问题
I'm having trouble formulating a MySQL query correctly. Everything I've tried doesn't give me what's needed, or gives a syntax error.
I have three tables: Clients, Courses, and CoursesForClients.
The Clients table just has basic coordinates for a person: ID, Name, Address, email, etc.
+----------+-----------------------------+------+
| ClientID | Name | Address | etc. |
+----------+-----------------------------+------+
| 10 | Joe Smith | 1 Main St. | ... |
| 20 | Bob Smith | 2 Main St. | ... |
| ... | ... ... | ... ... ... | ... |
+----------+-----------------------------+------+
The Courses table stores the course name and its ID.
+----------+-----------------------+
| CourseID | Name |
+----------+-----------------------+
| 100 | Intro. to Subject |
| 200 | Intermediate Subject |
| 300 | Advanced Subject |
| ... | ... ... ... ... |
+----------+-----------------------+
The CoursesForClients table has the CourseID and ClientID. A given Client can have taken multiple courses, so for every course that a Client has taken, there's a row, with the person's ID and the Course ID.
+----------+----------+
| CourseID | ClientID |
+----------+----------+
| 100 | 1 |
| 200 | 1 |
| 300 | 1 |
| 100 | 2 |
| 200 | 2 |
| ... | ... |
+----------+----------+
Now, what I need is to be able to list the Client - just once - together with all the Courses she has taken. So, the result of the query might look like this:
10:Joe Smith
1 Main St.
Somewhere, AL
Intro. to Subject
Intermediate Subject
Advanced Subject
---------------------------
20:Bob Smith
2 Main St.
Somewhere, AL
Intro. to Subject
Intermediate Subject
So this output reflects the relationships between the Client and the Course. The key thing here is that, no matter how many Courses a Client has taken, the Client's particulars appear only once, followed by the list of all the courses she's taken.
There's an additional twist in that there's another table that lists the Grade for the Course for the Client, and that GradeID is also stored in the CoursesForClients table, and there's another table of Grades, with ID and Grade Description. But I won't worry about this right now. For now, all I want is just the basic output shown, as described above.
It looks like it should be easy to set up a query for this, with a JOIN and maybe a GROUP BY, but I'm having a block here and can't seem to get it right. So, any help will be hugely appreciated. Thank you!
回答1:
SQL deals in tables. By definition a table has a bunch of rows, each of which has the same columns as each other. Your query is going to yield a result set that duplicates the client's information for each course she took.
Your presentation layer is going to format that table, by noticing the first row of each new client and breaking out the client header. You'll do that in php or Java or Crystal Reports or some such presentation tech.
Your query is something like this.
SELECT a.id, a.name, a.address, a.etc,
c.Name
FROM Clients a
JOIN CoursesForClients b USING(ClientID)
JOIN Courses c USING(CourseID)
ORDER BY a.id, c.CourseID
@Strawberry makes a good point about the pitfall of using USING(). Here is the same query on ON.
SELECT a.id, a.name, a.address, a.etc,
c.Name
FROM Clients a
JOIN CoursesForClients b ON a.ClientID = b.ClientID
JOIN Courses c ON b.CourseID = c.CourseID
ORDER BY a.id, c.CourseID
来源：https://stackoverflow.com/questions/30699602/how-to-formulate-query-to-show-all-courses-taken-by-a-person