SQL一对多联接问题

Question

Let's say I have two tables in Access. TableLetters and TableNumbers. TableLetters has one column TheLetter and 4 records, A, B, C, & D. TheNumbers is many for one TableLetters record. Say we have two columns in TheNumbersTable [TheLetter][TheNumber]. See below:

TheLetters
[TheLetter]
A
B
C
D

TheNumbers
[TheLetter][TheNumber]
A          1
A          2
A          3
B          1
B          2

How do I write a query that returns one record for each "TheLetters" record and the MAX "TheNumber" from TheNumbers table or blank if there's no match for TheLetter in TheNumbers table? So I want my result set to be:

[TheLetters.TheLetter][TheNumbers.TheNumber]
A                     3  
B                     2  
C                     <NULL>
D                     <NULL>

I can get A,3 - B,2 but it cuts out C & D because there's not a match in TheNumbers. I've tried switching my joins all around. I've tried putting an IF in the WHERE clause that says if we have a match return the record from TheNumbers or else give me blank. I can't seem to get the syntax right. Thanks for any help!

Answer 1

The key is to use a LEFT JOIN :

SELECT l.TheLetter, MAX(n.TheNumber)
FROM TheLetters l
LEFT JOIN TheNumbers n ON l.TheLetter = n.TheLetter
GROUP BY l.TheLetter

A left outer join returns all rows in the left table, returning data for any correlated rows in the right table, or a single row with the right table's columns set to NULL if there are no correlated rows.

Answer 2

Left Join should correct as below

SELECT l.TheLetter, MAX(n.TheNumber) FROM TheLetters l LEFT JOIN TheNumbers n ON l.TheLetter = n.TheLetter GROUP BY l.TheLetter