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：
| EmployeeID | EmployeeName | Salary |
|---|---|---|
| 1 | John Smith | 50000 |
| 2 | Maria Garcia | 60000 |
| 3 | Liam Johnson | 70000 |
| 4 | Emma Wilson | 80000 |
| 5 | Oliver Brown | 90000 |
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;
)
