MySQL Left Join not producing expected results

问题

I know there is a lot of good questions answered on Left Joins, but I am having trouble finding something pertaining to my particular problem. I apologize for a possible repeated topic, but help would be greatly appreciated.

I have two tables that contain gift card sales. I have to verify that the amounts match. I'm looking for help writing a query that will return everything from TABLE A even if there is no matching record in TABLE B.

TABLE A

| id | business_date | am_pm | gift_cards_sold | 
================================================
 103 | 2011-10-06    | PM    | 175.03
 104 | 2011-10-06    | PM    | 135.03
 105 | 2011-10-06    | PM    | 250.74
 106 | 2011-10-06    | PM    | 180.44
 107 | 2011-10-06    | PM    | 150.10
 108 | 2011-10-06    | PM    | 130.00

TABLE B

| id | business_date | am_pm | gift_cards_sold | 
================================================
 103 | 2011-10-06    | PM    | 100.03
 105 | 2011-10-06    | PM    | 250.74
 106 | 2011-10-06    | PM    | 180.44
 107 | 2011-10-06    | PM    | 150.10

Here is my query so far (obviously something wrong with it)

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD
FROM A
LEFT JOIN B
USING (id)
WHERE A.am_pm = 'PM'
AND A.business_date = '2011-10-06'
AND B.business_date = '2011-10-06'
GROUP BY A.id
ORDER BY A.id ASC

Here is the result:

| id | A_SOLD | B_SOLD | 
========================
 103 | 175.03 | 100.03
 105 | 250.74 | 250.74
 106 | 180.44 | 180.44
 107 | 150.10 | 150.10

As you can see, there is a variance with id of 103. However, I need to have the result show every id regardless if there is a match or not. The result set is what I would expect from using an Inner Join.

回答1:

Simply move the B.business_date = '2011-10-06' condition from the WHERE to the ON clause. When you have a LEFT join, a WHERE condition about the second table's columns (except IS (NOT) NULL ones) actually cancels the LEFT JOIN, it then acts as an INNER JOIN.

SELECT A.id AS ID
     , A.gift_cards_sold AS A_SOLD
     , B.gift_cards_sold AS B_SOLD
FROM A
  LEFT JOIN B
    ON  B.id = A.id
    AND B.business_date = '2011-10-06'
WHERE A.am_pm = 'PM'
  AND A.business_date = '2011-10-06'
ORDER BY A.id

回答2:

You should use something like this:

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD
FROM A
LEFT JOIN B
USING (id)
WHERE A.am_pm = 'PM'
AND A.business_date = '2011-10-06'
AND (B.business_date = '2011-10-06' or B.business_date is null)
GROUP BY A.id
ORDER BY A.id ASC

回答3:

I'm not sure why you were using a "group by" clause - it's not needed here, and in most other databases, would be a syntax error.

I'm assuming that you only need to match the rows on id.

This should work:

select A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD
 from A left join b on b.id = a.id
where A.business_date = '2011-10-06'  
  and A.am_pm = 'PM'
order by a.id

回答4:

Problem is with the where condition for B.business_date, it should also allow null

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD
FROM A LEFT JOIN B ON A.id = B.id
WHERE A.am_pm = 'PM'
AND A.business_date = '2011-10-06'
AND (B.business_date = '2011-10-06' OR B.business_date is null )
ORDER BY A.id ASC

来源：https://stackoverflow.com/questions/7680078/mysql-left-join-not-producing-expected-results

标签

mysql

sql

left-join