Maybe this is stupid question or maybe I have designed my code completely wrong but anyhow, here is my question...
I have a "dynamic" sql-query where its impossible to take all the parameters i need for making the query parameterized, therefore i get my data and put it in a dataview and after that i search for the rows I want to show in the dataview.
One of the columns are a column named id. Id is primary key and auto_increment in the table and therefore it's an int.
Now to my question, i want to present all my matching id with the number the user put in my textbox. Let us say my id consist of 5 numbers and the user put the 4 first, then in the perfect world i would have 10 matches (12340-12349 as an example). Doing this on a string is very easy using RowFilter and the operator LIKE combined with a wildcard. But how can i do something similar on integers? Do i have to convert it to strings and wont that ruin the rowfilter expression?
Not a live or death-situation... im more curious if the ice im walking is very thin... :)
Rowfilter expression supports CONVERT function, so technically you can convert your integer ID to string to do the LIKE command:
MyDataView.RowFilter = "Convert(ID, 'System.String') LIKE '1234*'";
But do try to offload the filtering to backend. It's unlikely that you have unlimited number of parameters and SQL is very flexible in allowing you different combinations.
Related
I want to create a MVCCKey with a timestamp and pretty value I know. But I realize a roachpb.key is not very straightforward; is there some prefix/suffix involved? Is the database name is also encoded in roachpb.key?
Can anyone please tell me how a MVCCKey is formed? What information does it have? In the documentation, it just says that it looks like /table/primary/key/column.
An engine.MVCCKey combines a regular key with a timestamp. MVCCKeys are encoded into byte strings for use as RockDB keys (RocksDB is configured with a custom comparator so MVCCKeys are sorted correctly even though the timestamp uses a variable-width encoding).
Regular keys are byte strings of type roachpb.Key. For ordinary data records, the keys are constructed from table, column, and index IDs, along with the values of indexed columns. (The database ID is not included here; the database to which a table belongs can be found in the system.descriptors table)
The function keys.PrettyPrint can convert a roachpb.Key to a human-readable form.
I am working on a sort function for a table column that holds desk numbers and names.
This is a legacy program and was designed so that this column is nvarchar.
Because of this, the sort function cannot sort numerically as shown below:
Should I go into the database and alter this column to add leading zeros to number-only entries? Is this even do-able since the column is nvarchar?
Or should I add code at the object-level to add leading zeros just before the data is presented?
I would add one getter property in my class so it look like
public int Ordering
{
get
{
return int.Parse(CharColumn);
}
}
and when getting list of those objects simply order by that new property.
SQL Server is built upon SET Theory which states that, if you order your result set, you get non-relational data.
I would create another database column with the numeric value (there may be some columns that don't convert well, make sure you identify these and work to convert them). Then I would work to convert your application to use the new values. If the conversion is too great you could make sure all new development uses the new column and old code is migrated the next time someone touches it.
The risk to your approach is too fold, storing them in varchar is inefficient with both memory and processing power. Adding zeroes will help with the sorting but not fix the root issue.
Here is another question that I think will help you: SQL Server : error converting data type varchar to numeric
What are they and how do they work?
Where are they used?
When should I (not) use them?
I've heard the word over and over again, yet I don't know its exact meaning.
What I heard is that they allow associative arrays by sending the array key through a hash function that converts it into an int and then uses a regular array. Am I right with that?
(Notice: This is not my homework; I go too school but they teach us only the BASICs in informatics)
Wikipedia seems to have a pretty nice answer to what they are.
You should use them when you want to look up values by some index.
As for when you shouldn't use them... when you don't want to look up values by some index (for example, if all you want to ever do is iterate over them.)
You've about got it. They're a very good way of mapping from arbitrary things (keys) to arbitrary things (values). The idea is that you apply a function (a hash function) that translates the key to an index into the array where you store the values; the hash function's speed is typically linear in the size of the key, which is great when key sizes are much smaller than the number of entries (i.e., the typical case).
The tricky bit is that hash functions are usually imperfect. (Perfect hash functions exist, but tend to be very specific to particular applications and particular datasets; they're hardly ever worthwhile.) There are two approaches to dealing with this, and each requires storing the key with the value: one (open addressing) is to use a pre-determined pattern to look onward from the location in the array with the hash for somewhere that is free, the other (chaining) is to store a linked list hanging off each entry in the array (so you do a linear lookup over what is hopefully a short list). The cases of production code where I've read the source code have all used chaining with dynamic rebuilding of the hash table when the load factor is excessive.
Good hash functions are one way functions that allow you to create a distributed value from any given input. Therefore, you will get somewhat unique values for each input value. They are also repeatable, such that any input will always generate the same output.
An example of a good hash function is SHA1 or SHA256.
Let's say that you have a database table of users. The columns are id, last_name, first_name, telephone_number, and address.
While any of these columns could have duplicates, let's assume that no rows are exactly the same.
In this case, id is simply a unique primary key of our making (a surrogate key). The id field doesn't actually contain any user data because we couldn't find a natural key that was unique for users, but we use the id field for building foreign key relationships with other tables.
We could look up the user record like this from our database:
SELECT * FROM users
WHERE last_name = 'Adams'
AND first_name = 'Marcus'
AND address = '1234 Main St'
AND telephone_number = '555-1212';
We have to search through 4 different columns, using 4 different indexes, to find my record.
However, you could create a new "hash" column, and store the hash value of all four columns combined.
String myHash = myHashFunction("Marcus" + "Adams" + "1234 Main St" + "555-1212");
You might get a hash value like AE32ABC31234CAD984EA8.
You store this hash value as a column in the database and index on that. You now only have to search one index.
SELECT * FROM users
WHERE hash_value = 'AE32ABC31234CAD984EA8';
Once we have the id for the requested user, we can use that value to look up related data in other tables.
The idea is that the hash function offloads work from the database server.
Collisions are not likely. If two users have the same hash, it's most likely that they have duplicate data.
I am trying to correct the sort order of my ASP.NET drop down list.
The problem I have is that I need to select a distinct Serial number and have these numbers organised by DateTime Desc.
However I cannot ORDER BY DateTime if using DISTINCT without selecting the DateTime field in my query.
However if I select DateTime this selects every data value associated with a single Serial number and results in duplications.
The purpose of my page is to display data for ALL Serials, or data associated to one serial. When a new cycle begins (because it is a new production run) the Serial reverts to 1. So I cannot simply organise by serial number either.
When I use the following SQL statement the list box is in the order I require but after a period of time (usually a few hours) the order changes and appears to have no organised structure.
alt text http://img7.imageshack.us/i/captureky.jpg/
I'm fairly new to ASP.NET / SQL, does anyone know of a solution to my problem.
If you have multiple date times for each serial number, then which do you want to use for ordering? If the most recent, try this:
SELECT SerialNumber,
MAX(DateTimeField)
FROM Table
GROUP BY SerialNumber
ORDER BY 2 DESC
I don´t know if everybody agrees with that, but when I see a DISTINCT in a query the first thought that goes trough my mind is "This is wrong". Generally, DISTINCT is not necessary and it´s used when the person writing the query doesnt know very well what he is doing and this might be the case since you said you are new with Sql.
Without complete knowledge of your model is difficult to assist you a hundred percente, but I would say that you should use a GROUP BY clause instead of DISTINCT, then you can order it correctly.
Developing a website and just trying to get back into the swing of (clever) SQL queries etc, my mind had totally gone tonight!
There is a website http://www.ufindus.com/ which has a textbox allowing you to enter either a place name or a postcode/zipcode. I am trying to do something similiar but I am rubbish at SQL - so how do you construct an SQL statement that could potentially look at 2 columns (i.e. place and postcode) because you can't query both fields for the same value e.g
place = 'YORK' AND postcode = 'YORK'
or
place = 'YO21 5EA' AND postcode = 'YO21 5EA'
so do you have to put some logic in to be intelligent enough to detect whether it looks like a place name or a postcode - that just seems too complicated to me!! Any help would be much appreciated.
You could use an "OR" to get the job done. For example,
place = 'YORK' or postcode = 'YORK'
You might also do better using the LIKE statement, as in
WHERE place LIKE 'YORK%' or postcode LIKE 'YORK%'
(this assumes both place and postcode are character-based columns)
why not use OR instead of AND?
place = #textboxvalue OR post = #textboxvalue
What's wrong with attempting to match on the place and postcode? If I put in 'York' and (somewhere) that happens to be a valid postcode, I should get that result. As for preventing the same thing being entered twice, well, you can handle that on the validation prior to doing the database call.
Ah. Guess I was a bit slow on the up-take. Yes... what the others suggested is right, 'OR' is what you were looking for. I misinterpreted.
Ok, first I'm assuming that you have a table with a mapping of postcodes to placenames.
Let's call this table 'postcode' with columns 'postcode' and 'postplace'. Both of these are of a char-type.
Then.. whatever you do, make sure the input from the user is not part of dynamic sql. Make sure it is a parameter. Otherwise, you are inviting SQL injection attacks that can really ruin your day. This is important.
Our user input is in #textboxstring.
Given this, you can get the postcode and postplace like this:
select #textboxstring = RTRIM(#textboxstring) + '%';
select postcode, postplace
from postcode
where postcode like #textboxstring or postplace like #textboxstring;
Note that I'm modifying #textboxstring to get wildcard match with like without having to use dynamic sql.
If the postcode was integer, you would need to convert the input to int before executing the sql. So with a #textboxint as well, you could do this:
select #textboxstring = RTRIM(#textboxstring) + '%';
select postcode, postplace
from postcode
where postcode = #textboxint or postplace like #textboxstring;
Oh, and you need to handle that your search can have multiple results. You probably only want the first row.