <if test="id != null"> AND id = #{id} </if> <if test="name != null"> AND name = #{name} </if> <if test="password != null"> AND password = #{password} </if></sql><select id="selectUser" resultType="com.fq.domain.User" parameterType="com.fq.domain.User"> SELECT * FROM user <where> <include refid="user_where"/> </where></select>
经验:最好基于单表定义SQL片段,而且在SQL片段中不要包含
<where>
/<set>
之类的标签,这样可以保证SQL片段重用度更高.
关联查询
数据模型分析思路
- 每张表的数据内容:分模块对每张表记录的内容进行熟悉,相当于学习系统需求/功能.
- 每张表重要的字段:非空字段/外键字段等.
- 表与表之间的数据库级别关系: 外键关系.
- 表与表之间的业务关系:建立在某个业务的基础上去分析.
订单/商品数据模型
- 表内容
- user: 购买商品的用户信息
- order: 用户创建的订单
- orderdetail: 订单详细(购买商品信息)
- item: 商品信息
- 表与表之间的业务关系:
- user/order:
- user -> order: 一对多
- order -> user: 一对一
- order/orderdetail:
- order -> orderdetail:一对多
- orderdetail -> order:一对一
- orderdetail/item:
- orderdetail -> item:一对一
- item -> orderdetail:一对多
- user/order:
- 表内容
‘一对一’查询
需求: 查询订单信息,关联查询(创建订单的)用户信息.
由以上分析可知主查询为order
表,而order -> user
关系为一对一, 因此使用resultMap
将查询结果的订单信息映射到Order
中,将用户信息映射到Order
中的User
属性.
- PO: 改造
User
public class User implements Serializable { private Integer id; private String username; private Date birthday; private Integer sex; private String address; // ...}
- PO: 新增
Order
, 将User
组合到Order
中:
public class Order implements Serializable { private Integer id; private Integer userId; private String number; private Date createTime; private String note; private User user; // ...}
- mapper
<?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.fq.mybatis.OrderDAO"> <cache type="org.mybatis.caches.ehcache.EhcacheCache"/> <resultMap id="order_user_map" type="com.fq.domain.Order"> <id column="id" property="id"/> <result column="user_id" property="userId"/> <result column="number" property="number"/> <result column="create_time" property="createTime"/> <result column="note" property="note"/> <association property="user" javaType="com.fq.domain.User"> <id column="user_id" property="id"/> <result column="username" property="username"/> <result column="birthday" property="birthday"/> <result column="sex" property="sex"/> <result column="address" property="address"/> </association> </resultMap> <select id="selectOrderWithUser" resultMap="order_user_map"> SELECT `order`.*, username, birthday, sex, address FROM `order`, user WHERE `order`.user_id = user.id AND `order`.id = #{0}; </select></mapper>
association
: 映射关联查询的单条记录(将关联查询信息映射到PO对象属性).
- OrderDAO
public interface OrderDAO { Order selectOrderWithUser(Integer id) throws Exception;}
- Client
@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")public class OrderDAOClient { @Autowired private OrderDAO dao; @Test public void client() throws Exception { Order order = dao.selectOrderWithUser(3); System.out.println(order); }}
‘一对多’查询
需求: 查询订单及订单明细的信息(一对多).
- PO: 定义
OrderDetail
,并在Order
中添加List<OrderDetail> orderDetails
订单明细属性:
public class OrderDetail implements Serializable { private Integer id; private Integer orderId; private Integer itemId; private Integer itemNumber; // ...}
- mapper
<resultMap id="order_user_detail_map" type="com.fq.domain.Order" extends="order_user_map"> <collection property="orderDetails" ofType="com.fq.domain.OrderDetail"> <id column="order_detail_id" property="id"/> <result column="item_id" property="itemId"/> <result column="item_num" property="itemNumber"/> <result column="order_id" property="orderId"/> </collection></resultMap><select id="selectOrderWithDetail" resultMap="order_user_detail_map"> SELECT `order`.*, username, birthday, sex, address, orderdetail.id order_detail_id, item_id, item_num, order_id FROM `order`, user, orderdetail WHERE `order`.user_id = user.id AND `order`.id = orderdetail.order_id AND `order`.id = #{0};</select>
property
指定关联查询的结果集存储到的属性 ofType
指定关联查询结果集中的对象类型- OrderDAO
Order selectOrderWithDetail(Integer id) throws Exception;
‘多对多’查询
需求: 查询用户及用户购买商品信息.
由于User表与Item表没有直接关联,因此只能通过Order表与OrderDetail表进行关联.
思路:
1) 将用户信息映射到User
中.
2) 在User
中添加List<Order>
订单列表属性,将用户创建的订单映射到orders
.
3) 在Order
中添加List<OrderDetail>
订单明细列表属性,将订单的明细映射到orderDetails
.
4) 在OrderDetail
中添加Item
属性,将订单明细所对应的商品映射到item
.PO: Item
public class Item { private Integer id; private String name; private Float price; private String detail; private String pic; private Date createTime; //...}
- mapper
<resultMap id="user_item_map" type="com.fq.domain.User"> <id column="id" property="id"/> <result column="username" property="username"/> <result column="birthday" property="birthday"/> <result column="sex" property="sex"/> <result column="address" property="address"/> <collection property="orders" ofType="com.fq.domain.Order"> <id column="order_id" property="id"/> <result column="id" property="userId"/> <result column="order_create_time" property="createTime"/> <result column="order_note" property="note"/> <result column="order_number" property="number"/> <collection property="orderDetails" ofType="com.fq.domain.OrderDetail"> <id column="order_detail_id" property="id"/> <result column="order_id" property="orderId"/> <result column="item_id" property="itemId"/> <result column="order_item_num" property="itemNumber"/> <association property="item" javaType="com.fq.domain.Item"> <id column="item_id" property="id"/> <result column="item_create_time" property="createTime"/> <result column="item_detail" property="detail"/> <result column="item_name" property="name"/> <result column="item_price" property="price"/> <result column="item_pic" property="pic"/> </association> </collection> </collection></resultMap><select id="selectUserItem" resultMap="user_item_map"> SELECT user.*, `order`.id order_id, `order`.create_time order_create_time, `order`.note order_note, `order`.number order_number, orderdetail.id order_detail_id, orderdetail.item_num order_item_num, item.id item_id, item.create_time item_create_time, item.detail item_detail, item.name item_name, item.price item_price, item.pic item_pic FROM user, item, `order`, orderdetail WHERE `order`.user_id = user.id AND orderdetail.order_id = `order`.id AND orderdetail.item_id = item.id ;</select>
- OrderDAO
List<User> selectUserItem() throws Exception;
resultMap小结:
使用<association/>
和<collection/>
可以完成一对一和一对多的高级映射.
- association: 将关联查询信息映射到一个PO对象中.
- collection: 将关联查询信息映射到一个集合中.
延迟加载
关联查询时,使用MyBatis 延迟加载 特性可有效减轻数据库压力.首次查询只查询主表信息,等需要时再去查询关联表信息.<resultMap/>
的<association/>
/<collection/>
具备延迟加载功能.
需求: 查询订单信息并关联查询用户信息.
延迟加载开关
在MyBatis核心配置文件(mybatis-configuration.xml)中配置:
1)lazyLoadingEnabled
: 设置是否懒加载.默认false
,则所有关联查询都会被初始化加载.
2)aggressiveLazyLoading
: 设置是否积极加载. 默认true
,所有关联属性被初始化加载.Settings配置:
<settings> <setting name="cacheEnabled" value="true"/> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/></settings>
Mapper
- 只查询订单信息
<resultMap id="order_user_map" type="com.fq.domain.Order"> <id column="id" property="id"/> <result column="user_id" property="userId"/> <result column="number" property="number"/> <result column="create_time" property="createTime"/> <result column="note" property="note"/> <association property="user" javaType="com.fq.domain.User" select="com.fq.mybatis.UserDAO.selectUserById" column="user_id"/></resultMap><select id="selectOrderWithUser" resultMap="order_user_map"> SELECT * FROM `order`;</select>
select
指定关联查询Statement为com.fq.mybatis.UserDAO.selectUserById
. column
指定关联查询时将users_id
值传入selectUserById
.- 关联查询用户信息(namespace为
com.fq.mybatis.UserDAO
)
<select id="selectUserById" parameterType="java.lang.Integer" resultType="com.fq.domain.User"> SELECT * FROM user WHERE id = #{id};</select>
将上面查询到的订单信息中的
user_id
传入selectUserById
来关联查询用户信息.
- OrderDAO(同前)
List<Order> selectOrderWithUser() throws Exception;
- Client
@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")public class OrderDAOClient { @Autowired private OrderDAO dao; @Test public void client() throws Exception { List<Order> orders = dao.selectOrderWithUser(); for (Order order : orders) { System.out.println(order.getUser()); } }}
debug上面
Client
, 观察log信息,会发现只有当确实需要User
信息时才会调用selectUserById
.
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