查询合并有相同列名的两个表

Question

I have these tables in my MySQL database:

BUYERS

ID|SELLER 
----------------
0 |Paul
1 |Jean
2 |David
3 |Jack
4 |John
5 |Fred
6 |Peter

PARIS

ID|CAR
---------
0 |Toyota
1 |BMW
2 |Honda

LONDON

ID|CAR
---------
3 |Ford
4 |BMW
5 |Honda
6 |Honda

I use the followinq query :

SELECT b.id, b.seller, p.car 
FROM buyers b
JOIN paris p
ON b.id = p.id
UNION ALL
SELECT b.id, b.seller, l.car 
FROM buyers b
JOIN london l
ON g.id = l.id;

To get the following result :

ID|SELLER |CAR 
----------------
0 |Paul   |Toyota
1 |Jean   |BMW
2 |David  |Honda
3 |Jack   |Ford
4 |John   |BMW
5 |Fred   |Honda
6 |Peter  |Honda

I wanted to retrieve rows with "Honda" as "CAR" and I Tried to append the query with "Where car = 'Honda'" but without success..

Thanks for any help

Answer 1

Just appending WHERE car = 'Honda' is ambiguous. Which car column should be checked?

The easiest way to achieve this is to wrap your existing query within another select statement so that the query is applied on the resulting table, ie

SELECT * FROM
(
    SELECT b.id, b.seller, p.car 
    FROM buyers b
    JOIN paris p
    ON b.id = p.id
    UNION ALL
    SELECT b.id, b.seller, l.car 
    FROM buyers b
    JOIN london l
    ON g.id = l.id;
)
WHERE car = 'Honda'

Answer 2

Adding

WHERE car = 'Honda';

to your query only refers to the second query, ie the one after UNION ALL .

So either:

SELECT b.id, b.seller, p.car FROM buyers b JOIN paris p ON b.id = p.id
  WHERE p.car = 'Honda'
UNION ALL
SELECT b.id, b.seller, l.car FROM buyers b JOIN london l ON b.id = l.id
  WHERE p.car = 'Honda'
;

or

SELECT id, seller, car
FROM
(
  SELECT b.id, b.seller, p.car FROM buyers b JOIN paris p ON b.id = p.id
  UNION ALL
  SELECT b.id, b.seller, l.car FROM buyers b JOIN london l ON b.id = l.id
) data
WHERE car = 'Honda';

Answer 3

Can you please try below query:

select * from (
SELECT b.id, b.seller, p.car 
FROM buyers b
JOIN paris p
ON b.id = p.id
UNION ALL
SELECT b.id, b.seller, l.car 
FROM buyers b
JOIN london l
ON g.id = l.id;
)
where car = 'HONDA'

Answer 4

This is not an answer, but mere advice.

Just in case this is your real database (I'm pretty sure it isn't), here is some advice:

• The table names tell us what entities you are dealing with. In your database this is buyers, parises, and londons. You probably see your mistake ;-)
• Then looking into the buyers table we see the main column is called seller. What the heck? Is it buyer or seller?
• Then a column called ID should be the table's ID and uniquely identify a record in the table. You, however, are using the buyers IDs in other tables and still call them ID.
• In your example each buyer has only one other record either in London or in Paris. If this is the case, then you can simply make the car a column in the buyers table instead. If you do need an n:m relation, then call the IDs by what they are, ie buyer_id or the like.
• In any case there should be a car table containing one record per car, in order to avoid misspellings like 'Homda' in some records.
• You are showing car brands, but call the table cars . If it's really about brands only, a better name would hence be brands or car_brands or the like.

Here is an example on how to design the database:

Cars

id_car | name
-------+-------
1      | BMW
2      | Ford
3      | Honda
4      | Toyota

Sellers

id_seller | name
----------+------
0         | Paul
1         | Jean
2         | David
3         | Jack
4         | John
5         | Fred
6         | Peter

Sellers_Cars

id_seller | id_car
----------+-------
0         | 1
0         | 2
0         | 4
1         | 1
2         | 3
3         | 2
4         | 1
5         | 3
6         | 3
6         | 4

A possible query:

select name as honda_seller
from sellers
where id_seller in
(
  select id_seller
  from sellers_cars
  where car_id = (select car_id from cars where name = 'Honda')
);