MySQL-链接多个表的建议-对吗？

Question

Would someone mind advising me please regarding this table setup.

Its the first time designing a database. This will be a part of it.

Its a report writing application. Multiple Engineers can be assigned to attend any job/report and multiple engineers can author the report as well as attending.

Is this the best way to do this. I would need to be able to search attendees and authors separately in the application.

Thanks very much for the assistance.

Answer 1

You have, I believe, two tables containing entities. The entities are employee and report .

These entities have two different many-to-many relationships: author and attendee .

So your tables are these

employee                report
--------                -----
employee_id (PK)        report_id (PK)
surname                 title
givenname               releasedate
whatever                whatever

Then you have two many:many relationship tables with the same columns as each other. One is author and the other is attendee .

author / attendee
------
employee_id PK, FK to employee.employee_id
report_id   PK, FK to report.report_id

Notice the compound (two-column) primary keys.

+---------------------+\   /+-------------+\   /+-----------------------+
|                     +-----+   author    +-----+                       |
|                     |/   \+-------------+/   \|                       |
|    employee         |                         |     report            |
|                     |                         |                       |
|                     |\   /+-------------+\   /|                       |
|                     +-----+  attendee   +-----+                       |
+---------------------+/   \+-------------+/   \+-----------------------+


           \   /
           -----    means a many-to-many relationship
           /   \

When you determine an employee is an attendee for a certain report, you insert a row into the attendee table with the correct employee and report.

If you want, for example, all authors for each report you can do this sort of thing:

  SELECT r.title, r.releasedate, 
         GROUP_CONCAT(e.surname ORDER BY e.surname SEPARATED BY ',')surnames
    FROM report r
    LEFT JOIN author a ON r.report_id = a.report_id
    LEFT JOIN employee e ON a.report_id = e.report_id
   GROUP BY r.title, r.releasedate
   ORDER BY r.releasedate DESC

The LEFT JOIN operations allow your query to find reports that have no authors. Ordinary inner JOIN operations would suppress those rows from your result set.

There is a limitation with this strict E:R design. For many kinds of reports, (scientific papers for example) the order of authors is critically important. (You want to start an academic food fight? List the authors of a paper in the wrong order.)

So you author table might also contain an ordinal value.

author
------
employee_id PK, FK to employee.employee_id
report_id   PK, FK to report.report_id
ordinal     INT

and your report query would contain this line.

         GROUP_CONCAT(e.surname ORDER BY e.ordinal SEPARATED BY ',')surnames