深入解析SQL SELECT TOP 语句的实现

SELECT TOP (n) [列列表]
FROM 表名
[WHERE 条件]
[ORDER BY 排序规则];

-- 返回工资最高的前3名员工
SELECT TOP (3) EmployeeID, Name, Salary
FROM Employees
ORDER BY Salary DESC;

-- 返回前10条订单
SELECT TOP 10 OrderID, OrderDate, CustomerID
FROM Orders
ORDER BY OrderDate DESC;

-- 返回销售额前10%的订单
SELECT TOP 10 PERCENT OrderID, TotalAmount
FROM Orders
ORDER BY TotalAmount DESC;

-- 返回分数前3名（如果有并列第3，也全部返回）
SELECT TOP (3) WITH TIES StudentID, Name, Score
FROM Students
ORDER BY Score DESC;

-- 第2页，每页10条（页码从1开始）
DECLARE @PageNumber INT = 2;
DECLARE @PageSize INT = 10;

SELECT *
FROM (
    SELECT TOP (@PageSize) *
    FROM (
        SELECT TOP (@PageSize * @PageNumber) *
        FROM Products
        ORDER BY ProductID ASC
    ) AS Inner1
    ORDER BY ProductID DESC  -- 反转顺序取最后几条
) AS Inner2
ORDER BY ProductID ASC;  -- 再反转回来

SELECT *
FROM Products
ORDER BY ProductID
OFFSET 10 ROWS        -- 跳过前10行（第2页开始）
FETCH NEXT 10 ROWS ONLY;  -- 取10行

-- 每个部门薪资最高的前2名员工
WITH Ranked AS (
    SELECT 
        EmployeeID, Name, DepartmentID, Salary,
        ROW_NUMBER() OVER (PARTITION BY DepartmentID ORDER BY Salary DESC) AS Rn
    FROM Employees
)
SELECT EmployeeID, Name, DepartmentID, Salary
FROM Ranked
WHERE Rn <= 2;  -- 比 TOP 更灵活

-- 但 TOP 也可以用在子查询
SELECT DepartmentID, 
       (SELECT TOP 1 Name FROM Employees e2 
        WHERE e2.DepartmentID = e1.DepartmentID 
        ORDER BY Salary DESC) AS TopEarner
FROM Departments e1;