SQL中的CASE WHEN语句：从基础到高级应用指南

SQL中的CASE WHEN语句：从基础到高级应用指南

准备工作 - 表1: products 示例数据：

我们使用一个名为"Products"的表，包含以下列：ProductID、ProductName、CategoryID、UnitPrice、StockQuantity。

-- 建表
CREATE TABLE `products` (
  `productID` int(11) NOT NULL,
  `productName` varchar(255) DEFAULT NULL,
  `categoryID` int(11) DEFAULT NULL,
  `unitPrice` int(11) DEFAULT NULL,
  `stockQuantity` int(11) DEFAULT NULL,
  PRIMARY KEY (`productID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

-- 样例数据
INSERT INTO `products` VALUES (1, 'Laptop', 1, 800, 50);
INSERT INTO `products` VALUES (2, 'Smartphone', 1, 500, 100);
INSERT INTO `products` VALUES (3, 'T-shirt', 2, 20, 200);
INSERT INTO `products` VALUES (4, 'Jeans', 2, 40, 150);
INSERT INTO `products` VALUES (5, 'Headphones', 1, 100, 75);
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• 示例展示

一. CASE WHEN 基础使用

1. CASE WHEN-基本使用

SELECT
	ProductName,
	UnitPrice,
CASE
		
		WHEN UnitPrice > 100 THEN
		'Expensive' ELSE 'Affordable' 
	END AS PriceCategory 
FROM
	Products;
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查询结果：

2. CASE WHEN-多条件

SELECT
	productName,
	stockQuantity,
CASE
		
		WHEN stockQuantity > 100 THEN
		'In Stock' 
		WHEN stockQuantity > 50 THEN
		'Limited Stock' ELSE 'Out of Stock' 
	END AS StockStatus 
FROM
	products;
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查询结果：

3. CASE WHEN-聚合函数

SELECT
	categoryID,
	AVG( unitPrice ) AS AvgPrice,
CASE
		
		WHEN AVG( unitPrice ) > 50 THEN
		'High Price' ELSE 'Low Price' 
	END AS PriceCategory 
FROM
	products 
GROUP BY
	categoryID;
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• 查询结果

4. CASE WHEN-日期条件

SELECT
	productName,
CASE
		
		WHEN EXTRACT( MONTH FROM CURRENT_DATE ) = 8 THEN
		( SELECT NOW() ) ELSE 'Other Month' 
	END AS CurrentTime 
FROM
	products;
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• 查询结果

5. CASE WHEN-用于排序

SELECT
    ProductName,
    UnitPrice,
    CASE
        WHEN UnitPrice > 50 THEN 'Expensive'
        ELSE 'Affordable'
    END AS PriceCategory
FROM Products
ORDER BY UnitPrice DESC;
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• 查询结果

6. CASE WHEN-子查询

SELECT
	productName,
	unitPrice,
	( CASE WHEN unitPrice > ( SELECT AVG( unitPrice ) FROM products ) THEN 'Above Avg' ELSE 'Below Avg' END ) AS PriceComparison 
FROM
	products;
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• 查询结果

7. CASE WHEN-计算字段

SELECT
    ProductName,
    UnitPrice,
    StockQuantity,
    CASE
        WHEN StockQuantity > 0 THEN UnitPrice / StockQuantity
        ELSE 0
    END AS PricePerUnit
FROM Products;
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• 查询结果

8. CASE WHEN-动态列名

SELECT
	productName,
	unitPrice,
	stockQuantity,
CASE
		
		WHEN stockQuantity > 150 THEN
		'High' 
		WHEN stockQuantity > 100 THEN
		'Medium' ELSE 'Low' 
	END AS StockCategory,
CASE
		
		WHEN stockQuantity > 100 THEN
		stockQuantity * 1.1 ELSE stockQuantity * 1.05 
	END AS AdjustedStock 
FROM
	products;
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• 查询结果

9. CASE WHEN-带有嵌套逻辑

SELECT
    ProductName,
    UnitPrice,
    CASE
        WHEN StockQuantity > 100 THEN
            CASE
                WHEN UnitPrice > 50 THEN 'High Demand, High Price'
                ELSE 'High Demand, Affordable'
            END
        ELSE 'Low Demand'
    END AS ProductStatus
FROM Products;
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• 查询结果

10. CASE WHEN-处理字符串匹配

SELECT
    ProductName,
    CASE
        WHEN ProductName LIKE '%Laptop%' THEN 'Electronics'
        WHEN ProductName LIKE '%T-shirt%' THEN 'Clothing'
        ELSE 'Other'
    END AS Category
FROM Products;
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• 查询结果

11. CASE WHEN-用于条件合并

SELECT
    ProductName,
    UnitPrice,
    CASE
        WHEN UnitPrice > 50 AND StockQuantity > 50 THEN 'High Price, High Stock'
        WHEN UnitPrice > 50 OR StockQuantity > 50 THEN 'High Price or High Stock'
        ELSE 'Low Price and Low Stock'
    END AS ProductStatus
FROM Products;
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• 查询结果

12. CASE WHEN-处理多列

SELECT
    ProductName,
    UnitPrice,
    StockQuantity,
    CASE
        WHEN StockQuantity > 50 AND UnitPrice <

 30 THEN 'Popular and Affordable'
        WHEN StockQuantity <= 50 AND UnitPrice < 30 THEN 'Limited Stock, Affordable'
        WHEN StockQuantity > 50 AND UnitPrice >= 30 THEN 'Popular and Expensive'
        ELSE 'Limited Stock, Expensive'
    END AS ProductCategory
FROM Products;
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• 查询结果

13. CASE WHEN-加入窗口函数

SELECT
    ProductName,
    UnitPrice,
    StockQuantity,
    CASE
        WHEN StockQuantity > AVG(StockQuantity) OVER () THEN 'Above Avg Stock'
        ELSE 'Below Avg Stock'
    END AS StockComparison
FROM Products;
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• 查询结果

二. CASE WHEN 进阶使用

1. 基于历史数据的趋势预测

样例SQL：

SELECT
    p.ProductID,
    p.ProductName,
    s.SaleDate,
    s.QuantitySold,
    CASE
        WHEN s.QuantitySold > LAG(s.QuantitySold) OVER (PARTITION BY p.ProductID ORDER BY s.SaleDate) THEN 'Increased'
        WHEN s.QuantitySold < LAG(s.QuantitySold) OVER (PARTITION BY p.ProductID ORDER BY s.SaleDate) THEN 'Decreased'
        ELSE 'Stable'
    END AS Trend
FROM Products p
JOIN SalesHistory s ON p.ProductID = s.ProductID;
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2. 基于不同维度的复杂分析

样例SQL：

SELECT
    o.OrderID,
    o.OrderDate,
    SUM(CASE WHEN p.CategoryID = 1 THEN o.Quantity ELSE 0 END) AS ElectronicsQuantity,
    SUM(CASE WHEN p.CategoryID = 2 THEN o.Quantity ELSE 0 END) AS ClothingQuantity,
    SUM(CASE WHEN p.CategoryID = 3 THEN o.Quantity ELSE 0 END) AS OtherQuantity
FROM Orders o
JOIN Products p ON o.ProductID = p.ProductID
GROUP BY o.OrderID, o.OrderDate;
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3. 多层次CASE WHEN分析

样例SQL：

SELECT
    EmployeeID,
    FirstName,
    LastName,
    Salary,
    CASE
        WHEN Salary > 70000 THEN 'High'
        WHEN Salary > 60000 THEN 'Medium'
        WHEN Salary > 50000 THEN 'Low'
        ELSE 'Very Low'
    END AS SalaryLevel,
    CASE
        WHEN Salary > 60000 THEN 'Above Average'
        ELSE 'Below Average'
    END AS SalaryComparison
FROM Employees;
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4. 使用CASE WHEN进行数据分桶

样例SQL：

SELECT
    CustomerID,
    Age,
    Gender,
    CASE
        WHEN Age < 30 THEN 'Young'
        WHEN Age >= 30 AND Age < 40 THEN 'Middle-aged'
        ELSE 'Senior'
    END AS AgeGroup,
    CASE
        WHEN Gender = 'Male' THEN 'Male'
        WHEN Gender = 'Female' THEN 'Female'
        ELSE 'Other'
    END AS GenderCategory
FROM Customers;
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5. 基于多条件的复杂逻辑判断

样例SQL：

SELECT
    OrderID,
    OrderDate,
    SUM(CASE WHEN Quantity * Price > 500 THEN Quantity ELSE 0 END) AS HighValueItems,
    SUM(CASE WHEN Quantity * Price > 100 AND Quantity * Price <= 500 THEN Quantity ELSE 0 END) AS MediumValueItems,
    SUM(CASE WHEN Quantity * Price <= 100 THEN Quantity ELSE 0 END) AS LowValueItems
FROM Orders
GROUP BY OrderID, OrderDate;
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三. CASE WHEN 业务场景常用技巧

1. 数据重编码

您可以使用CASE WHEN来对现有数据进行重新编码，例如将文本值转换为数字编码或将某些字符串转换为更易于处理的标识符。

SELECT
    customerName,
    CASE
        WHEN customerType = 'Individual' THEN 1
        WHEN customerType = 'Corporate' THEN 2
        ELSE 0
    END AS CustomerTypeCode
FROM Customers;
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2. 条件分组

使用CASE WHEN可以在查询结果中创建不同的数据分组，而无需在实际数据中创建新的列。

SELECT
    productName,
    SUM(quantity) AS totalQuantity,
    CASE
        WHEN SUM(quantity) > 100 THEN 'High'
        WHEN SUM(quantity) > 50 THEN 'Medium'
        ELSE 'Low'
    END AS QuantityGroup
FROM Sales
GROUP BY productName;
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3. 动态排序规则

通过在ORDER BY子句中使用CASE WHEN，您可以根据不同条件动态调整查询结果的排序规则。

SELECT
    productName,
    unitPrice
FROM Products
ORDER BY
    CASE
        WHEN category = 'Electronics' THEN unitPrice
        WHEN category = 'Clothing' THEN unitPrice * 0.9
        ELSE unitPrice * 1.1
    END;
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4. 分位数分析

使用CASE WHEN可以在查询结果中对数据进行分位数分析，识别哪些数据点位于不同的分位数区间。

SELECT
    productName,
    unitPrice,
    CASE
        WHEN unitPrice <= PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY unitPrice) THEN 'Q1'
        WHEN unitPrice <= PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY unitPrice) THEN 'Q2'
        WHEN unitPrice <= PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY unitPrice) THEN 'Q3'
        ELSE 'Q4'
    END AS PriceQuartile
FROM Products;
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5. 缺失数据填充

使用CASE WHEN可以根据条件将缺失的数据点填充为特定值，从而更好地处理数据缺失情况。

SELECT
    orderID,
    orderDate,
    CASE
        WHEN orderAmount IS NULL THEN 0
        ELSE orderAmount
    END AS FilledOrderAmount
FROM Orders;
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6. 日期区间分析

使用CASE WHEN可以对日期进行区间分析，例如判断每个日期属于哪个季节、哪个月份等。

SELECT
    orderDate,
    CASE
        WHEN EXTRACT(MONTH FROM orderDate) IN (12, 1, 2) THEN 'Winter'
        WHEN EXTRACT(MONTH FROM orderDate) IN (3, 4, 5) THEN 'Spring'
        WHEN EXTRACT(MONTH FROM orderDate) IN (6, 7, 8) THEN 'Summer'
        ELSE 'Fall'
    END AS Season
FROM Orders;
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7. 业务阶段分析

使用CASE WHEN可以根据特定业务规则判断数据所处的不同阶段，如用户生命周期阶段、订单处理阶段等。

SELECT
    userID,
    registrationDate,
    CASE
        WHEN NOW() - registrationDate < INTERVAL '30 days' THEN 'New User'
        WHEN NOW() - registrationDate < INTERVAL '90 days' THEN 'Regular User'
        ELSE 'Inactive User'
    END AS UserStage
FROM Users;
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8. 动态列选择

使用CASE WHEN可以在查询结果中根据条件选择不同的列，从而根据业务需求定制查询结果。

SELECT
    orderID,
    orderDate,
    CASE
        WHEN displayPrice = 'Gross' THEN grossPrice
        ELSE netPrice
    END AS SelectedPrice
FROM Orders;
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9. 异常值标记

使用CASE WHEN可以根据条件识别和标记异常数据点，帮助进行数据质量分析。

SELECT
    customerID,
    orderDate,
    orderAmount,
    CASE
        WHEN orderAmount < 0 THEN 'Negative'
        WHEN orderAmount > 10000 THEN 'High Amount'
        ELSE 'Normal'
    END AS DataQuality
FROM Orders;
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10. 数据格式转换

使用CASE WHEN可以在不同的数据格式之间进行转换，例如将布尔值转换为文本标签。

SELECT
    productID,
    productName,
    inStock,
    CASE
        WHEN inStock THEN 'Available'
        ELSE 'Out of Stock'
    END AS StockStatus
FROM Products;
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