SELECT FROM a database named from a value in a JOIN - php

Question

I have a SQL statement that has to pull information from two databases, one is a constant and known database, and the other is dynamically found in the first database. The databases structure looks like this:
(database) lookup
(table) test
(fields) key, database_name
(row sample) "foo", "database_foo"
(database) database_foo
(table) bar
(fields) important1, important2
(row sample) "silly", "test"
So my SQL statement looks like this:
SELECT
test.key as key,
test.database_name as database_name,
bar.important1 as important1,
bar.importnat2 as important2,
FROM
lookup.test as test,
(database_name).bar as bar, # this, obviously, doesn't work
WHERE
key = 'foo'
LIMIT 1;
Is there a way I can make this work, or is it better for me to just do two separate SQL statements, one for the lookup, and one for the database?

Answer 1

If you must do it this way then you need to use dynamic sql and a two statements.
You have your query built as a string and then you run an EXEC on the query once it's constructed.
In this case you would have a string variable for the db name, then you would create a query from this variable and your literal query, then you would simply execute it.
Be aware, though, this makes you vulnerable to SQL Injection if you don't control the input parameters.
Erland Sommarskog has a great primer on using dynamic SQL:
http://www.sommarskog.se/dynamic_sql.html
EDIT: From #BryanMoyle comment below, you will likely need to do both a separate query and dynamic sql. You need to extract the value in order to determine the other DB name... Since you cannot use the DB name as a variable otherwise, you'll need to SELECT this information first, then stick it into the subsequent query.

Answer 2

I personally go for 2 separate statements; it would make it easier to control for errors such as the the lookup provides a row, that the row provides a valid database, etc.
As Matthew pointed out, beware of SQLIA and sanitize all user input. I like to MD5 hash inputs and compare to the hash of the value looked up.

Answer 3

I'd go for two separate queries, mediated by php: It's simpler, more robust, and will keep you sane and productive. Write one query to find out which database you should be talking to. Use php (not SQL) to direct your query to that database:
$row = $lookup->prepare("SELECT database_name WHERE key = 'foo'")->execute()->fetch();
$db = $row[0];
Then you contact $db and ask for the row with key foo. Use PHP code to select the right open connection, or switch databases inside the connection with USE:
$query2 = "USE " . $db . "; SELECT * FROM bar where key == 'foo'"