I wonder if it is possible to query a specific part of a comma separated string, something like the following:
$results = mysql_query("SELECT * FROM table1 WHERE $pid=table1.recordA[2] ",$con);
$pid is a number
and recordA contains data like
34,9008,606,,416,2
where i want to check the third part (606)
Thank you in advance
Having comma seperated lists or any data seperation within a mySQL field is frowned upon and is to all extents bad practice.
Rather than looking at querying an element of a delimetered list within a mySQL field consider breaking the field into its own table and then creating an adjacency list to create a 1:many relationship between table1 and it's associated variables.
If you are commited to this route, the simplest method would be to use PHP to manage it as mySQL has very few tools (above and beyond regex / text searches) to drill down to the data you want to extract. $results = explode(',',$query); would create an array of your variables from the returned field allowing you to run as many conditional checks against it as needed.
However, consider adding this to your 'need to re-write / re-think' list. A relational tables structure would allow you to query the database for $pid's value directly as it would be contained within it's own field and linked
If the delimetered variable list is of an inderterminate length or the relationships between the variables are heirarchical you'd be better off searching stackoverflow for information on Directed Acyclic Graphs in mySQL to find a better solution to the problem.
Without knowing the nature or the intended purpose for this script I can't answer in any more detail. I hope this has helped a little.
How about this:
SELECT * FROM table1 WHERE FIND_IN_SET({$pid}, recordA) = 3
Make sure to index recordA. I love normalization as much as the next guy, but sometimes breaking it up is just more trouble than it's worth ;)
Related
This question already has answers here:
Many database rows vs one comma separated values row
(4 answers)
Closed 8 years ago.
I'm interested how and why many to many relationship is better than storing the information in one row.
Example: I have two tables, Users and Movies (very big data). I need to establish a relationship "view".
I have two ideas:
Make another column in Users table called "views", where I will store the ids of the movies this user has viewed, in a string. for example: "2,5,7...". Then I will process this information in PHP.
Make new table users_movies (many to many), with columns user_id and movie_id. row with user_id=5 and movie_id=7 means that user 5 has viewed movie 7.
I'm interested which of this methods is better and WHY. Please consider that the data is quite big.
The second method is better in just about every way. Not only will you utilize your DBs indexes to find records faster, it will make modification far far easier.
Approach 1) could answer the question "Which movies has User X viewed" by just having an SQL like "...field_in_set(movie_id, user_movielist) ...". But the other way round ("Which user do have viewed movie x") won't work on an sql basis.
That's why I always would go for approach 2): clear normalized structure, both ways are simple joins.
It's just about the needs you have. If you need performance then you must accept redundancy of the information and add a column. If your main goal is to respect the Normalization paradigma then you should not have redundancy at all.
When I have to do this type of choice I try to estimate the space loss of redundancy vs the frequency of the query of interest and its performance.
A few more thoughts.
In your first situation if you look up a particular user you can easily get the list of ids for the films they have seen. But then would need a separate query to get the details such as the titles of those movies. This might be one query using IN with the list of ids, or one query per film id. This would be inefficient and clunky.
With MySQL there is a possible fudge to join in this situation using the FIND_IN_SET() function (although a down side of this is you are straying in to non standard SQL). You could join your table of films to the users using ON FIND_IN_SET(film.id, users.film_id) > 0 . However this is not going to use an index for the join, and involves a function (which while quick for what it does, will be slow when performed on thousands of rows).
If you wanted to find all the users who had view any film a particular user had viewed then it is a bit more difficult. You can't just use FIND_IN_SET as it requires a single string and a comma separated list. As a single query you would need to join the particular user to the film table to get a lot of intermediate rows, and then join that back against the users again (using FIND_IN_SET) to find the other users.
There are ways in SQL to split up a comma separated list of values, but they are messy and anyone who has to maintain such code will hate it!
These are all fudges. With the 2nd solution these easy to do, and any resulting joins can easily use indexes (and possibly the whole queries can just use indexes without touching the actual data).
A further issue with the first solution is data integretity. You will have to manually check that a film doesn't appear twice for a user (with the 2nd solution this can easily be enforced using a unique key). You also cannot just add a foreign key to ensure that any film id for a user does actually exist. Further you will have to manually ensure that nothing enters a character string in your delimited list of ids.
I need some help please! Basically I have a system that has an unlimited amount of categories and the way in which it works is through unique IDs. So basically the system will find the root folder and match all subfolders based on its parent's UID. An endless loop...
But now I want to do the opposite of that in a single MySQL statement (if possible).
Basically I want it to do this.. (By the way this isn't my actual code, it's just how I want it to work)
SELECT UID FROM Table
WHERE UID = 'value'
--AND ALSO:
SELECT * FROM SameTable
WHERE UID = The Parent UID just fetched...
And do this until the UID = 'Specified Value'.
I seriously hope that makes sense!
Is it even possible? I could do it using multiple queries in a PHP loop I know, but that just feels like a long way around, and bad practice.
What you have is called "Hierarchical data". You have to read on it on google. In short, there are three main ways to represent it in a 2-dimensional table:
Adjacency list (what you have). You scarcely can make it with single query
Materialized path (my favorite). Natural and readable. Not so efficient though.
Nested set (Most complicated) yet most powerful.
You can choose any system you like ir stick to your current one. Single query is not Holy grail to pursue at any cost.
I have a feed that comes from the State of Florida in a CSV that I need to load daily into MySQL. It is a listing of all homes for sale in my area. One field has a list of codes, separated by commas. Here's one such sample:
C02,C11,U01,U02,D02,D32,D45,D67
These codes all mean something (pool, fenced in area, etc) and I have the meanings in a separate table. My question is, how should I handle loading these? Should I put them in their own field as they are in the CSV? Should I create a separate table that holds them?
If I do leave them as they are in a field (called feature_codes), how could I get the descriptions out of a table that has the descriptions? That table is simply feature_code, feature_code_description. I don't know how to break them apart in my first query to do the join to bring the description in.
Thank you
As a general rule, csv data should never stored in a field, especially if you actually need to consider individual bits of the csv data, instead of just the csv string as a whole.
You SHOULD normalize the design and split each of those sub "fields" into their own table.
That being said, MySQL does have find_in_set() which allows you sort-of search those csv strings and treat each as its own distinct datum. It's not particularly efficient to use this, but it does put a bandaid on the design.
You should keep the information about feature codes in a separate table, where each row is a pair of house identifier, and feature identifier
HouseID FeatureID
1 C07
1 D67
2 D02
You can use explode() to separate your CSV string : http://php.net/manual/en/function.explode.php
$string = 'C02,C11,U01,U02,D02,D32,D45,D67';
$array = explode(',', $string);
Then with your list of feature_codes you can easily retrieve your feature_code_description but you need to do another query to get an array with all your feature_codes and feature_code_description.
Or split your field and put it in another table with the home_id.
You can save it in your DB as is and when you read it out you can run the php function explode. Go check that function out. It will build an array for you out of a string separating the values by whatever you want . In your case you can use:
$array_of_codes = explode(",", $db_return_string);
This will make an array out of each code separating them by the commas between them. Good luck.
I'm in need of some quick help on matching a field in my database that stores all of the "parent" categories for my online store. Here's an example of how my "parents" are stored in the table via one field named Parent:
MENS MENS-BRANDS MENS-SHIRTS MENS-T-SHIRTS
Here is my query in PHP to perform the call:
$query = "SELECT id FROM $usertable where parent like '".strtoupper($parent)."'";
The problem is, if I am on MENS-BRANDS, this will also return those products who are listed in every other category because it contains the word "MENS." Since all of the parents are stored in one field, how can I make my SQL query only recognize each physical word that is separated by spaces in the field itself, instead of it trying to find every instance of different fragments of a word throughout the field?
I hope this makes sense, and any help is surely appreciated.
Ideally you can change your schema so that you have a separate table linking these categories to your existing entries. This way you can have one row per product and you can easily write a SQL query that looks for the specific word you want without the need for a LIKE match. Added bonus: this will improve performance.
However, if you absolutely cannot change this schema, your best bet is probably to use a regular expression like WHERE parent REGEXP '[[:<:]]MENS[[:>:]]'
I'm here using MySQL regular expressions. If you're using a different database management system the same concept will work, but the exact syntax may be different.
I have a field in a table recipes that has been inserted using mysql_real_escape_string, I want to count the number of line breaks in that field and order the records using this number.
p.s. the field is called Ingredients.
Thanks everyone
This would do it:
SELECT *, LENGTH(Ingredients) - LENGTH(REPLACE(Ingredients, '\n', '')) as Count
FROM Recipes
ORDER BY Count DESC
The way I am getting the amount of linebreaks is a bit of a hack, however, and I don't think there's a better way. I would recommend keeping a column that has the amount of linebreaks if performance is a huge issue. For medium-sized data sets, though, I think the above should be fine.
If you wanted to have a cache column as described above, you would do:
UPDATE
Recipes
SET
IngredientAmount = LENGTH(Ingredients) - LENGTH(REPLACE(Ingredients, '\n', ''))
After that, whenever you are updating/inserting a new row, you could calculate the amounts (probably with PHP) and fill in this column before-hand. Or, if you're into that sort of thing, try out triggers.
I'm assuming a lot here, but from what I'm reading in your post, you could change your database structure a little bit, and both solve this problem and open your dataset up to more interesting uses.
If you separate ingredients into its own table, and use a linking table to index which ingredients occur in which recipes, it'll be much easier to be creative with data manipulation. It becomes easier to count ingredients per recipe, to find similarities in recipes, to search for recipes containing sets of ingredients, etc. also your data would be more normalized and smaller. (storing one global list of all ingredients vs. storing a set for each recipe)
If you're using a single text entry field to enter ingredients for a recipe now, you could do something like break up that input by lines and use each line as an ingredient when saving to the database. You can use something like PHP's built-in levenshtein() or similar_text() functions to deal with misspelled ingredient names and keep the data as normalized as possbile without having to hand-groom your [users'] data entry too much.
This is just a suggestion, take it as you like.
You're going a bit beyond the capabilities and intent of SQL here. You could write a stored procedure to scan the string and return the number and then use this in your query.
However, I think you should revisit the design of whatever is inserting the Ingredients so that you avoid searching strings in of every row whenever you do this query. Add a 'num_linebreaks' column, calculate the number of line breaks and set this column when you're adding the Indgredients.
If you've no control over the app that's doing the insertion, then you could use a stored procedure to update num_linebreaks based on a trigger.
Got it thanks, the php code looks like:
$check = explode("\r\n", $_POST['ingredients']);
$lines = count($check);
So how could I update all the information in the table so Ingred_count based on field Ingredients in one fellow swoop for previous records?