mysql - 基于空值生成值的情况

Question

使用表格CARS:

Make   | Model   | Color | Year
------- --------- ------- -----
Honda  | Accord  | Red   | 09
Nissan | Skyline | Blue  | 15
null   | Skyline | Red   | 15
Toyota | null    | Black | 15

当 Make 包含值但 Model 时，我尝试在 Make 列中生成字符串 ERROR> 没有。我的声明目前没有产生任何结果

SELECT 
CASE
WHEN (SELECT Count(*) 
    FROM CARS
    WHERE a.make=make
    AND a.year=year
    AND a.color=color
    AND a.model = NULL) AND (SELECT Count(*) 
    FROM CARS
    WHERE a.year=year
    AND a.color=color
    AND a.make = make) > 1 THEN '**ERROR**'
ELSE a.make
END as make
FROM a.CARS

Answer 1

如果您希望将 ERROR 作为 make 的值，而 make 没有任何值，但 model 那么这可能就是您想要的:

SELECT 
    CASE 
       WHEN make IS NULL AND model IS NOT NULL 
       THEN 'ERROR' 
       ELSE make 
    END AS make, 
    model, color, year 
FROM cars;

也许我误解了你的意图。您的问题似乎与您在另一个答案中发布的评论所说的不同。