使用MySQL联接表和数据

Question

Objective 1: Your sales data is stored in the Purchases table. Your sales staff wants to see the sales data in a pivoted form, broken down by quarter.

If your Purchases table doesn't have sales data, create some. Be sure the data spans four quarters. Next, write a query to pivot the data as follows:

Album              Q1   Q2   Q3   Q4

OK Computer        2    5    3    7

 Sea Change        8    6    2    1 

Do not create a separate table or a view. Do not alter any tables.

Save your query as dba1lesson10project1.sql and hand in the project.

This is What I need to do. But, the table it wants me to work with looks like this. And it states in the assignment I cannot alter it at all.

CustomerID    DateOfPurchase   SongID

1              2007-03-31        3
3              2007-06-30        4
4              2007-09-30        4
5              2007-12-31        5

I know I need to join three tables together so I can group by the title. Which are my Songs, Albums, and Purchases tables.

 SELECT Albums.Title FROM Albums
 LEFT JOIN Songs
 INNER JOIN Purchases
 ON Songs.SongID = Purchases.SongID
 ON Albums.Title = Purchases.SongID,
 SELECT Title,
 SUM(CASE WHEN QUARTER(DateOfPurchase) = 1 THEN 1 ELSE 0 END) AS 'Q1',
 SUM(CASE WHEN QUARTER(DateOfPurchase) = 2 THEN 1 ELSE 0 END) AS 'Q2',
 SUM(CASE WHEN QUARTER(DateOfPurchase) = 3 THEN 1 ELSE 0 END) AS 'Q3',
 SUM(CASE WHEN QUARTER(DateOfPurchase) = 4 THEN 1 ELSE 0 END) AS 'Q4'
 From Purchases
 GROUP BY Title;

I'm kind of at a loss here when it comes to Joining three separate tables then pivoting the data

I've tried the code above in multiple other variants which has failed me past the table joining portion.

Any help would be much appreciated.

Answer 1

My suggestion before attempting to PIVOT the data would be to first, write the query to return the columns that you need, this will involve joining your tables. You didn't provide your table definitions so I am making a few guesses on the structure. If your tables are structured similar to the following:

CREATE TABLE Purchases
    (`CustomerID` int, `DateOfPurchase` datetime, `SongID` int)
;

CREATE TABLE Albums
    (`AlbumId` int, `Title` varchar(11))
;

CREATE TABLE Songs
    (`SongID` int, `AlbumID` int)
;

Then you would SELECT from the tables using a JOIN similar to this code:

select a.title,
  p.DateOfPurchase
from albums a
inner join songs s
  on a.albumid = s.albumId
inner join purchases p
  on s.songid = p.songid

This query will return to you the album Title as well as the DateOfPurchase which you need to pivot the data. Once you have the JOINS worked out, then you can replace the p.DateOfPurchase with your aggregate function and CASE expression and add the GROUP BY clause to get the final result:

select a.title,
  SUM(CASE WHEN Quarter(p.DateOfPurchase) = 1 THEN 1 ElSE 0 END) AS Q1,
  SUM(CASE WHEN Quarter(p.DateOfPurchase) = 2 THEN 1 ELSE 0 END) AS Q2,
  SUM(CASE WHEN Quarter(p.DateOfPurchase) = 3 THEN 1 ELSE 0 END) AS Q3,
  SUM(CASE WHEN Quarter(p.DateOfPurchase) = 4 THEN 1 ELSE 0 END) AS Q4
from albums a
inner join songs s
  on a.albumid = s.albumId
inner join purchases p
  on s.songid = p.songid
group by a.title;

See SQL Fiddle with Demo . This gives a result:

|       TITLE | Q1 | Q2 | Q3 | Q4 |
| OK Computer |  1 |  0 |  0 |  0 |
|  Sea Change |  0 |  1 |  1 |  0 |