Question

我需要根据某种优先级从表中选择数据，如下所示：

select product, price from table1 where project = 1

-- pseudo: if no price found, do this:
select product, price from table1 where customer = 2

-- pseudo: if still no price found, do this:
select product, price from table1 where company = 3

也就是说，如果我发现3种产品的价格基于project = X，我不想在customer = Y上选择。我只想返回结果3行并完成。

你怎么在SQL中做这样的事情？对pseudo-if使用某种CASE语句？做工会还是其他聪明的事情？

编辑：我正在使用MS SQL。

谢谢！

Answer 1

您可以进行以下sql查询

IF ((SELECT COUNT(*) FROM table1 WHERE project = 1) > 0) 
    SELECT product, price FROM table1 WHERE project = 1
ELSE IF ((SELECT COUNT(*) FROM table1 WHERE project = 2) > 0) 
    SELECT product, price FROM table1 WHERE project = 2
ELSE IF ((SELECT COUNT(*) FROM table1 WHERE project = 3) > 0)
    SELECT product, price FROM table1 WHERE project = 3

Answer 2

CASE语句最接近SQL中的IF语句，并且在所有版本的SQL Server上都受支持：

SELECT CASE <variable> 
           WHEN <value>      THEN <returnvalue> 
           WHEN <othervalue> THEN <returnthis> 
           ELSE <returndefaultcase> 
       END 
  FROM <table>

Answer 3

不要使用EXISTS和COUNT，而只需使用@@ROWCOUNT：

select product, price from table1 where project = 1

IF @@ROWCOUNT = 0
BEGIN
    select product, price from table1 where customer = 2

    IF @@ROWCOUNT = 0
    select product, price from table1 where company = 3
END

Answer 4

使用SQL服务器，您可以使用CTE而不是IF / THEN逻辑，以便从现有查询中轻松映射并更改所涉及查询的数量;

WITH cte AS (
    SELECT product,price,1 a FROM table1 WHERE project=1   UNION ALL
    SELECT product,price,2 a FROM table1 WHERE customer=2  UNION ALL
    SELECT product,price,3 a FROM table1 WHERE company=3
)
SELECT TOP 1 WITH TIES product,price FROM cte ORDER BY a;

An SQLfiddle to test with

或者，您可以将它们全部合并为一个SELECT，以简化优化程序;

SELECT TOP 1 WITH TIES product,price FROM table1 
WHERE project=1 OR customer=2 OR company=3
ORDER BY CASE WHEN project=1  THEN 1 
              WHEN customer=2 THEN 2
              WHEN company=3  THEN 3 END;

Another SQLfiddle

Answer 5

请检查这是否有帮助：

select TOP 1
    product, 
    price 
from 
    table1 
where 
    (project=1 OR Customer=2 OR company=3) AND
    price IS NOT NULL
ORDER BY company

Answer 6

有一个案例陈述，但我认为以下内容更准确/更有效/更容易阅读您想要的内容。

select 
  product
  ,coalesce(t4.price,t2.price, t3.price) as price
from table1 t1
left join table1 t2 on t1.product = t2.product and t2.customer =2
left join table1 t3 on t1.product = t3.product and t3.company =3
left join table1 t4 on t1.product = t4.product and t4.project =1

Answer 7

- 大部分内容与上述类似。包含的代码用于测试

DROP TABLE table1
GO
CREATE TABLE table1 (project int, customer int, company int, product int, price money)
GO
INSERT INTO table1 VALUES (1,0,50, 100, 40),(1,0,20, 200, 55),(1,10,30,300, 75),(2,10,30,300, 75)
GO
SELECT TOP 1 WITH TIES product
        , price
        , CASE WhereFound WHEN 1 THEN 'Project'
                WHEN 2 THEN 'Customer'
                WHEN 3 THEN 'Company'
            ELSE 'No Match'
            END AS Source
FROM 
    (
     SELECT product, price, 1 as WhereFound FROM table1 where project = 11
     UNION ALL
     SELECT product, price, 2 FROM table1 where customer = 0
     UNION ALL
     SELECT product, price, 3 FROM table1 where company = 30
    ) AS tbl
ORDER BY WhereFound ASC

你能在SQL中使用if-then-else逻辑吗？

7 个答案: