Question

我有两张桌子。表"alldates"由名为"nextdate"的日期列组成。表"availability"由多列组成，包括名为"availdate"的日期列。 "alldates"表主键是列"nextdate"。 "availability"表主键是一个名为"event_num"的列。

为了便于说明，我们假设表"alldates"填充了16行，日期10/01/2014 through 10/16/2014和表"availability"填充了9行，如下所示：

Availdate      Event_num   Event Description
10/01/2014  3                    Joe's birthday
10/04/2014 12                   Bill's dentist appt
10/04/2014  5                    Buy pizza
10/05/2014  6                    Clean the house
10/07/2014  7                    Go to theater
10/07/2014  8                    Continue forward
10/09/2014  9                    Mow the grass 
10/11/2014 10                   Take a nap
10/15/2014 11                   Fix the clock

我需要创建一个如下所示的新表：

Availdate      Event_num   Event Description
10/01/2014  3                    Joe's birthday
10/02/2014   (from table "alldates")
10/03/2014   (from table "alldates")
10/04/2014 12                   Bill's dentist appt
10/04/2014  5                    Buy pizza
10/05/2014  6                    Clean the house
10/06/2014   (from table "alldates")
10/07/2014  7                    Go to theater
10/07/2014  8                    Continue forward
10/08/2014   (from table "alldates")
10/09/2014  9                    Mow the grass 
10/10/2014   (from table "alldates")
10/11/2014 10                   Take a nap
10/12/2014   (from table "alldates")
10/13/2014   (from table "alldates")
10/14/2014   (from table "alldates")
10/15/2014 11                   Fix the clock
10/16/2014   (from table "alldates")

Answer 1

看起来像是经典的左派加入我。

select a.nextdate as availdate,
b.event_num,
b.event_description
from alldates a
left join availability b
on a.nextdate = b.availdate
;

Answer 2

如果表格的列数相同，那么您可以在它们与UNION order by列之间Availdate进行select * from ( select Availdate, Event_num, Event Description from alldates UNION select Availdate, Event_num, Event Description from availability ) tab order by Availdate

{{1}}

Answer 3

你想要的是LEFT OUTER JOIN。 LEFT OUTER告诉SQL包含第一个表中的行，即使它们在第二个表中没有相应的条目。 “对应”由谓词决定，这是告诉数据库一个表中的行如何与另一个表中的行相关联的条件。

以下查询将从alldates中选择所有行。对于nextdate与availdate中的行availability匹配的用户，每个匹配行将显示一次。对于nextdate与availability不匹配的人，event_num和event_description将列为NULL。

SELECT
    alldates.nextdate,
    availability.event_num,
    availability.event_description
FROM
    ALLDATES
    LEFT JOIN availability
ON alldates.nextdate = availability.availdate;

如何合并两个SQL表中的数据？

3 个答案: