小编给大家分享一下MyBatis中动态SQL及关联查询怎么实现，希望大家阅读完这篇文章之后都有所收获，下面让我们一起去探讨吧！
序言
MyBatis,大家都知道，半自动的ORM框架，原来叫ibatis，后来好像是10年apache软件基金组织把它托管给了goole code，就重新命名了MyBatis，功能相对以前更强大了。它相对全自动的持久层框架Hibernate，更加灵活，更轻量级，这点我还是深有体会的。
MyBatis的一个强大特性之一就是动态SQL能力了，能省去我们很多串联判断拼接SQL的痛苦，根据项目而定，在一定的场合下使用，能大大减少程序的代码量和复杂程度，不过还是不是过度太过复杂的使用，以免不利于后期的维护和扩展。
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
<code class=" hljs vbnet"><select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">SELECT * FROM userAND id = #{id}AND name = #{name}AND password = #{password}</select></code>会自动处理第一个AND(MyBatis还提供了自定义行为的元素, 详细可参考MyBatis文档).
UserDAO
<code class="hljs vbnet"><code class="hljs php">List<user> selectUser(User user) throws Exception;</user></code></code>Client
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java">@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);}</code></code></code>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提供了元素,他有点像Java中的switch.
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">SELECT * FROM userAND id = #{id}AND name = #{name}AND password = #{password}</select></code></code></code></code>set
用于动态更新语句的解决方案为,set元素可以被用于动态包含需要更新的列, 而舍去其他的.
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><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></code></code></code></code></code>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:
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql">SELECT * FROM user WHERE (id = ? OR id = ? OR id = ?);SELECT * FROM user WHERE id IN (?, ?, ?, ?);</code></code></code></code></code></code>因此其foreach的定义也有如下两种方案:
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><select id="selectUser" parametertype="java.util.List" resulttype="com.fq.domain.User">SELECT *FROM userid = #{id}</select></code></code></code></code></code></code></code><code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><select id="selectUser" parametertype="java.util.List" resulttype="com.fq.domain.User">SELECT *FROM user#{id}</select></code></code></code></code></code></code></code></code>元素 描述
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
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica">List<user> selectUser(List<integer> ids) throws Exception;</integer></user></code></code></code></code></code></code></code></code></code>批量插入用户案例
mapper
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><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></code></code></code></code></code></code></code></code></code></code>UserDAO
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java">void insertUserList(List<user> users) throws Exception;</user></code></code></code></code></code></code></code></code></code></code></code>Client
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java">@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")));}</code></code></code></code></code></code></code></code></code></code></code></code>传入数组
mapper
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><select id="selectUser" parametertype="Object[]" resulttype="com.fq.domain.User">SELECT *FROM user#{id}</select></code></code></code></code></code></code></code></code></code></code></code></code></code>与List类似,传递数组作为parameterType时,SQL解析参数名固定为array.
UserDAO
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica">List<user> selectUser(Integer[] ids) throws Exception;</user></code></code></code></code></code></code></code></code></code></code></code></code></code></code>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:
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs vbnet"><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</select></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>经验:最好基于单表定义SQL片段,而且在SQL片段中不要包含/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
‘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
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java">public class User implements Serializable {private Integer id;private String username;private Date birthday;private Integer sex;private String address;// ..}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>PO: 新增Order, 将User组合到Order中:
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java">public class Order implements Serializable {private Integer id;private Integer userId;private String number;private Date createTime;private String note;private User user;// ...}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>mapper
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs vbnet"><code class=" hljs java"><code class=" hljs java"><code class=" hljs xml"><!--{cke_protected}{C}%3C!%2D%2D%3Fxml%20version%3D%221.0%22%20encoding%3D%22UTF-8%22%20%3F%2D%2D%3E--><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"></result></result></result></result></id></association></result></result></result></result></id></resultmap><select id="selectOrderWithUser" resultmap="order_user_map">SELECT`order`.*,username,birthday,sex,addressFROM `order`, userWHERE `order`.user_id = user.id AND `order`.id = #{0};</select></cache></mapper></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>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
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java"><code class="hljs xml"><code class="hljs cs">public interface OrderDAO {Order selectOrderWithUser(Integer id) throws Exception;}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>Client
<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java"><code class="hljs xml"><code class="hljs cs"><code class="hljs java">@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")public class OrderDAOClient {@Autowiredprivate OrderDAO dao;@Testpublic void client() throws Exception {Order order = dao.selectOrderWithUser(3);System.out.println(order);}}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>‘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 orderDetails订单明细属性:
<code class="hljs java">public class OrderDetail implements Serializable {private Integer id;private Integer orderId;private Integer itemId;private Integer itemNumber;// ...}</code>mapper
<code class=" hljs java"><code class=" hljs vbnet"><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"></result></result></result></id></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_idFROM `order`, user, orderdetailWHERE `order`.user_id = user.id AND `order`.id = orderdetail.order_id AND `order`.id = #{0};</select></code></code>元素 描述
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
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica">Order selectOrderWithDetail(Integer id) throws Exception;</code></code></code>‘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
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs">public class Item {private Integer id;private String name;private Float price;private String detail;private String pic;private Date createTime;//...}</code></code></code></code>mapper
<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><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"></result></result></result></result></result></id></association></result></result></result></id></collection></result></result></result></result></id></collection></result></result></result></result></id></resultmap><select id="selectUserItem" resultmap="user_item_map">SELECTuser.*,`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_picFROM user, item, `order`, orderdetailWHERE `order`.user_id = user.id AND orderdetail.order_id = `order`.id AND orderdetail.item_id = item.id ;</select></code></code></code></code></code>OrderDAO
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs"><code class="hljs applescript"><code class="hljs php">List<user> selectUserItem() throws Exception;</user></code></code></code></code></code></code>resultMap小结:
使用和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.的/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:
<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php"><code class=" hljs xml"><settings><setting name="cacheEnabled" value="true"><setting name="lazyLoadingEnabled" value="true"><setting name="aggressiveLazyLoading" value="false"></setting></setting></setting></settings></code></code></code></code></code></code></code>Mapper
只查询订单信息
<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php"><code class=" hljs xml"><code class=" hljs applescript"><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"></association></result></result></result></result></id></resultmap><select id="selectOrderWithUser" resultmap="order_user_map">SELECT *FROM `order`;</select></code></code></code></code></code></code></code></code>元素 描述
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)
<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php"><code class=" hljs xml"><code class=" hljs applescript"><code class=" hljs vbnet"><select id="selectUserById" parametertype="java.lang.Integer" resulttype="com.fq.domain.User">SELECT *FROM userWHERE id = #{id};</select></code></code></code></code></code></code></code></code></code>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)
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs"><code class="hljs applescript"><code class="hljs php"><code class="hljs xml"><code class="hljs applescript"><code class="hljs vbnet"><code class="hljs mathematica">List<order> selectOrderWithUser() throws Exception;</order></code></code></code></code></code></code></code></code></code></code>Client
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs"><code class="hljs applescript"><code class="hljs php"><code class="hljs xml"><code class="hljs applescript"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs java">@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")public class OrderDAOClient {@Autowiredprivate OrderDAO dao;@Testpublic void client() throws Exception {List<order> orders = dao.selectOrderWithUser();for (Order order : orders) {System.out.println(order.getUser());}}}</order></code></code></code></code></code></code></code></code></code></code></code>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.
看完了这篇文章，相信你对“MyBatis中动态SQL及关联查询怎么实现”有了一定的了解，如果想了解更多相关知识，欢迎关注亿速云行业资讯频道，感谢各位的阅读！
