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SQL SELECT TOP, LIMIT, ROWNUM 子句

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SQL SELECT TOP, LIMIT, ROWNUM 子句

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：

EmployeeIDEmployeeNameSalary
1John Smith50000
2Maria Garcia60000
3Liam Johnson70000
4Emma Wilson80000
5Oliver Brown90000

SQL 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 SQL，数据如下所示：

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SQL 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;

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