I've created a SQLite table using:
CREATE TABLE T1 (
CN INTEGER PRIMARY KEY ASC,
Name TEXT
);
If I do:
SELECT * FROM T1
Will I get the rows order by CN even without specifying a ORDER BY clause?
Is CN an alias to ROWID?
There is no such thing as a default order, if you need your results ordered add an explicit order by clause.
The dbms is simply optimised to look for the best way to quickly get the required data based on the query. In this case it's the primary key on CN, but that's only because your example is so simple. Never ever rely on the dbms choosing the order you want.
The second question might be useful to others.
From the SQLite documentation:
Except for WITHOUT ROWID tables, all rows within SQLite tables have a 64-bit signed integer key that uniquely identifies the row within its table. This integer is usually called the "rowid".
... if a rowid table has a primary key that consists of a single column and the declared type of that column is "INTEGER" in any mixture of upper and lower case, then the column becomes an alias for the rowid.
This also holds for columns that are declared of type "INTEGER PRIMARY KEY ASC", so in your table CN is an alias for "rowid"
Further information can be found here:
http://www.sqlite.org/lang_createtable.html#rowid
Related
If I have a single column integer PK on a SQLite table - do I need to create explicit index or is it take care of by the engine?
Same question for string/text single field PK
From UNIQUE constraints:
In most cases, UNIQUE and PRIMARY KEY constraints are implemented by
creating a unique index in the database. (The exceptions are INTEGER
PRIMARY KEY and PRIMARY KEYs on WITHOUT ROWID tables.)
If a column is defined as INTEGER PRIMARY KEY it is actually an alias of the rowid of the column:
The data for rowid tables is stored as a B-Tree structure containing
one entry for each table row, using the rowid value as the key. This
means that retrieving or sorting records by rowid is fast. Searching
for a record with a specific rowid, or for all records with rowids
within a specified range is around twice as fast as a similar search
made by specifying any other PRIMARY KEY or indexed value.
The exception mentioned above is that if the declaration of a column with
declared type "INTEGER" includes an "PRIMARY KEY DESC" clause, it does
not become an alias for the rowid and is not classified as an integer
primary key.
Ie, are the following two SQL statements equivalent in SQLite?
CREATE TABLE posts (
id INTEGER PRIMARY KEY
);
CREATE TABLE posts (
id INTEGER PRIMARY KEY ASC
);
Yes they are.
There is no need to specify ASC and beware that if you were to specify DESC, then NO they are then not equivalent (see 4 below) as id INTEGER PRIMARY KEY DESC is an exclusion to the column being an alias of the rowid column as per :-
The exception mentioned above is that if the declaration of a column
with declared type "INTEGER" includes an "PRIMARY KEY DESC" clause, it
does not become an alias for the rowid and is not classified as an
integer primary key. This quirk is not by design. It is due to a bug
in early versions of SQLite. But fixing the bug could result in
backwards incompatibilities. Hence, the original behavior has been
retained (and documented) because odd behavior in a corner case is far
better than a compatibility break.
ROWIDs and the INTEGER PRIMARY KEY
You can use id INTEGER, PRIMARY KEY(id, DESC), but still the order defaults to ASC when retrieving the column as it is an alias of the rowid (see 5 below )
Perhaps consider the following :-
DROP TABLE IF EXISTS posts1;
CREATE TABLE posts1 (
id INTEGER PRIMARY KEY
);
DROP TABLE IF EXISTS posts2;
CREATE TABLE posts2 (
id INTEGER PRIMARY KEY ASC
);
DROP TABLE IF EXISTS posts3;
CREATE TABLE posts3 (
id INTEGER PRIMARY KEY DESC
);
DROP TABLE IF EXISTS posts4;
CREATE TABLE posts4 (
id INTEGER, PRIMARY KEY (id DESC)
);
INSERT INTO posts1 VALUES(null),(null),(null);
INSERT INTO posts2 VALUES(null),(null),(null);
INSERT INTO posts3 VALUES(null),(null),(null);
INSERT INTO posts4 VALUES(null),(null),(null);
SELECT * FROM sqlite_master WHERE name LIKE '%posts%';
SELECT * FROM posts1;
SELECT * FROM posts2;
SELECT * FROM posts3;
SELECT * FROM posts4;
Results
1
The query SELECT * FROM sqlite_master WHERE name LIKE '%posts%'; results in :-
As you can see posts3 is significantly different as the index sqlite_autoindex_posts3_1 has been created
The others do not have a specific index created as the id column is an alias of the rowid column
The data for rowid tables is stored as a B-Tree structure containing
one entry for each table row, using the rowid value as the key. This
means that retrieving or sorting records by rowid is fast. Searching
for a record with a specific rowid, or for all records with rowids
within a specified range is around twice as fast as a similar search
made by specifying any other PRIMARY KEY or indexed value.
ROWIDs and the INTEGER PRIMARY KEY
2
The query SELECT * FROM posts1; results in :-
3
The query SELECT * FROM posts2;, confirms the initial YES answer as per :-
4
The query SELECT * FROM posts3;, may be a little confusing, but shows that id INTEGER PRIMARY KEY DESC does not result in an alias of the rowid and in the case of no value or null being inserted into the column, the value is null rather than an auto generated value. There is no UNIQUE constraint conflict (as nulls are considered as being different values).
5
The query SELECT * FROM posts4; produces the same result as for 1 and 2 even though id INTEGER, PRIMARY KEY (id DESC) was used. Confirming that even if DESC is applied via the column definition that the sort order is still defaults to ASC (unless the ORDER BY clause is used).
Note that this peculiarity is specific to the rowid column or an alias thereof.
See both https://www.sqlite.org/lang_createtable.html#rowid and https://www.sqlite.org/lang_createindex.html for a more complete answer. Shawn's link is specific to INTEGER PRIMARY KEY which matches the example code, but the more general question is not answered explicitly in either location, but can be deduced by reading both.
Under SQL Data Constraints, the first link says
In most cases, UNIQUE and PRIMARY KEY constraints are implemented by creating a unique index in the database. (The exceptions are INTEGER PRIMARY KEY and PRIMARY KEYs on WITHOUT ROWID tables.)
The CREATE INDEX page explains that originally the sort order was ignored and all indices were generated in ascending order. Only as of version 3.3.0 is the DESC order "understood". But even that description is somewhat vague, however altogether it is apparent that ASC is the default.
I have a problem with a sqlite command.
I have a table with three columns: Id, user, number.
The id is continuing. Now if I put a user and a number inside my list, my app should compare if such a user with this number already exist. The problem is, if I use a standard "insert or ignore" command, the Id column is not fixed, so I will get a new entry every time.
So is it possible just two compare two of three columns if they are equal?
Or do I have to use a temporary list, where are only two columns exist?
The INSERT OR IGNORE statement ignores the new record if it would violate a UNIQUE constraint.
Such a constraint is created implicitly for the PRIMARY KEY, but you can also create one explicitly for any other columns:
CREATE TABLE MyTable (
ID integer PRIMARY KEY,
User text,
Number number,
UNIQUE (User, Number)
);
You shouldn't use insert or ignore unless you are specifying the key, which you aren't and in my opinion never should if your key is an Identity (Auto number).
Based on User and Number making a record in your table unique, you don't need the id column and your primary key should be user,number.
If for some reason you don't want to do that, and bearing in mind in that case you are saying that User,Number is not your uniqueness constraint then something like
if not exists(Select 1 From MyTable Where user = 10 and Number = 15)
Insert MyTable(user,number) Values(10,15)
would do the job. Not a SqlLite boy, so you might have to rwiddle with the syntax and wrap escape your column names.
I have a table with unique usernames and a bunch of string data I am keeping track of. Each user will have 1000 rows and when I select them I want to return them in the order they were added. Is the following code a necessary and correct way of doing this:
CREATE TABLE foo (
username TEXT PRIMARY KEY,
col1 TEXT,
col2 TEXT,
...
order_id INTEGER NOT NULL
);
CREATE INDEX foo_order_index ON foo(order_id);
SELECT * FROM foo where username = 'bar' ORDER BY order_id;
Add a DateAdded field and default it to the date/time the row was added and sort on that.
If you absolutely must use the order_ID, which I don't suggest. Then at least make it an identity column. The reason I advise against this is because you are relying on side affects to do your sorting and it will make your code harder to read.
If each user will have 1000 rows, then username should not be the primary key. One option is to use the int identity column which all tables have (which optimizes I/O reads since it's typically stored in that order).
Read under "RowIds and the Integer Primary Key" # http://www.sqlite.org/lang_createtable.html
The data for each table in SQLite is stored as a B-Tree structure
containing an entry for each table row, using the rowid value as the
key. This means that retrieving or sorting records by rowid is fast.
Because it's stored in that order in the B-tree structure, it should be fast to order by the int primary key. Make sure it's an alias for rowid though - more in that article.
Also, if you're going to be doing queries where username = 'bob', you should consider an index on the username column - especially there's going to be many users which makes the index effective because of high selectivity. In contrast, adding an index on a column with values like 1 and 0 only leads to low selectivity and renders the index very ineffective. So, if you have 3 users :) it's not worth it.
You can remove the order_id column & index entirely (unless you need them for something other than this sorting).
SQLite tables always have a integer primary key - in this case, your username column has silently been made a unique key, so the table only has the one integer primary key. The key column is called rowid. For your sorting purpose, you'll want to explicitly make it AUTOINCREMENT so that every row always has a higher rowid than older rows.
You probably want to read http://www.sqlite.org/autoinc.html
CREATE TABLE foo (
rowid INTEGER PRIMARY KEY AUTOINCREMENT,
username TEXT UNIQUE KEY,
...
Then your select becomes
select * from foo order by rowed;
One advantage of this approach is that you're re-using the index SQLite will already be placing on your table. A date or order_id column is going to mean an extra index, which is just overhead here.
I have an SQLite table that I need to sort. I am familiar with the ORDER BY command but this is not what I am trying to accomplish. I need the entire table sorted within the database.
Explanation:
My table uses a column called rowed which sets the order of the table (a key?). I need to sort the table by another column called name and then re-assign rowid numbers in alphabetical order according to name. Can this be done?
Assuming you created your original table like so:
CREATE TABLE my_table (rowid INTEGER PRIMARY KEY, name TEXT, somedata TEXT) ;
You can create another sorted table like so:
CREATE TABLE my_ordered_table (rowid INTEGER PRIMARY KEY, name TEXT, somedata TEXT) ;
INSERT INTO my_ordered_table (name, somedata) SELECT name,somedata FROM my_table
ORDER BY name ;
And if you then want to replace the original table:
DROP TABLE my_table ;
ALTER TABLE my_ordered_table RENAME TO my_table;
I think this issue relates to wanting the primary key to mean something. Avoid that trap. Choose an arbitrarily generated primary key that uniquely identifies a row of data and has no other meaning. Otherwise you will eventually run into the problem of wanting to alter the primary key values to preserve the meaning.
For a good explanation of why you should rely on ORDER BY to retrieve the data in the desired order instead of assuming the data will otherwise appear in a sequence determined by the primary key see Cruachan's answer to a similar question