Oracle：如何在选择语句中使用 if then
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Oracle : How to use if then in a select statement
提问by Nick
select ma.TITLE,ma.ID as aid,ur.USER_ID
from LEO_MENU_ACTIVITY_RELATION mr
inner join LEO_MENU_MASTER mm on mm.ID=mr.MENU_ID
INNER join LEO_MENUACTIVITY ma on mr.ACTIVITY_ID=ma.ID
LEFT OUTER JOIN LEO_USER_RIGHTS ur on ma.ID=ur.MENU_RELATION_ID and ur.MENU_ID=mm.ID and ur.USER_ID='141'
where mm.ID='1'
UNION (SELECT
'List' as TITLE,
1 as ID,
case (WHEN ur.MENU_RELATION_ID=1 THEN NULL ELSE USER_ID END)as USER_ID
from
LEO_USER_RIGHTS)
In the UNION i want perform a conditional select like if ur.MENU_RELATION_ID=1 then the USER_ID should be selected as NULL otherwise the the original value from the 'LEO_USER_RIGHTS' table must be retrieved.
在 UNION 中，我想执行一个条件选择，比如 ur.MENU_RELATION_ID=1 那么应该将 USER_ID 选为 NULL，否则必须检索“LEO_USER_RIGHTS”表中的原始值。
How can i do this ? Please help
我怎样才能做到这一点 ？请帮忙
Krishnik
克里什尼克
回答by hol
If you want to combine in a UNION something based on the first table I think you can only do it by repeating the whole thing like this:
如果你想在 UNION 中结合基于第一个表的东西，我认为你只能通过像这样重复整个事情来做到这一点：
select ma.TITLE,ma.ID as aid,ur.USER_ID
from LEO_MENU_ACTIVITY_RELATION mr
inner join LEO_MENU_MASTER mm on mm.ID=mr.MENU_ID
INNER join LEO_MENUACTIVITY ma on mr.ACTIVITY_ID=ma.ID
LEFT OUTER JOIN LEO_USER_RIGHTS ur on ma.ID=ur.MENU_RELATION_ID and ur.MENU_ID=mm.ID and ur.USER_ID='141'
where mm.ID='1'
UNION (SELECT
'List' as TITLE,
1 as ID,
case (WHEN ur.MENU_RELATION_ID=1 THEN NULL ELSE USER_ID END)as USER_ID
from
LEO_MENU_ACTIVITY_RELATION mr
inner join LEO_MENU_MASTER mm on mm.ID=mr.MENU_ID
INNER join LEO_MENUACTIVITY ma on mr.ACTIVITY_ID=ma.ID
LEFT OUTER JOIN LEO_USER_RIGHTS ur on ma.ID=ur.MENU_RELATION_ID and ur.MENU_ID=mm.ID and ur.USER_ID='141'
where mm.ID='1'
)
If this is used often I would create a view to avoid duplicate code. In ORACLE (I do not know for other SQL dialects) there is a WITH statement enables you to make a sort of "temporary view".
如果经常使用，我会创建一个视图以避免重复代码。在 ORACLE（我不知道其他 SQL 方言）中有一个 WITH 语句使您能够创建一种“临时视图”。
