I'm quite new to iphone programming. I would like some information with you. Here is the structure of my database:
ID_song : auto-int (primary key)
txt_song : text
When we do delete some data from a table view, and when the data is deleted from the SQLite database, let's say I have only 3 songs in my database
id_song:1 txt_song:Song_A
id_song:2 txt_song:Song_B <------ (To be deleted)
id_song:3 txt_song:Song_C
After deleting the rows, does the data in the table looks like:
id_song:1 txt_song:Song_A
id_song:3 txt_song:Song_C
or
id_song:1 txt_song:Song_A
id_song:2 txt_song:Song_C
I mean, does sqlite reorganise the index?
Why would it do that ?
First of all, that could make a huge performance hit and a real risk to violate the integrity of the data.
Changing the primary key, means that all related foreign keys have to be changed as well.
No, it does not. (which you probably could've figured out in about 5 minutes by trying it yourself. :))
No. No Database will rewrite an ID field after deletion of a row. If you wish to number your songs, you will need to do this yourself in code.
it looks like
id_song:1 txt_song:Song_A
id_song:3 txt_song:Song_C
Related
I am attempting to create a DB for my app and one thing I'd like to find the best way of doing is creating a one-to-many relationship between my Users and Items tables.
I know I can make a third table, ReviewedItems, and have the columns be a User id and an Item id, but I'd like to know if it's possible to make a column in Users, let's say reviewedItems, which is an integer array containing foreign keys to Items that the User has reviewed.
If PostgreSQL can do this, please let me know! If not, I'll just go down my third table route.
It may soon be possible to do this: https://commitfest.postgresql.org/17/1252/ - Mark Rofail has been doing some excellent work on this patch!
The patch will (once complete) allow
CREATE TABLE PKTABLEFORARRAY (
ptest1 float8 PRIMARY KEY,
ptest2 text
);
CREATE TABLE FKTABLEFORARRAY (
ftest1 int[],
FOREIGN KEY (EACH ELEMENT OF ftest1) REFERENCES PKTABLEFORARRAY,
ftest2 int
);
However, author currently needs help to rebase the patch (beyond my own ability) so anyone reading this who knows Postgres internals please help if you can.
No, this is not possible.
PostgreSQL is a relational DBMS, operating most efficiently on properly normalized data models. Arrays are not relational data structures - by definition they are sets - and while the SQL standard supports defining foreign keys on array elements, PostgreSQL currently does not support it. There is an (dormant? no activity on commitfest since February 2021) effort to implement this - see this answer to this same question - so the functionality might one day be supported.
For the time being you can, however, build a perfectly fine database with array elements linking to primary keys in other tables. Those array elements, however, can not be declared to be foreign keys and the DBMS will therefore not maintain referential integrity. Using an appropriate set of triggers (both on the referenced and referencing tables, as a change in either would have to trigger a check and possible update on the other) one would in principle be able to implement referential integrity over the array elements but the performance is unlikely to be stellar (because indexes would not be used, for instance).
Friends, here is my problem: I populate a table in Xcode using SQLite database. Here is what I want - If my table has 20 rows and all of them are populated, how to update the table if new data is saved in my SQLite database? I want the new information to overwrite the old one. How to do that in Xcode, any ideas? Thanks in advance! I am not sure if the info that I provided is enough, so let me know if you want some code or other stuff.
The key to what you're doing is in the routine you furnish for your table: cellForRowAtIndexPath. Basically, you can write that routine to furnish whatever data you want it to furnish. Once you know that your data has been updated, you can cause the entire table to refresh with a call to reloadData.
Hopefully an easy question for someone with more experience than me. I have a stored procedure that Inserts records into a table. Like all databases that I have worked with, when you insert a record it inserts it into the bottom of the table. I would like to insert it to the top of the table and then move all the existing records down by one (I assume this would happen automatically with the insert).
I want to to do this because I'm using the 'Top #' keyword. I am pretty sure that I could just leave it the way it is, and instead of using the 'Top" keyword, I could use the 'Bottom" keyword. But I want to make it easier for people reading it that aren't familiar with it, so they can instantly see the newest entries. I'm going to keep researching this on my own, but If someone knew off the top of their head and could save me the time that would be appreciated.
is there any incremental id on that table.If yes then create clustered index on that id with descending order
I am obtaining a json array from a url and inserting data into a table. Since the contents of the url are subject to change, I want to make a second connection to a url and check for updates and insert new records in y table using sqlite3.
The issues that I face are:
1) My table doesn't have a primary key
2) The url lists the changes on the same day. Hence, if I run my app multiple times, when I insert values in my database, I get duplicate entries. I want to keep a check for the day duplicated entries that should be removed. The problem can be solved by adding a constraint, but since the url itself has duplicated values, I find it difficult.
The only way I can see you can do it if you have no primary key or something you can use that is unique to each record, is when you get your new data in you go through the new entries where for each one you check if the exact same data exists in the database already. If it doesn't then you add it, if it does then you skip over it.
You could even do something like create a unique key yourself for each entry which is a concatenation of each column of the table. That way you can quickly do the check for if the entry already exists in the database.
I see two possibilities depending on your setup:
You have a column setup as UNIQUE (this can be through a PRIMARY KEY or not). In this case, you can use the ON CONFLICT clause:
http://www.sqlite.org/lang_conflict.html
If you find this construct a little confusing, you can instead use "INSERT OR REPLACE" or "INSERT OR IGNORE" as described here:
http://www.sqlite.org/lang_insert.html
You do not have a column setup as UNIQUE. In this case, you will need to SELECT first to verify for duplicate data, and based on the result INSERT, UPDATE, or do nothing.
A more common & robust way to handle this is to associate a timestamp with each data item on the server. When your app interrogates the server it provides the timestamp corresponding to the last time it synced. The server then queries its database and returns all values that are timestamped later than the timestamp provided by the app. Then it also returns a new timestamp value for the app to store, to use on the next sync.
this is my first time using SQL at all, so this might sound basic. I'm making an iPhone app that creates and uses a sqlite3 database (I'm using the libsqlite3.dylib database as well as importing "sqlite3.h"). I've been able to correctly created the database and a table in it, but now I need to know the best way to get stuff back from it.
How would I go about retrieving all the information in the table? It's very important that I be able to access each row in the order that it is in the table. What I want to do (if this helps) is get all the info from the various fields in a single row, put all that into one object, and then store the object in an array, and then do the same for the next row, and the next, etc. At the end, I should have an array with the same number of elements as I have rows in my sql table. Thank you.
My SQL is rusty, but I think you can use SELECT * FROM myTable and then iterate through the results. You can also use a LIMIT/OFFSET(1) structure if you do not want to retrieve all elements at one from your table (for example due to memory concerns).
(1) Note that this can perform unexpectedly bad, depending on your use case. Look here for more info...
How would I go about retrieving all the information in the table? It's
very important that I be able to access each row in the order that it
is in the table.
That is not how SQL works. Rows are not kept in the table in a specific order as far as SQL is concerned. The order of rows returned by a query is determined by the ORDER BY clause in the query, e.g. ORDER BY DateCreated, or ORDER BY Price.
But SQLite has a rowid virtual column that can be used for this purpose. It reflects the sequence in which the rows were inserted. Except that it might change with a VACUUM. If you make it an INTEGER PRIMARY KEY it should stay constant.
order by rowid