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网站建设与维护设计报告,网站wap转换,crm管理系统登录入口官网,做访问的公司网站SQL SELECT TOP, LIMIT, ROWNUM 子句SQL SELECT TOP 子句SELECT TOP 语句用于在 SQL 中限制返回的结果集中的行数#xff0c; 它通常用于只需要查询前几行数据的情况#xff0c;尤其在数据集非常大时#xff0c;可以显著提高查询性能。SELECT TOP 子句对于拥有数千条记录的大…SQL SELECT TOP, LIMIT, ROWNUM 子句SQL SELECT TOP 子句SELECT TOP 语句用于在 SQL 中限制返回的结果集中的行数 它通常用于只需要查询前几行数据的情况尤其在数据集非常大时可以显著提高查询性能。SELECT TOP 子句对于拥有数千条记录的大型表来说是非常有用的。说明SELECT TOP在 SQL Server 和 MS Access 中使用而在 MySQL 和 PostgreSQL 中使用LIMIT关键字。Oracle 在 12c 版本之前没有直接等效的关键字可以通过ROWNUM实现类似功能但在 12c 及以上版本中引入了FETCH FIRST。当使用TOP或LIMIT时最好结合ORDER BY子句以确保返回的行是特定顺序的前几行。SQL Server / MS Access 语法SELECT TOP number|percent column1, column2, ... FROM table_name;number|percent指定返回的行数或百分比。number具体的行数。percent数据集的百分比。MySQL 语法SELECT column1, column2, ... FROM table_name LIMIT number;Oracle 语法SELECT column1, column2, ... FROM table_name FETCH FIRST number ROWS ONLY;PostgreSQL 语法SELECT column1, column2, ... FROM table_name LIMIT number;实例假设我们有一个名为 Employees 的表其中包含以下数据EmployeeIDEmployeeNameSalary1John Smith500002Maria Garcia600003Liam Johnson700004Emma Wilson800005Oliver Brown90000SQL Server 和 MS Access 返回前 3 行数据SELECT TOP 3 EmployeeName, Salary FROM Employees;返回前 10% 的数据SELECT TOP 10 PERCENT EmployeeName, Salary FROM Employees;MySQL 返回前 3 行数据SELECT EmployeeName, Salary FROM Employees LIMIT 3;PostgreSQL 返回前 3 行数据SELECT EmployeeName, Salary FROM Employees LIMIT 3;Oracle 返回前 3 行数据SELECT EmployeeName, Salary FROM Employees FETCH FIRST 3 ROWS ONLY;演示数据库在本教程中我们将使用 RUNOOB 样本数据库。下面是选自 Websites 表的数据mysql SELECT * FROM Websites; -------------------------------------------------------------- | id | name | url | alexa | country | -------------------------------------------------------------- | 1 | Google | https://www.google.cm/ | 1 | USA | | 2 | 淘宝 | https://www.taobao.com/ | 13 | CN | | 3 | 菜鸟教程 | http://www.runoob.com/ | 5000 | USA | | 4 | 微博 | http://weibo.com/ | 20 | CN | | 5 | Facebook | https://www.facebook.com/ | 3 | USA | | 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND | --------------------------------------------------------------MySQL SELECT LIMIT 实例下面的 SQL 语句从 Websites 表中选取头两条记录实例SELECT * FROM Websites LIMIT 2;执行以上 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SELECT TOP PERCENT 实例在 Microsoft SQL Server 中还可以使用百分比作为参数。下面的 SQL 语句从 websites 表中选取前面百分之 50 的记录实例以下操作在 Microsoft SQL Server 数据库中可执行。SELECT TOP 50 PERCENT * FROM Websites;