你能在SQL中使用if-then-else逻辑吗？

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,如果我找到3个基于价格的产品project = X,我不想选择customer = Y.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

而不是使用EXISTS,COUNT只是使用@@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