为啥这个表我这样写sql不能查出姓名只有两个汉字的学生信息

Posted 2023-05-10

-- 查询姓名为两个汉字的记录
SELECT * FROM stu_student WHERE stu_name LIKE '__';

几个下划线就是几个汉字，这是字符通配符，也可以判断由几个字母组成

参考技术A 你这种写法查出来的是姓名等于“--”的人，从你的表里看是没有的。
你应该这样写where length(s.name) = 4追问

不是一个－可以代表一个字吗

不管怎么查，查出来都为空

本回答被提问者和网友采纳 参考技术B 应该用substring函数
语句中where SUBSTRING(' sname',2,2)
这样应该可以追答

不对，这样错了。你的方式肯定是对的，有可能是哪里打错了吧

参考技术C select * from student where name =“??”;追问

不行

追答

把等号改成like 呢？

追问

name我改为姓名了

引号是什么状态下的，会不会是引号的影响

单引还是双引

追答

不会，你的数据库编码是utf-8吗？

如果作者写了多篇文章，为啥这个 SQL 查询将一篇文章的作者姓名返回为 NULL？

【中文标题】如果作者写了多篇文章，为啥这个 SQL 查询将一篇文章的作者姓名返回为 NULL？

【英文标题】：Why is this SQL query returning the author name for an article as NULL, if the author has written more than one article?如果作者写了多篇文章，为什么这个 SQL 查询将一篇文章的作者姓名返回为 NULL？

【发布时间】：2016-12-07 23:21:16

【问题描述】：

这是我的创作声明：

            String createFeedTable = "CREATE TABLE  rss_feed ("
                + "channel_ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1), "
                + "channelTitle VARCHAR(255), "
                + "channelLink VARCHAR(255), "
                + "channelDescription VARCHAR(255), "
                + "channelPubDate TIMESTAMP, "
                + "channelLastBuildDate TIMESTAMP,"
                + "channelIcon BLOB(1M),"
                + "PRIMARY KEY (channel_ID))";

        String createFeedItemTable = "CREATE TABLE  feed_item ("
                + "  item_ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1),"
                + "  itemTitle VARCHAR(255),"
                + "  itemLink VARCHAR(255),"
                + "  itemDescription VARCHAR(255),"
                + "  itemGuid VARCHAR(255),"
                + "  itemPubDate TIMESTAMP,"
                + "  channel_ID INTEGER REFERENCES rss_feed(channel_ID),"
                + "  haveRead BOOLEAN DEFAULT FALSE,"
                + "  PRIMARY KEY (item_ID))";

        String createCategoryTable = "CREATE TABLE  category ("
                + "category_ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1),"
                + "categoryName VARCHAR(255), "
                + "CONSTRAINT category_constraint UNIQUE (categoryName),"
                + "PRIMARY KEY (category_ID))";

        String createFeedItemCategoryTable = "CREATE TABLE  feed_item_category ("
                + "  item_ID INTEGER REFERENCES feed_item(item_ID),"
                + "  category_ID INTEGER REFERENCES category(category_ID))";

        String createFeedCategoryTable = "CREATE TABLE  rss_feed_category ("
                + "feed_ID INTEGER REFERENCES rss_feed(channel_ID),"
                + "category_ID INTEGER REFERENCES category(category_ID))";

        String createAuthorTable = "CREATE TABLE  author ("
                + "author_ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1),"
                + "authorName VARCHAR(255),"
                + "CONSTRAINT author_constraint UNIQUE (authorName),"
                + "PRIMARY KEY (author_ID))";

        String createFeedItemAuthorTable = "CREATE TABLE  feed_item_author ("
                + "item_ID INTEGER REFERENCES feed_item(item_ID),"
                + "author_ID INTEGER REFERENCES author(author_ID))";

我的查询：

SELECT i.ITEMTITLE, i.ITEMLINK, i.ITEMDESCRIPTION, i.ITEMPUBDATE, i.CHANNEL_ID,
  i.ITEM_ID, a.AUTHOR_ID, a.AUTHORNAME

    FROM FEED_ITEM i
    LEFT JOIN FEED_ITEM_AUTHOR fia
         ON i.ITEM_ID = fia.ITEM_ID
    LEFT JOIN AUTHOR a
         ON fia.ITEM_ID = a.AUTHOR_ID;

结果：

<table border =1>
<tr>  
<th>ITEMTITLE</th>
<th>ITEM_ID</th>
<th>AUTHOR_ID</th>
<th>AUTHORNAME</th>
</tr>
<tr>
<td align="center">Lawyers...</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">Joe Mullin</td>
</tr>
<tr>
<td align="center">AT&T/Time Warner...</td>
<td align="center">2</td>
<td align="center">2</td>
<td align="center">Jon Brodkin</td>
</tr>
<tr>
<td align="center">The “technosphere” ...</td>
<td align="center">19</td>
<td align="center">19</td>
<td align="center">Annalee Newitz</td>
</tr>
  <tr>
<td align="center">Decrypted: ...</td>
<td align="center">20</td>
<td align="center">&lt;null&gt;</td>
<td align="center">&lt;null&gt;</td>
</tr>


</table>

feed_item_author 表具有正确的作者和项目数 - 15 个作者和 20 个项目，并且逐行正确匹配数据。

author 和 feed_items 表也有正确的行数 - 15 个作者和 20 个 feed_items。

我的代码中也有验证所有内容是否输入正确：

  try 

        feedItem_AuthorStatement = conn.prepareStatement(feed_item_author_SQL, PreparedStatement.RETURN_GENERATED_KEYS);

        feedItem_AuthorStatement.setInt(1, item_ID);
        feedItem_AuthorStatement.setInt(2, author_ID);
        int entrySuccess = feedItem_AuthorStatement.executeUpdate();

        System.out.println("SUCCESS? :" + entrySuccess);
     catch (SQLException ex) 
        Logger.getLogger(FeedItemCategoryHelper.class.getName()).log(Level.SEVERE, null, ex);
    

我在这里缺少什么？如果作者写了不止一篇文章，为什么我的查询返回的作者姓名和 ID 为 null？

【参考方案1】：

您不应该加入 ON fia.AUTHOR_ID = a.AUTHOR_ID；

您希望作者 ID 相同。除非我遗漏了什么，否则希望 itemID 与 authorID 匹配是没有意义的

【讨论】：

宾果游戏。谢谢，Learning2Code。