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动态SQL1

發布時間：2025/3/20 数据库 14 豆豆

生活随笔收集整理的這篇文章主要介紹了动态SQL1 小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

If標簽

動態SQL可以說是MyBatis最強大之處了，這塊的應用主要有四個方面if,choose,trim和foreach，接下來先說說if。

顧名思義，if是用來判斷條件的，現在假設我們有個需求，原先我們查詢員工是靠id查詢的，現在提出新的要求，要求用戶輸入攜帶了哪個字段查詢條件就帶上這個字段的值，

List<Employee>?getEmpsByConditionIf(Employee?e);??

那么這個sql語句該怎么寫呢？

<select?id="EmployeeMapperDynamicTest"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee?where?id?=?#{id}?and?last_name?like?#{lastName}??

????and?email?=?#{email}?and?gender=#{gender}??

</select>?

這種寫法顯然是有問題的，如果傳入的Employee中lastName為空，這個select語句就查不出我們想要的結果，還有很多類似的情況，此時，就要使用到if，<if test=""></if>，test屬性中采用的是OGNL表達式，與EL表達式和你像，大家可以去官網查找具體的內容[http://commons.apache.org/proper/commons-ognl/download_ognl.cgi]

在書寫test屬性中的OGNL時要注意遇到特殊符號應該寫轉移字符，比如&要寫成&;

具體的對應關系可以查找W3C的手冊:

[https://www.w3cschool.cn/htmltags/ref-entities.html]

<select?id="getEmpsByConditionIf"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee??

????<if?test="id!=null">??

????????id?=?#{id}??

????</if>??

????<if?test="lastName!=null?and?lastName!=""">??

????????and?last_name?like?#{lastName}??

????</if>??

????<if?test="email!=null?and?email.trim()!=""">??

????????and?email?=?#{email}??

????</if>??

????<if?test="gender==0?or?gender==1">??

????????gender=#{gender}??

????</if>??

</select>

注意兩點1.email判斷時用到了email.trim() 2.gender原本是字符串，可此處將它當做是是數字,OGNL會進行字符串與數字的轉化判斷.

????做一下測試：

public?static?SqlSessionFactory?getSqlSessionFactory()?throws?IOException?{??

????String?resource?=?"mybatis-conf.xml";??

????InputStream?inputStream?=?Resources.getResourceAsStream(resource);??

????return?new?SqlSessionFactoryBuilder().build(inputStream);??

}??

@Test??

public?void?testGetEmpsByConditionIf()?throws?IOException?{??

????SqlSessionFactory?sqlSessionFactory?=?getSqlSessionFactory();??

????SqlSession?openSession?=?sqlSessionFactory.openSession();??

????try?{??

????????EmployeeMapperDynamic?mapper?=?openSession.getMapper(EmployeeMapperDynamic.class);??

????????Employee?e?=?new?Employee();??

????????e.setId(5);??

????????e.setLastName("%青%");??

????????List<Employee>?employeeList?=?mapper.getEmpsByConditionIf(e);??

????????for?(Employee?employee:employeeList){??

????????????System.out.println(employee);??

????????}??

????}finally?{??

????????openSession.close();??

????}??

}??

結果：Employee{id=5, lastName='吳青寧', gender='1', email='wq', department=null}

接著，再試試創建的Employee對象，只給lastName一個屬性賦值的情況

@Test??

public?void?testGetEmpsByConditionIf()?throws?IOException?{??

????SqlSessionFactory?sqlSessionFactory?=?getSqlSessionFactory();??

????SqlSession?openSession?=?sqlSessionFactory.openSession();??

????try?{??

????????EmployeeMapperDynamic?mapper?=?openSession.getMapper(EmployeeMapperDynamic.class);??

????????Employee?e?=?new?Employee();??

????????e.setLastName("%青%");??

????????List<Employee>?employeeList?=?mapper.getEmpsByConditionIf(e);??

????????for?(Employee?employee:employeeList){??

????????????System.out.println(employee);??

????????}??

????}finally?{??

????????openSession.close();??

????}??

}??

結果：

????報錯！！！

????分析一下，為什么？如果傳入的Employee對象中沒有id值，但是有lastName時，SQL語句會變成下面這個樣子：

????select * from tb_employee and last_name like #{lastName}

顯然是個有問題的查詢語句，我們該如何解決這個問題？

????我們可以將<select>查詢中添加1=1，在之后的每個屬性前都加上and XXXXXXX，就像下面這樣：

<select?id="getEmpsByConditionIf"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee?where?1=1??

????<if?test="id!=null">??

????????and?id?=?#{id}??

????</if>??

????<if?test="lastName!=null?and?lastName!=""">??

????????and?last_name?like?#{lastName}??

????</if>??

????<if?test="email!=null?and?email.trim()!=""">??

????????and?email?=?#{email}??

????</if>??

????<if?test="gender==0?or?gender==1">??

????????gender=#{gender}??

????</if>??

</select>??

這顯然沒有問題，另外一種方法，MyBatis的<where>標簽

where標簽

????使用方法很簡單，將之前的<if>標簽都包含在<where></where>之中即可

<select?id="getEmpsByConditionIf"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee??

????<where>??

????????<if?test="id!=null">??

????????????and?id?=?#{id}??

????????</if>??

????????<if?test="lastName!=null?and?lastName!=""">??

????????????and?last_name?like?#{lastName}??

????????</if>??

????????<if?test="email!=null?and?email.trim()!=""">??

????????????and?email?=?#{email}??

????????</if>??

????????<if?test="gender==0?or?gender==1">??

????????????gender=#{gender}??

????????</if>??

????</where>??

</select>?

DEBUG [main] - ==> Preparing: select * from tb_employee WHERE last_name like ?

可以發現<where>能將多余的and或者or去掉，當然where也有不好使的時候，有些人習慣將and放在后面，如下:

<select?id="getEmpsByConditionIf"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee??

????<where>??

????????<if?test="id!=null">??

????????????and?id?=?#{id}?and??

????????</if>??

????????<if?test="lastName!=null?and?lastName!=""">??

????????????last_name?like?#{lastName}?and??

????????</if>??

????????<if?test="email!=null?and?email.trim()!=""">??

????????????email?=?#{email}?and??

????????</if>??

????????<if?test="gender==0?or?gender==1">??

????????????gender=#{gender}??

????????</if>??

????</where>??

</select>??

DEBUG [main] - ==> Preparing: select * from tb_employee WHERE last_name like ? and

SQL語句明顯出錯，可見<where>只能去掉頭部的and或者or

trim標簽

????如果我就想把and 放在后面，要怎么辦？這是我們該用<trim>標簽了，<trim>可以自定義截取標簽

????<trim>標簽包含4個屬性:

前綴：prefix

前綴覆蓋：prefixOverrides

后綴：suffix

后綴覆蓋：suffixOverrides

trim標簽體中是整個字符串拼串后的結果，prefix給拼串后的整個字符串加一個前綴，比如我們可以不在SQL語句中寫where而選擇在<trim>中寫：<trim prefix="where">……

????前綴覆蓋指的是去掉拼串前面多余的字符

后綴和后綴覆蓋同理，那么我們在后面添加and就有可能出現后面多出and的情況，因此我們只要指定后綴覆蓋的屬性值為and，就可以處理上述情況：

<select?id="getEmpsByConditionTrim"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee??

????<trim?prefix="where"?suffixOverrides="and">??

????????<if?test="id!=null">??

?????????????id?=?#{id}?and??

????????</if>??

????????<if?test="lastName!=null?and?lastName!=""">??

?????????????last_name?like?#{lastName}?and??

????????</if>??

????????<if?test="email!=null?and?email.trim()!=""">??

?????????????email?=?#{email}?and??

????????</if>??

????????<if?test="gender==0?or?gender==1">??

?????????????gender=#{gender}??

????????</if>??

????</trim>??

</select>??

DEBUG [main] - ==> Preparing: select * from tb_employee where last_name like ?

????雖然看似功能強大的<trim>其實并不常用，大家作為了解即可

choose標簽

????這是一個分支選擇標簽，類似于Java中的switch-case，之前我們在<if>標簽中是逐一判斷哪個條件是合法存在的，然后使用該標簽，接下來我們換一種要求，已知傳入的Employee對象只會有一種屬性是存在值的，那么我們就可以使用choose，當然也可以用if

????List<Employee> getEmpsByConditionChoose(Employee e);

<select?id="getEmpsByConditionChoose"?resultType="com.figsprite.bean.Employee">??

????select?*?from?tb_employee??

????<where>??

????????<choose>??

????????????<when?test="id!=null">??

????????????????id=#{id}??

????????????</when>??

????????????<when?test="lastName!=null?and?lastName!=""">??

????????????????last_name?like?#{lastName}??

????????????</when>??

????????????<when?test="email!=null?and?email.trim()!=""">??

????????????????email?=?#{email}??

????????????</when>??

????????????<otherwise>??

????????????????1=1??

????????????</otherwise>??

????????</choose>??

????</where>??

</select>??

(其中test屬性中的""要改成")

????<otherwise>標簽，就是當前面所有的when條件都沒中的時候，拼接上這個標簽里的語句

@Test??

public?void?testGetEmpsByConditionIf()?throws?IOException?{??

????SqlSessionFactory?sqlSessionFactory?=?getSqlSessionFactory();??

????SqlSession?openSession?=?sqlSessionFactory.openSession();??

????try?{??

????????EmployeeMapperDynamic?mapper?=?openSession.getMapper(EmployeeMapperDynamic.class);??

????????Employee?e?=?new?Employee();??

????????e.setLastName("%青%");??

????????List<Employee>?employeeList?=?mapper.getEmpsByConditionChoose(e);??

????????for?(Employee?employee:employeeList){??

????????????System.out.println(employee);??

????????}??

????}finally?{??

????????openSession.close();??

????}??

DEBUG [main] - ==> Preparing: select * from tb_employee WHERE last_name like ?

如果傳入的對象同時包含id和lastName的話會查誰？顯然是只會用id作為條件

轉載于:https://www.cnblogs.com/figsprite/p/10745050.html

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