SQL GROUP BY 实例
2010-04-03 00:33
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GROUP BY 语法:
SELECT column_name, function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
实例
表结构如下:
BookComments
BookList表:
Connection conn = DatabaseConn.getConnection(); //此处调用的是前面篇章中的连接池的方法
Statement stm = conn.createStatement();
ResultSet result=stm.executeQuery("" +
"select BookComments.BookNo,BookList.BookName,BookList.Auther," +
"COUNT(BookComments.BookNo) AS CountNo " +
"from BookComments,BookList " +
"where BookList.BookNo=BookComments.BookNo " +
"GROUP BY BookComments.BookNo,BookList.BookName,BookList.Auther " +
"order by CountNo desc ");
实现功能:统计图书的评论数,然后按照从多到少的顺序排列起来,首先使用BookNo将BookComments,BookList连接起来,然后将BookComments.BookNo,BookList.BookName,BookList.Auther这些列聚组,COUNT(BookComments.BookNo) AS CountNo统计不同的BookNo出现的次数,即实现记录评论的数量的目的
SELECT column_name, function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
实例
表结构如下:
BookComments
列 名 | 类型 | 长度 | 是否允许为空 | 说明 | ||
CommentsNo | bigint | 8 | 否 | 主键 非负 自增 | ||
BookNo | varchar | 50 | 否 | 外键 book表主键 | ||
| Datetime | 是 | 评论生成时间 | |||
Uid | Varchar | 10 | 是 | 评论人ID | ||
Text | Text | 是 | 评论内容 |
列 名 | 类 型 | 大 小 | 是否允许为空 | 说明 |
BookNo | Varchar | 10 | 否 | 书号 主键 |
BookName | Varchar | 200 | 否 | 书名 |
Auther | Varchar | 200 | 否 | 作者 |
Introduction | Text | 是 | 简介 |
Statement stm = conn.createStatement();
ResultSet result=stm.executeQuery("" +
"select BookComments.BookNo,BookList.BookName,BookList.Auther," +
"COUNT(BookComments.BookNo) AS CountNo " +
"from BookComments,BookList " +
"where BookList.BookNo=BookComments.BookNo " +
"GROUP BY BookComments.BookNo,BookList.BookName,BookList.Auther " +
"order by CountNo desc ");
实现功能:统计图书的评论数,然后按照从多到少的顺序排列起来,首先使用BookNo将BookComments,BookList连接起来,然后将BookComments.BookNo,BookList.BookName,BookList.Auther这些列聚组,COUNT(BookComments.BookNo) AS CountNo统计不同的BookNo出现的次数,即实现记录评论的数量的目的
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