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 user
AND 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">@Test
public 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 user
AND 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 = "htmlcode">
<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 user
id = #{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">@Test
public 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

MyBatis实践之动态SQL及关联查询

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,
address
FROM `order`, user
WHERE `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 {
@Autowired
private OrderDAO dao;
@Test
public 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_id
FROM `order`, user, orderdetail
WHERE `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">
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></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 user
WHERE 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 {
@Autowired
private OrderDAO dao;
@Test
public 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.

以上所述是小编给大家介绍的mybatis实践之动态sql及关联查询,希望对大家以上帮助!

标签:
mybatis动态sql,sql关联查询

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稳了!魔兽国服回归的3条重磅消息!官宣时间再确认!

昨天有一位朋友在大神群里分享,自己亚服账号被封号之后居然弹出了国服的封号信息对话框。

这里面让他访问的是一个国服的战网网址,com.cn和后面的zh都非常明白地表明这就是国服战网。

而他在复制这个网址并且进行登录之后,确实是网易的网址,也就是我们熟悉的停服之后国服发布的暴雪游戏产品运营到期开放退款的说明。这是一件比较奇怪的事情,因为以前都没有出现这样的情况,现在突然提示跳转到国服战网的网址,是不是说明了简体中文客户端已经开始进行更新了呢?