MS Access：使用级联字段加入

Question

I have a query with a JOIN on a couple Fields (ID_1 & ID_2).

• ID_1 can never be Null
• ID_2 can be Null or Not Null.
• ID_1 & ID_2 in combination is Unique

My issue: The JOIN is not producing the desired result when ID_2 is Null

Can I JOIN on the concatenation of ID_1 & ID_2 at query time? A few feeble attempts have not helped.

I'd prefer not to create a new Index in the 2 tables as one of them does not belong to me.

Thanks for any advice.

Simplified version of my query:

SELECT Table1.ID_1, Table1.ID_2, Table1.Name, Table2.ID_1, Table2.ID_2, Table2.Owner
FROM Table1 
LEFT JOIN Table2 ON (Table1.ID_2 = Table2.ID_2) AND (Table1.ID_1 = Table2.ID_1);

Using the query above, I see ID_1, ID_2 & Name from Table1, but nothing from Table2 when ID_2 is null

Sample ("Alternative") Data: :)

Table1:

ID_1   ID_2    Name
===========================
A001           ACME Corp
A001   B001    ACME Medical
A001   B002    ACME Pharmacy
A002           General Electric Corp
A003           Philips

Table2:

ID_1   ID_2    Owner
===========================
A001           Bob
A001   B001    Lori
A001   B002    Becky
A002           Ravi
A003           Joe

When I LEFT JOIN, I expect to marry the info across the 2 tables. The issue as describe presents the following result - ONLY joining in Owner when both ID_1 and ID_2 are both non-Null (Owner = Null is not the correct result):

Joined (Incorrect Result):

ID_1   ID_2    Name              Owner   
=========================================
A001           ACME Corp
A001   B001    ACME Medical      Lori
A001   B002    ACME Pharmacy     Becky
A002           General Electric
A003           Philips

Answer 1

Lets go over JOINS:

INNER JOIN - Only displays COMMON ROWS between both tables. If the joined table has NULL in the COMMON FIELD YOUR JOINING ON , the row is not displayed.

LEFT JOIN - All rows from the left table included. Non-Matched entries from the right table will have NULL

RIGHT JOIN - All rows from the Right Table included. Non-Matched entries from the left table will have NULL

FULL JOIN - All rows from left and right table included. Non-Matched entries from left and right table will have NULL.

Using a LEFT JOIN will give your desired result.

Select A.ID_1
      ,B.ID_2
From tableA A
LEFT JOIN tableB B
ON A.ID_1 = B.ID_2  

Answer 2

If you must live with the deficient table design, you might try a query like this:

SELECT Table1.ID_1, Table1.ID_2, Table2.OtherInfo
FROM Table1 INNER JOIN Table2 ON ((Table1.ID_2 = Table2.ID_2) AND (Table1.ID_1 = Table2.ID_1))
OR ((Table1.ID_1 = Table2.ID_1) AND Table1.ID_2 IS NULL AND Table2.ID_2 IS NULL)

Answer 3

Consider the following variations. Choose the easiest to maintain or most efficient type.

Union Query

First SELECT capture records with matching ids ( ID_1 = ID_2 ) and second SELECT captures records where only ID_1 match but ID_2 IS NULL in both tables.

SELECT t1.ID_1, t1.ID_2, t1.[Name], t2.Owner
FROM [Table1] t1 
INNER JOIN [Table2] t2 ON t1.ID_1 = t2.ID_1 AND t1.ID_2 = t2.ID_2

UNION ALL

SELECT t1.ID_1, t1.ID_2, t1.[Name], t2.Owner
FROM [Table1] t1 
INNER JOIN [Table2] t2 ON t1.ID_1 = t2.ID_1
WHERE t1.ID_2 IS NULL AND t2.ID_2 IS NULL;

Alternatively, consider single SELECT statements:

Explicit Join (with OR in ON clause)

SELECT t1.ID_1, t1.ID_2, t1.[Name], t2.Owner
FROM [Table1] t1
INNER JOIN  [Table2] t2 ON (t1.ID_1 = t2.ID_1 AND t1.ID_2 = t2.ID_2)
      OR (t1.ID_1 = t2.ID_1 AND t1.ID_2 IS NULL AND t2.ID_2 IS NULL);

Implicit Join (query with no explicit JOIN )

SELECT t1.ID_1, t1.ID_2, t1.[Name], t2.Owner
FROM [Table1] t1, [Table2] t2
WHERE (t1.ID_1 = t2.ID_1 AND t1.ID_2 = t2.ID_2)
   OR (t1.ID_1 = t2.ID_1 AND t1.ID_2 IS NULL AND t2.ID_2 IS NULL);

Each above should return (but not necessarily in same order) the following output:

-- ID_1     ID_2    Name                    Owner
-- A001             ACME Corp               Bob
-- A001     B001    ACME Medical            Lori
-- A001     B002    ACME Pharmacy           Becky
-- A002             General Electric Corp   Ravi
-- A003             Philips                 Joe