Question

想知道是否有人知道小sql脚本的技巧。

我希望只返回有手动变速器且福特制造的行。如果不存在任何行，则返回所有福特制造的车辆。我目前正在使用IF EXIST条件。我考虑使用临时表来存储第一组数据，然后查看表的rowcount（）（插入行== 0）以查看是否需要插入更多数据。可能没有其他方法可以做到这一点，然后我描述了我的两个选项。也许社区对此有一些想法。

示例DDL

CREATE TABLE Cars
(
  Make varchar(50),
  Model varchar(50),
  ManualTransmission bit
);


INSERT INTO Cars
(Make, Model, ManualTransmission)
VALUES
('Ford', 'Taurus', 0),
('Ford', 'Contour', 0),
('Ford', 'Mustang', 0),
('Jeep', 'Liberty', 1),
('Jeep', 'Cherokee', 0);

Answer 1

单程

WITH CTE
     AS (SELECT *,
                RANK() OVER (ORDER BY ManualTransmission DESC) AS Rnk
         FROM   Cars
         WHERE  Make = 'Ford')
SELECT Make,
       Model,
       ManualTransmission
FROM   CTE
WHERE  Rnk = 1

或另一个

SELECT TOP 1 WITH TIES Make,
                       Model,
                       ManualTransmission
FROM   Cars
WHERE  Make = 'Ford'
ORDER  BY ManualTransmission DESC

这两个答案都利用了ManualTransmission是BIT数据类型的事实，而1是它可以拥有的最大可能值。如果ManualTransmission可以为空，那么您需要将它们更改为

ORDER BY ISNULL(ManualTransmission,0) DESC

或者

ORDER BY CASE WHEN ManualTransmission = 1 THEN 0 ELSE 1 END

CASE版本也适用于更复杂的条件。

Answer 2

试试这个：

select *
from (select *,
             max(cast(ManualTransmission as int)) over (partition by make) as hasManual
      from cars
      where make = 'Ford'
     ) t
where hasManual = 0 or ManualTransmission = 1

Answer 3

我想我发现了一种相当便宜的方法？它似乎更便宜，因为它不需要进行任何排序

;WITH CTE AS
(
    SELECT   *
            ,HasManual = SUM(CAST(ManualTransmission AS INT)) OVER (PARTITION BY Make)
    FROM Cars
)
SELECT Make, Model, ManualTransmission
FROM CTE 
WHERE   (Make = 'Ford' AND ManualTransmission = 1)
OR      (Make = 'Ford' AND HasManual = 0)

Answer 4

您可以尝试以下方法：

SELECT * FROM Cars
 WHERE Make = COALESCE('Ford', Make)
   AND ManualTransmission = COALESCE(1,ManualTransmission)

替代方法接近SQL语句如果不存在行，则显示全部

示例DDL

4 个答案: