sql-server - mssql - 如果第一个表没有结果，则从另一个表中选择

Question

我必须运行查询才能找到价格。我有多个价目表，每个价目表都有自己的表。customer_price、default_price 和 minimum_price。

并非所有价目表都包含所有产品，有时产品可能不会出现在任何价目表上，然后价格需要为价格返回 0。

所以我想：

select price from customer_price where customer='walmart' and product='whitebread'

如果这返回结果，那么一切都很好。如果结果为 0,NULL 或查询不返回任何行，我希望它运行：

select price from default_price where customer='walmart' and product='whitebread'

如果这返回结果，那么一切都很好。如果结果为 0，NULL 或查询不返回任何行，我只想返回 0。

我不确定如何进行。我尝试了一个 case 语句，但如果在结果中找不到任何行，case 语句就会失败。我该怎么做或说if 0 results then

预先感谢，一如既往。

Answer 1

select top 1 price from (
    select 1 as qorder, price from customer_price where price > 0 and customer='walmart' and product='whitebread'
    union
    select 2 as qorder, price from default_price where price > 0 and customer='walmart' and product='whitebread'
    union 
    select 3 as qorder, 0 
    ) as sq
order by qOrder

Answer 2

case when
(select count(price) from customer_price where customer='walmart' and product='whitebread' and price is not null and price > 0) > 0 
then 
    select price from customer_price where customer='walmart' and product='whitebread'
else 
    select price from default_price where customer='walmart' and product='whitebread'   
end 

Answer 3

我正在编写一个双向解决方案，其中包括Empty Select Queries或Tables。您可以替换您的查询

if OBJECT_ID('tempdb..#temp1') is not null
drop table #temp1

select * into #temp1 from (select col1, col2 from First_Table) as firstTableVariable
if (@@ROWCOUNT > 0)
    begin
    print 'data is displayed from the first table/query'
        select * from #temp1 
    end
else
    begin
    print 'Data displayed from second table/query '
    select col1, col2 from second_Table 
    end
end