序言
MyBatis,大家都知道,半自動的ORM框架,原來叫ibatis,後來好像是10年apache軟件基金組織把它托管給了goole code,就重新命名了MyBatis,功能相對以前更強大了。它相對全自動的持久層框架Hibernate,更加靈活,更輕量級,這點我還是深有體會的。
MyBatis的一個強大特性之一就是動態SQL能力了,能省去我們很多串聯判斷拼接SQL的痛苦,根據項目而定,在一定的場合下使用,能大大減少程序的代碼量和復雜程度,不過還是不是過度太過復雜的使用,以免不利於後期的維護和擴展。
動態SQL
動態SQL提供了對SQL語句的靈活操作,通過表達式進行判斷,對SQL進行拼接/組裝.
if
對查詢條件進行判斷,如果輸入參數不為空才進行查詢條件的拼接.
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>
會自動處理第一個AND(MyBatis還提供了自定義行為的元素, 詳細可參考MyBatis文檔).
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>
由於id與name為null, 因此這兩個條件不會拼接在SQL中,這一點可以調試時日志中看出.
choose/when/otherwise
有些時候,我們並不想用到所有的條件語句,而只想從中選擇一二.針對這種情況,MyBatis提供了元素,他有點像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
用於動態更新語句的解決方案為,set元素可以被用於動態包含需要更新的列, 而捨去其他的.
<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可以實現向SQL中傳遞數組或List:
傳入List
查詢多個id的用戶信息, 可以由下面兩種SQL實現:
<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的定義也有如下兩種方案:
<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 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解析的參數名
index 循環下標
item 單個元素的名
open 循環開始輸出
close 循環結束輸出
separator 中間分隔輸出
傳遞List作為parameterType時,SQL解析參數名固定為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>
批量插入用戶案例
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>
傳入數組
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>
與List類似,傳遞數組作為parameterType時,SQL解析參數名固定為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
可以將一段公共的SQL語句抽取出來, 作為一個SQL片段, 供其他SQL調用:
<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>
經驗:最好基於單表定義SQL片段,而且在SQL片段中不要包含/之類的標簽,這樣可以保證SQL片段重用度更高.
關聯查詢
數據模型分析思路
每張表的數據內容:分模塊對每張表記錄的內容進行熟悉,相當於學習系統需求/功能. 每張表重要的字段:非空字段/外鍵字段等. 表與表之間的數據庫級別關系: 外鍵關系. 表與表之間的業務關系:建立在某個業務的基礎上去分析.
訂單/商品數據模型
表內容
user: 購買商品的用戶信息 order: 用戶創建的訂單 orderdetail: 訂單詳細(購買商品信息) item: 商品信息 表與表之間的業務關系:
user/order:
user -> order: 一對多 order -> user: 一對一 order/orderdetail:
order -> orderdetail:一對多 orderdetail -> order:一對一 orderdetail/item:
orderdetail -> item:一對一 item -> orderdetail:一對多
‘一對一'查詢
需求: 查詢訂單信息,關聯查詢(創建訂單的)用戶信息.
由以上分析可知主查詢為order表,而order -> user關系為一對一, 因此使用resultMap將查詢結果的訂單信息映射到Order中,將用戶信息映射到Order中的User屬性.
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, 將User組合到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: 映射關聯查詢的單條記錄(將關聯查詢信息映射到PO對象屬性).
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>
‘一對多'查詢
需求: 查詢訂單及訂單明細的信息(一對多).
PO: 定義OrderDetail,並在Order中添加List orderDetails訂單明細屬性:
<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 指定關聯查詢的結果集存儲到的屬性
ofType 指定關聯查詢結果集中的對象類型OrderDAO
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica">Order selectOrderWithDetail(Integer id) throws Exception;</code></code></code>
‘多對多'查詢
需求: 查詢用戶及用戶購買商品信息.
由於User表與Item表沒有直接關聯,因此只能通過Order表與OrderDetail表進行關聯.
思路:
1) 將用戶信息映射到User中.
2) 在User中添加List訂單列表屬性,將用戶創建的訂單映射到orders.
3) 在Order中添加List訂單明細列表屬性,將訂單的明細映射到orderDetails.
4) 在OrderDetail中添加Item屬性,將訂單明細所對應的商品映射到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小結:
使用和可以完成一對一和一對多的高級映射.
association: 將關聯查詢信息映射到一個PO對象中. collection: 將關聯查詢信息映射到一個集合中.
延遲加載
關聯查詢時,使用MyBatis 延遲加載 特性可有效減輕數據庫壓力.首次查詢只查詢主表信息,等需要時再去查詢關聯表信息.的/具備延遲加載功能.
需求: 查詢訂單信息並關聯查詢用戶信息.
延遲加載開關
在MyBatis核心配置文件(mybatis-configuration.xml)中配置:
1) lazyLoadingEnabled : 設置是否懶加載.默認false,則所有關聯查詢都會被初始化加載.
2) aggressiveLazyLoading : 設置是否積極加載. 默認true,所有關聯屬性被初始化加載.
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
只查詢訂單信息
<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 指定關聯查詢Statement為com.fq.mybatis.UserDAO.selectUserById.
column 指定關聯查詢時將users_id值傳入selectUserById.關聯查詢用戶信息(namespace為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>
將上面查詢到的訂單信息中的user_id傳入selectUserById來關聯查詢用戶信息.
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, 觀察log信息,會發現只有當確實需要User信息時才會調用selectUserById.
以上所述是小編給大家介紹的mybatis實踐之動態sql及關聯查詢,希望對大家以上幫助!