Issue with mysql query that calls column name from another table - mysql

Question

I have two tables, one is an index (or map) which helps when other when pulling queries.
SELECT v.*
FROM smv_ v
WHERE (SELECT p.network
FROM providers p
WHERE p.provider_id = v.provider_id) = 'RUU='
AND (SELECT p.va
FROM providers p
WHERE p.provider_id = v.provider_id) = 'MjU='
LIMIT 1;
Because we do not know the name of the column that holds the main data, we need to look it up, using the provider_id which is in both tables, and then query.
I am not getting any errors, but also no data back. I have spent the past hour trying to put this on sqlfiddle, but it kept crashing, so I just wanted to check if my code is really wrong, hence the crashing?
In the above example, I am looking in the providers table for column network, where the provider_id matches, and then use that as the column on smv.
I am sure i have done this before just like this, but after the weekend trying I thought i would ask on here.
Thanks in Advance.
UPDATE
Here is an example of the data:
THis is the providers, this links so no matter what the name of the column on the smv table, we can link them.
+---+---+---------------+---------+-------+--------+-----+-------+--------+
| | A | B | C | D | E | F | G | H |
+---+---+---------------+---------+-------+--------+-----+-------+--------+
| 1 | 1 | Home | network | batch | bs | bp | va | bex |
| 2 | 2 | Recharge | code | id | serial | pin | value | expire |
+---+---+---------------+---------+-------+--------+-----+-------+--------+
In the example above, G will mean in the smv column for recharge we be value. So that is what we would look for in our WHERE clause.
Here is the smv table:
+---+---+-----------+-----------+---+----+---------------------+-----+--+
| | A | B | C | D | E | F | value | va |
+---+---+-----------+-----------+---+----+---------------------+-----+--+
| 1 | 1 | X2 | Home | 4 | 10 | 2016-09-26 15:20:58 | | 7 |
| 2 | 2 | X2 | Recharge | 4 | 11 | 2016-09-26 15:20:58 | 9 | |
+---+---+-----------+-----------+---+----+---------------------+-----+--+
value in the same example as above would be 9, or 'RUU=' decoded.
So we do not know the name of the rows, until the row from smv is called, once we have this, we can look up what column name we need to get the correct information.
Hope this helps.
MORE INFO
At the point of triggering, we do not know what the row consists of the right data because some many of the fields would be empty. The map is there to help we query the right column, to get the right row (smv grows over time depending on whats uploaded.)
1) SELECT p.va FROM providers p WHERE p.network = 'Recharge' ;
2) SELECT s.* FROM smv s, providers p WHERE p.network = 'Recharge';
1) gives me the correct column I need to look up and query smv, using the above examples it would come back with "value". So I need to now look up, within the smv table, C = Recharge, and value = '9'. This should bring me back row 2 of the smv table.
So individually both 1 and 2 queries work, but I need them put together so the query is done on the database server.
Hope this gives more insight
Even More Info
From reading other posts, which are not really doing what I need, i have come up with this:
SELECT s.*
FROM (SELECT
(SELECT p.va
FROM dh_smv_providers p
WHERE p.provider_name = 'vodaphone'
LIMIT 1) AS net,
(SELECT p.bex
FROM dh_smv_providers p
WHERE p.provider_name = 'vodaphone'
LIMIT 1) AS bex
FROM dh_smv_providers) AS val, dh_smv_ s
WHERE s.provider_id = 'vodaphone' AND net = '20'
ORDER BY from_base64(val.bex) DESC;
The above comes back blank, but if i replace net, in the WHERE clause with a column I know exists, I do get the results expected:
SELECT s.*
FROM (SELECT
(SELECT p.va
FROM dh_smv_providers p
WHERE p.provider_name = 'vodaphone'
LIMIT 1) AS net,
(SELECT p.bex
FROM dh_smv_providers p
WHERE p.provider_name = 'vodaphone'
LIMIT 1) AS bex
FROM dh_smv_providers) AS val, dh_smv_ s
WHERE s.provider_id = 'vodaphone' AND value = '20'
ORDER BY from_base64(val.bex) DESC;
So what I am doing wrong, which is net, not showing the value derived from the subquery "value" ?
Thanks

Answer 1

SELECT
v.*,
p.network, p.va
FROM
smv_ v
INNER JOIN
providers p ON p.provider_id = v.provider_id
WHERE
p.network = 'RUU=' AND p.va = 'MjU='
LIMIT 1;
The tables talk to each other via the JOIN syntax. This completely circumvents the need (and limitations) of sub-selects.
The INNER JOIN means that only fully successful matches are returned, you may need to adjust this type of join for your situation but the SQL will return a row of all v columns where p.va = MjU and p.network = RUU and p.provider_id = v.provider_id.
What I was trying to explain in comments is that subqueries do not have any knowledge of their outer query:
SELECT *
FROM a
WHERE (SELECT * FROM b WHERE a)
AND (SELECT * FROM c WHERE a OR b)
This layout (as you have in your question) is that b knows nothing about a because the b query is executed first, then the c query, then finally the a query. So your original query is looking for WHERE p.provider_id = v.provider_id but v has not yet been defined so the result is false.