你能在 SQL 中有 if-then-else 逻辑吗? [英] Can you have if-then-else logic in SQL?

问题描述

我需要根据某种优先级从表中选择数据，如下所示:

I need to do select data from a table based on some kind of priority like so:

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.

That is, if I found 3 products with prices based on project = X, I don't want to select on customer = Y. I just want to return the resulting 3 rows and be done.

你应该如何在 SQL 中做这样的事情?对伪 if 使用某种 CASE 语句?做一个工会或其他一些聪明的事情?

How are you supposed to do stuff like this in SQL? Use some kind of CASE-statement for the pseudo-if's? Do a union or some other smart thing?

我使用的是 MS SQL.

I'm using MS SQL.

谢谢！

推荐答案

可以进行如下sql查询

You can make the following sql query

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

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