查询以基于另一列的值从单个列返回多个值

Question

I have a table with an EntryDate and a ChecklistDay column. They are Date and Integer columns. I want to return two columns, one named StartDate and another EndDate, both getting their values from the EntryDate column. StartDate would be returned from...

SELECT EntryDate AS StartDate FROM TABLE WHERE ChecklistDay = 1

EndDate would be returned from...

SELECT EntryDate AS EndDate FROM TABLE WHERE ChecklistDay = 10

Now I only want to return a row if a value is returned for both StartDate and EndDate, meaning that a ChecklistDay must have both values 1 and 10 in order for it to select the two EntryDates and return a row. What kind of query do I use here?

Answer 1

You can join to the same table twice.

select startDt.EntryDate as StartDate,
       endDt.EntryDate as EndDate
from table startDt
inner join table endDt
on startDt.id = endDt.id
where startDt.ChecklistDay = 1
and endDt.CheckListDay = 10

Answer 2

Would this help:

Select CASE ChecklistDay WHEN 1 THEN EntryDate ELSE NULL as StartDate,   
CASE CheckListDay WHEN 10 THEN DateAdd(day, ChecklistDay, StartDate) ELSE NULL END as EndDate   
from Table

Answer 3

This should run in any ANSI compatible DBMS (Oracle, MySql, Sql Server, Postgresql, Informix, DB2, etc)

SELECT
   CASE ChecklistDay WHEN 1 THEN EntryDate ELSE NULL END as StartDate
 , CASE CheckListDay WHEN 10 THEN EntryDate ELSE NULL END as EndDate
FROM
   TABLE
;