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);
- 示例展示
| productID | productName | categoryID | unitPrice | stockQuantity |
|---|---|---|---|---|
| 1 | Laptop | 1 | 800 | 50 |
| 2 | Smartphone | 1 | 500 | 100 |
| 3 | T-shirt | 2 | 20 | 200 |
| 4 | Jeans | 2 | 40 | 150 |
| 5 | Headphones | 1 | 100 | 75 |
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;
查询结果：
| ProductName | UnitPrice | PriceCategory |
|---|---|---|
| Laptop | 800 | Expensive |
| Smartphone | 500 | Expensive |
| T-shirt | 20 | Affordable |
| Jeans | 40 | Affordable |
| Headphones | 100 | Affordable |
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;
查询结果：
| productName | stockQuantity | StockStatus |
|---|---|---|
| Laptop | 50 | Out of Stock |
| Smartphone | 100 | Limited Stock |
| T-shirt | 200 | In Stock |
| Jeans | 150 | In Stock |
| Headphones | 75 | Limited Stock |
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;
- 查询结果
| categoryID | AvgPrice | PriceCategory |
|---|---|---|
| 1 | 466.6667 | Hign Price |
| 2 | 30 | low Price |
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;
- 查询结果
| productName | CurrentTime |
|---|---|
| Laptop | 2023/8/30 19:14 |
| Smartphone | 2023/8/30 19:14 |
| T-shirt | 2023/8/30 19:14 |
| Jeans | 2023/8/30 19:14 |
| Headphones | 2023/8/30 19:14 |
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;
- 查询结果
| productName | unitPrice | PriceCategory |
|---|---|---|
| Laptop | 1902/3/10 0:00 | Expensive |
| Smartphone | 1901/5/14 0:00 | Expensive |
| Headphones | 1900/4/9 0:00 | Expensive |
| Jeans | 1900/2/9 0:00 | Affordable |
| T-shirt | 1900/1/20 0:00 | Affordable |
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;
- 查询结果
| productName | unitPrice | PriceComparison |
|---|---|---|
| Laptop | 800 | Above Avg |
| Smartphone | 500 | Above Avg |
| T-shirt | 20 | Below Avg |
| Jeans | 40 | Below Avg |
| Headphones | 100 | Below Avg |
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;
- 查询结果
| productName | unitPrice | stockQuantity | PricePerUnit |
|---|---|---|---|
| Laptop | 800 | 50 | 16 |
| Smartphone | 500 | 100 | 5 |
| T-shirt | 20 | 200 | 0.1 |
| Jeans | 40 | 150 | 0.2667 |
| Headphones | 100 | 75 | 1.3333 |
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;
- 查询结果
| productName | unitPrice | stockQuantity | StockCategory | AdjustedStock |
|---|---|---|---|---|
| Laptop | 800 | 50 | Low | 52.5 |
| Smartphone | 500 | 100 | Low | 105 |
| T-shirt | 20 | 200 | High | 220 |
| Jeans | 40 | 150 | Medium | 165 |
| Headphones | 100 | 75 | Low | 78.75 |
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;
- 查询结果
| productName | unitPrice | ProductStatus |
|---|---|---|
| Laptop | 800 | Low Demand |
| Smartphone | 500 | Low Demand |
| T-shirt | 20 | High Demand, Affordable |
| Jeans | 40 | High Demand, Affordable |
| Headphones | 100 | Low Demand |
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;
- 查询结果
| productName | Category |
|---|---|
| Laptop | Electronics |
| Smartphone | Other |
| T-shirt | Clothing |
| Jeans | Other |
| Headphones | Other |
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;
- 查询结果
| productName | unitPrice | ProductStatus |
|---|---|---|
| Laptop | 800 | High Price or High Stock |
| Smartphone | 500 | High Price, High Stock |
| T-shirt | 20 | High Price or High Stock |
| Jeans | 40 | High Price or High Stock |
| Headphones | 100 | High Price, High Stock |
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;
- 查询结果
| productName | unitPrice | stockQuantity | ProductCategory |
|---|---|---|---|
| Laptop | 800 | 50 | Limited Stock, Expensive |
| Smartphone | 500 | 100 | Popular and Expensive |
| T-shirt | 20 | 200 | Popular and Affordable |
| Jeans | 40 | 150 | Popular and Expensive |
| Headphones | 100 | 75 | Popular and Expensive |
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;
- 查询结果
| productName | unitPrice | stockQuantity | StockComparison |
|---|---|---|---|
| T-shirt | 20 | 200 | Above Avg Stock |
| Laptop | 800 | 50 | Below Avg Stock |
| Jeans | 40 | 150 | Above Avg Stock |
| Smartphone | 500 | 100 | Below Avg Stock |
| Headphones | 100 | 75 | Below Avg Stock |
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;