What you do here is called a JOIN
[although you do it implicitly because you select from multiple tables]. This means, if you didn't put any conditions in your WHERE clause, you had all combinations of those tables. Only with your condition you restrict your join to those rows where the drink id matches.
But there are still X multiple rows in the result for every drink, if there are X photos with this particular drinks_id. Your statement doesn't restrict which photo[s] you want to have!
If you only want one row per drink, you have to tell SQL what you want to do if there are multiple rows with a particular drinks_id. For this you need grouping and an aggregate function. You tell SQL which entries you want to group together [for example all equal drinks_ids] and in the SELECT, you have to tell which of the distinct entries for each grouped result row should be taken. For numbers, this can be average, minimum, maximum [to name some].
In your case, I can't see the sense to query the photos for drinks if you only want one row. You probably thought you could have an array of photos in your result for each drink, but SQL can't do this. If you only want any photo and you don't care which you'll get, just group by the drinks_id [in order to get only one row per drink]:
SELECT name, price, photo
FROM drinks, drinks_photos
WHERE drinks.id = drinks_id
GROUP BY drinks_id
name price photo
fanta 5 ./images/fanta-1.jpg
dew 4 ./images/dew-1.jpg
In MySQL, we also have GROUP_CONCAT, if you want the file names to be concatenated to one single string:
SELECT name, price, GROUP_CONCAT[photo, ',']
FROM drinks, drinks_photos
WHERE drinks.id = drinks_id
GROUP BY drinks_id
name price photo
fanta 5 ./images/fanta-1.jpg,./images/fanta-2.jpg,./images/fanta-3.jpg
dew 4 ./images/dew-1.jpg,./images/dew-2.jpg
However, this can get dangerous if
you have ,
within the field values, since most likely you want to split this again on the client side. It is also not a standard SQL aggregate function.
4.4.9 Using More Than one Table
The pet
table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs to contain the following
information:
The pet name so that you know which animal each event pertains to.
A date so that you know when the event occurred.
A field to describe the event.
An event type field, if you want to be able to categorize events.
Given these considerations, the CREATE
TABLE
statement for the event
table might look like this:
mysql> CREATE TABLE event [name VARCHAR[20], date DATE,
type VARCHAR[15], remark VARCHAR[255]];
As with the pet
table, it is easiest to load the initial records by
creating a tab-delimited text file containing the following information.
1995-05-15 | litter | 4 kittens, 3 female, 1 male |
1993-06-23 | litter | 5 puppies, 2 female, 3 male |
1994-06-19 | litter | 3 puppies, 3 female |
1999-03-21 | vet | needed beak straightened |
1997-08-03 | vet | broken rib |
1991-10-12 | kennel | |
1991-10-12 | kennel | |
1998-08-28 | birthday | Gave him a new chew toy |
1998-03-17 | birthday | Gave him a new flea collar |
1998-12-09 | birthday | First birthday |
Load the records like this:
mysql> LOAD DATA LOCAL INFILE 'event.txt' INTO TABLE event;
Based on what you have learned from the queries that you have run on the pet
table, you should be able to perform retrievals on the records in the event
table; the principles are the same. But when is the event
table by itself insufficient to answer questions you might ask?
Suppose that you want to find out the ages at which each pet had its litters. We saw earlier how to calculate ages from two dates. The litter
date of the mother is in the event
table, but to calculate her age on that date you need her birth date, which is stored in the pet
table. This means the query requires both tables:
mysql> SELECT pet.name,
TIMESTAMPDIFF[YEAR,birth,date] AS age,
remark
FROM pet INNER JOIN event
ON pet.name = event.name
WHERE event.type = 'litter';
+--------+------+-----------------------------+
| name | age | remark |
+--------+------+-----------------------------+
| Fluffy | 2 | 4 kittens, 3 female, 1 male |
| Buffy | 4 | 5 puppies, 2 female, 3 male |
| Buffy | 5 | 3 puppies, 3 female |
+--------+------+-----------------------------+
There are several things to note about this query:
The
FROM
clause joins two tables because the query needs to pull information from both of them.When combining [joining] information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a
name
column. The query uses anON
clause to match up records in the two tables based on thename
values.The query uses an
INNER JOIN
to combine the tables. AnINNER JOIN
permits rows from either table to appear in the result if and only if both tables meet the conditions specified in theON
clause. In this example, theON
clause specifies that thename
column in thepet
table must match thename
column in theevent
table. If a name appears in one table but not the other, the row does not appear in the result because the condition in theON
clause fails.Because the
name
column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name.
You need not have two different tables to perform a join. Sometimes it is useful to join a
table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet
table with itself to produce candidate pairs of live males and females of like species:
mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
FROM pet AS p1 INNER JOIN pet AS p2
ON p1.species = p2.species
AND p1.sex = 'f' AND p1.death IS NULL
AND p2.sex = 'm' AND p2.death IS NULL;
+--------+------+-------+------+---------+
| name | sex | name | sex | species |
+--------+------+-------+------+---------+
| Fluffy | f | Claws | m | cat |
| Buffy | f | Fang | m | dog |
+--------+------+-------+------+---------+
In this query, we specify aliases for the table name to refer to the columns and keep straight which instance of the table each column reference is associated with.