SQL 中 CASE WHEN 及 SELECT CASE WHEN 的用法详解

CASE 
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE default_result
END

CASE expression 
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ...
    ELSE default_result
END

SELECT 
    employee_name,
    salary,
    CASE 
        WHEN salary > 10000 THEN '高薪'
        WHEN salary > 5000 THEN '中等'
        ELSE '一般'
    END AS salary_level
FROM employees;

SELECT 
    product_name,
    CASE category_id
        WHEN 1 THEN '电子产品'
        WHEN 2 THEN '服装'
        WHEN 3 THEN '食品'
        ELSE '其他'
    END AS category_name
FROM products;

UPDATE orders
SET discount = 
    CASE 
        WHEN order_amount > 1000 THEN 0.1
        WHEN order_amount > 500 THEN 0.05
        ELSE 0
    END;

SELECT * FROM customers
ORDER BY 
    CASE 
        WHEN country = 'China' THEN 1
        WHEN country = 'USA' THEN 2
        ELSE 3
    END;

SELECT 
    CASE 
        WHEN age < 20 THEN '青少年'
        WHEN age BETWEEN 20 AND 30 THEN '青年'
        ELSE '成年'
    END AS age_group,
    COUNT(*) AS count
FROM users
GROUP BY age_group;

SELECT 
    student_name,
    score,
    CASE 
        WHEN score >= 90 THEN 'A'
        WHEN score >= 80 THEN 
            CASE 
                WHEN attendance_rate > 0.9 THEN 'B+'
                ELSE 'B'
            END
        WHEN score >= 70 THEN 'C'
        ELSE 'D'
    END AS grade
FROM students;

SELECT 
    department_id,
    SUM(CASE WHEN gender = 'M' THEN 1 ELSE 0 END) AS male_count,
    SUM(CASE WHEN gender = 'F' THEN 1 ELSE 0 END) AS female_count
FROM employees
GROUP BY department_id;