Is there a way that I can do a select as such
select * from attributes where product_id = 500
would return
id name description
1 wheel round and black
2 horn makes loud noise
3 window solid object you can see through
and the query
select * from attributes where product_id = 234
would return the same results as would any query to this table.
Now obviously I could just remove the where clause and go about my day. But this involves editing code that I don't really want to modify so i'm trying to fix this at the database level.
So is there a "magical" way to ignore what is in the where clause and return whatever I want using a view or something ?
Even if it was possible, I doubt it would work. Both of those WHERE clauses expect one thing to be returned, therefore the code would probably just use the first row returned, not all of them.
It would also give the database a behaviour that would make future developers pull their hair out trying to understand.
Do it properly and fix the code.
or you could pass "product_id" instead of an integer, if there's no code checking for that...so the query would become:
select * from attributes where product_id = product_id;
this would give you every row in the table.
If you can't edit the query, maybe you can append to it? You could stick
OR 1=1
on the end.
You may be able to use result set metadata to get what you want, but a result set won't have descriptions of fields. The specific API to get result set metadata from a prepared query varies by programming language, and you haven't said what language you're using.
You can query the INFORMATION_SCHEMA for the products table.
SELECT ordinal_position, column_name, column_comment
FROM INFORMATION_SCHEMA.columns
WHERE table_name = 'products' AND schema_name = 'mydatabase';
You can restructure the database into an Entity-Attribute-Value design, but that's a much more ambitious change than fixing your code.
Or you can abandon SQL databases altogether, and use a semantic data store like RDF, which allows you to query metadata of an entity in the same way you query data.
As far out as this idea seems I'm always interested in crazy ways to do things.
I think the best solution I could come up with is to use a view that uses the products table to get all the products then the attributes table to get the attributes, so every possible product is accounted for and all will get the same result
Related
I'm currently thinking about a database schema in MySQL where I store SELECT queries into a certain table column, just to execute them on-the-fly when getting selected, and having the result passed instead of the actual query.
Would this be possible somehow? Or may this be bad practice? Is it even technically possible to have a result table passed to a single field, at least so I could run the query through PDO to get back a nested result array? Are there any alternatives?
I've read that this may be achieved through stored procedures, and although I grip the concept of those I can't think of how I could use those to achieve that.
You could do this, but what purpose do you have for doing it?
I would suggest using views:
The syntax should be valid when the view is created, unlike storing
the SQL in a field which may have invalid syntax.
It's easier to debug and modify.
For example, let's say one of the queries you want to store is:
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category;
You can create a new "view" object that defines this query:
CREATE VIEW prod_cat_count AS
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category;
Now, the object called "prod_cat_count" is stored in the database. Internally, the database just knows that "prod_cat_count" is equal to the SELECT query we mentioned. When the view is created, the database validates the syntax (checks that all columns exist, checks you haven't forgotten the GROUP BY, for example)
Then, whenever you want to get this data/run this query, you can run this statement (in SQL or in application code, for example):
SELECT product_category, category_count
FROM prod_cat_count;
If you then decide you want to change the way the product categories are counted, you can adjust the view:
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category
ORDER BY product_category;
Hope that helps!
Ok, for a moment, throw out of your mind "good database design". Let's say I have two tables, and they have some of the same columns.
item
-------
id
title
color
and
item_detail
-------
id
weight
color
In a good normal query, you'd choose the columns you want within the query, like so:
SELECT item.title, item_detail.color, item_detail.weight ...
But what if you are stuck with a query that was built with star/all:
SELECT * ...
In this case you would get two color columns pulled back in your results, one for each table. Is there a way in MySQL to chose one color column over the other, so only one shows up in the results, without a full rewrite of the statement? So that I could say that the table item_detail takes priority?
Probably not but I thought I'd ask.
Err. No there is not.
But define "without a full rewrite of the statement". As far as I can see you'd just need to rewrite the select * portion of the query.
If you cannot touch the statement at all, then you are free to ignore the column in your application (the order of the columns does not change between calls)... or you could create a view...
It's hard to know which constraints you are dealing with when you say "But what if you are stuck with a query".
After selecting data from multiple tables, like this:
SELECT games.name, scores.score
FROM games, scores
WHERE players.id = scores.player_id
..can I extract the column headings of this newly generated table?
The statement I'd normally use would be as follows:
SELECT column_name
FROM information_schema.columns
WHERE table_name=table
But naturally this would not work for a dynamically generated table with no name
Help much appreciated!
Edit: I'm using the MariaDB client
..can I extract the column headings of this newly generated table?
No. Mostly because you've not created a table. Just a result set. I think you already know this, because you've already looked at the information schema :)
Unfortunately it seems even creating a temporary table won't help - because those aren't stored in the information schema either. I don't think you can declare cursors for SHOW COLUMN... statements either.
I don't think you've got a way to do it I'm afraid.
If it's a prepared statement (with the select statement held in a variable) you could probably chop it up using some ugly string manipulation...?
It might at this point be worth asking "more abstractly, what problem are you trying to solve?"
I have a situation where I'm assembling a query based on user provided criteria and want to figure out what the most efficient way is to do this.
If I have a table that looks like this:
int id | varchar phone | varchar email | varchar RFID
and the user will pass in an array which defines the order (and items) with which they'd like to look up a user which *could look like this:
["id","email","phone"]
or it could look like this:
["email"]
or it could look like this:
["phone","rfid"]
or any other possible combination of those 4 fields.
Based on what I receive I need to look the user up in the order in which these fields arrived and if I find a match, I don't want to keep looking.
In other words if the input is
["email","rfid","phone"]
and I look into the db and find a user with the provided email, I don't want to keep looking to see if their rfid also matches, I just want to return said user.
However, if I don't find such an email, then I want to move on to the rfid.
So, in the various tests I've done (mostly playing with a case statement in the where clause) my results have been really terrible. Frequently taking almost a second to return a value, as opposed to taking <50ms when I simplify the where to search for the individual field.
I should note that all these fields are indexed.
So... my question is, should I just bite the bullet and make as many sql calls as there are items in the incoming array, or is there some really efficient way to structure a single query that will not bog down the system as my various attempts have.
I recognize that this may be too abstract a question, but am hoping that there's some mechanism for just such a use that I'm simply overlooking.
I don't think there's any good way to do a short-circuit in SQL. You can construct a WHERE clause that uses OR to combine the critiera, but doing this generally prevents it from using the indexes. You can use a UNION like this:
SELECT * FROM
(SELECT 1 precedence, table.*
FROM table
WHERE field1 = 'value'
UNION
SELECT 2 precedence, table.*
FROM table
WHERE field2 = 'value'
...
) x
ORDER BY precedence
LIMIT 1
where you replace field1, field2, etc. with the field names from the input array. This will produce the desired results in one query, but it will have to perform all the sub-queries, it won't short-circuit.
The best solution is probably to solve it in the application code. Loop through the fields in the input, and perform a query for just that field. When you get a result, break out of the loop and return it.
I'd like to select * from 2 tables, but have each table's column name be prefixed with a string, to avoid duplicate column name collissions.
For example, I'd like to have a view like so:
CREATE VIEW view_user_info as (
SELECT
u.*,
ux.*
FROM
user u,
user_ex ux
);
where the results all had each column prefixed with the name of the table:
e.g.
user_ID
user_EMAIL
user_ex_ID
user_ex_TITLE
user_ex_SIN
etc.
I've put a sql fiddle here that has the concept, but not the correct syntax of course (if it's even possible).
I'm using MySql, but would welcome generic solutions if they exist!
EDIT: I am aware that I could alias each of the fields, as mentioned in one of the comments. That's what I'm currently doing, but I find at the start of a project I keep having to sync up my tables and views as they change. I like the views to have everything in them from each table, and then I manually select out what I need. Kind of a lazy approach, but this would allow me to iterate quicker, and only optimize when it's needed.
I find at the start of a project I keep having to sync up my tables and views as they change.
Since the thing you're trying to do is not really supported by standard SQL, and you keep modifying database structures in development, I wonder if your best approach would be to write a little script that recreates that SELECT statement for you. Maybe wrap it in a method call in the development language of your choice?
Essentially you'd need to query INFORMATION_SCHEMA for the tables and columns of interest, probably via a join, and write the results out in SQL style.
Then just run the script every time you make database structural changes that are important to you, and watch your code magically keep up.