MyBatis 实践 -动态SQL/关联查询

2021-08-14 01:49:03

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
@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);
}

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>
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
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
@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")));
}

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
    MyBatis 实践 -动态SQL/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:
      1. user/order:
        • user -> order: 一对多
        • order -> user: 一对一
      2. order/orderdetail:
        • order -> orderdetail:一对多
        • orderdetail -> order:一对一
      3. 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 
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>
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
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>
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)
<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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