一對多查詢

實體類

package com.zucc.model;


import java.util.List;


public class District{
    private Integer did;


    private String dname;
    
    private List<Street> streets;


    public Integer getDid() {
        return did;
    }


    public void setDid(Integer did) {
        this.did = did;
    }


    public String getDname() {
        return dname;
    }


    public void setDname(String dname) {
        this.dname = dname;
    }


<span style="white-space:pre">	</span>public List<Street> getStreets() {
<span style="white-space:pre">		</span>return streets;
<span style="white-space:pre">	</span>}


<span style="white-space:pre">	</span>public void setStreets(List<Street> streets) {
<span style="white-space:pre">		</span>this.streets = streets;
<span style="white-space:pre">	</span>}
    
}
 

<resultMap type="com.zucc.model.District" id="districtResultMap">  
        <id column="did" property="id"/>  
        <result column="dname" property="name"/>  
        <collection property="streets" ofType="cn.wd.entity.Street" column="district_id">  
            <id column="sid" property="id"/>  
            <result column="sname" property="name"/>  
        </collection>  
</resultMap>  	

<select id="findAllDistrictById" parameterType="int" resultMap="districtResultMap" >  
        select d.*,s.* from district d,street s where s.district_id=d.did and d.did=#{id} 
    </select>
 

多對一

實體類

package com.zucc.model;

public class Street {
    private Integer sid;

    private String sname;

    private Integer disId;
    
    private District district;

    public Integer getSid() {
        return sid;
    }

    public void setSid(Integer sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public Integer getDisId() {
        return disId;
    }

    public void setDisId(Integer disId) {
        this.disId = disId;
    }

	public District getDistrict() {
		return district;
	}

	public void setDistrict(District district) {
		this.district = district;
	}
    
    
}
 

配置檔案

<resultMap type="com.zucc.model.Street" id="StreetResultMap">  
        <id column="sid" property="sid"/>  
        <result column="sname" property="sname"/>  
        <association property="district" javaType="com.zucc.model.District">  
            <id column="did" property="did"/>  
            <result column="dname" property="dname"/>  
        </association>  
    </resultMap>  

<select id="findStreetById" parameterType="int" resultMap="StreetResultMap">  
        select d.*,s.* from district d,street s where d.did=s.district_id and s.sid=#{id}   
    </select>  

總結：

在MyBatis進行查詢對映的時候，其實查詢出來的每一個屬性都是放在一個對應的Map裡面的，其中鍵是屬性名，值則是其對應的值。

使用association和collection完成一對一和一對多高階對映。

association:將關聯查詢資訊對映到一個pojo物件中。

collection:將關聯對映資訊對映到一個集合中。

延遲載入

延遲載入（Lazy Load）實現的功能很好理解，就是在資料與物件進行Mapping操作時，只有當真正使用該物件時，才進行Mapping操作，以減少不必要的資料庫查詢開銷,從而提升了程式的效率。

首先進行配置

<!-- 全域性配置引數，需要時再設定 -->
	<settings>
		<!-- 開啟延遲載入的開關 -->
		<setting name="lazyLoadingEnabled" value="true" />
		<!-- 將積極載入改為消極載入即按需載入 -->
		<setting name="aggressiveLazyLoading" value="false" />
	</settings>

<resultMap type="com.zucc.model.Street" id="StreetResultMapLazyLoadingMap">  
        <id column="sid" property="id"/>  
        <result column="sname" property="name"/>  
        <association property="district" javaType="com.zucc.model.District"
        select="cn.wd.dao.DistrictMapper.findAllDistrictById" column="district_id">  
        </association>  
</resultMap>  


<select id="selectStreetLazyLoading" parameterType="int" resultMap="StreetResultMapLazyLoadingMap">
    <span style="white-space:pre">	</span>select * from street where sid=#{id}
    </select>