I am using the DB Browser for SQLite to try and figure this out. I've opened Northwind.sqlite and in it it shows me the following for a table:
CREATE TABLE `Order Details` (
`OrderID` int,
`ProductID` int,
`UnitPrice` float ( 26 ),
`Quantity` int,
`Discount` float ( 13 ),
PRIMARY KEY(`OrderID`,`ProductID`)
);
However, in the Sql Server Northwind OrderID and ProductID are foreign keys, not primary keys. Does this work differently in SQLite? And if so, how do the relationships work?
thanks - dave
The above will create a table that has no FOREIGN keys but 2 indexes.
One a largely hidden index according to rowid.
The other, PRIMARY KEY(OrderID,ProductID) will be an index according to the combination of OrderId and ProductID.
some things about rowid (aka id)
rowid is an automatically created column called rowid (it can also be referenced using oid or rowid (case independent)) and if present is really the primary key.
rowid will be a unique signed integer using up to 64 bits. The lowest value and also the first value will be 1, the highest value being 9223372036854775807.
In later versions of SQLite 3.8.2 on the WITHOUT ROWID keyword was added to allow suppression of the rowid column/index (your Order Details table may benefit being a without rowid table).
if a column is defined with the type INTEGER PRIMARY KEY or INTEGER PRIMARY KEY AUTOINCREMENT then that column (there can only be 1 such column per table) is an alias of for the rowid column.
AUTOINCREMENT introduces a rule that when inserting a row the rowid must be greater than any that exist or existed.
It DOES NOT guarantee that the rowid will monotonically increase, although generally the id will (even without AUTOINCREMENT (perhaps the most misused/misunderstood keyword in SQLite)).
Without AUTOINCREMENT SQlite may find a lower rowid and use that, but not until a rowid of 9223372036854775807 has been reached.
AUTOINCREMENT, if a rowid of 9223372036854775807 has been reached will is an SQLITE_FULL exception.
AUTOINCREMENT results in overheads (e.q. a table named sqlite_sequence is then maintained recording the highest given sequence number). The documentation recommends that it not be used unless required, which is rarely the case.
Some limited testing I did resulted in an 8-12% greater processing time for AUTOINCREMENT. What are the overheads of using AUTOINCREMENT for SQLite on Android?
For more about rowid see SQLite Autoincrement and also Clustered Indexes and the WITHOUT ROWID Optimization
Coding PRIMARY KEY (if not on an INTEGER column i.e. not an alias of rowid) implies a UNIQUE constraint. It is not saying/checking that the value or any of the values in a clustered index exists in any other table.
Note null is not considered to be the same value, so in your Order Details table it is possible to have any combination of the values as null.
Coding a FOREIGN KEY introduces a constraint that the referenced value(s) must exist in the respective table/column. Additionally :-
Usually, the parent key of a foreign key constraint is the primary key
of the parent table. If they are not the primary key, then the parent
key columns must be collectively subject to a UNIQUE constraint or
have a UNIQUE index. If the parent key columns have a UNIQUE index,
then that index must use the collation sequences that are specified in
the CREATE TABLE statement for the parent table.
SQLite Foreign Key Support
Considering all of this you may want to do make some changes to the Order Details table :-
You could make it a WITHOUT ROWID table.
You could make both the OrderID and the ProductID columns NOT NULL.
You could add FOREIGN KEY's to both the OrderID and the ProductID columns.
So perhaps you could have :-
CREATE TABLE `Order Details` (
`OrderID` int NOT NULL REFERENCES `Orders` (`OrderId`), -- ADDED NOT NULL and FKEY
`ProductID` int NOT NULL REFERENCES `Products`(`ProductId`) , -- ADDED NOT NULL and FKEY
`UnitPrice` float ( 26 ),
`Quantity` int,
`Discount` float ( 13 ),
PRIMARY KEY(`OrderID`,`ProductID`)
)
WITHOUT ROWID -- ADDED WITHOUT ROWID
;
The above uses column constraints
Alternately, utilising TABLE constraints, you could do :-
CREATE TABLE `Order Details` (
`OrderID` int NOT NULL, -- ADDED NOT NULL
`ProductID` int NOT NULL, -- ADDED NOT NULL
`UnitPrice` float ( 26 ),
`Quantity` int,
`Discount` float ( 13 ),
PRIMARY KEY(`OrderID`,`ProductID`),
FOREIGN KEY (`OrderId`) REFERENCES `Orders`(`OrderId`), -- ADDED FKEY AS TABLE CONSTRAINT
FOREIGN KEY (`ProductID`) REFERENCES `Products`(`ProductID`) -- ADDED FKEY AS TABLE CONSTRAINT
)
WITHOUT ROWID -- ADDED WITHOUT ROWID
;
Both have the same outcome, the only difference being where the FOREIGN KEY constraints are defined.
Both the above assumes that the referenced tables are Orders and Products.
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.
SQLite's documentation says:
A UNIQUE constraint is similar to a PRIMARY KEY constraint, except that a single table may have any number of UNIQUE constraints.
What I'm wondering is, if I declare something like:
CREATE TABLE Example (
id INTEGER PRIMARY KEY UNIQUE);
Does SQLite create two indexes or one? Is the behavior different if I'm not using the rowid (i.e. if the column was id TEXT PRIMARY KEY UNIQUE)?
I realize the simplest thing to do is just remove UNIQUE but I'm curious what effect this will have.
When you define a primary key it will be unique, no need to define another index for unique column.
The point of primary keys is so that they are unique for any given row.
Meaning you can choose one or more fields to be a primary key, but a single field doesn't have to be unique.
Unique on the other hand has to be unique for the specified fields.
So it's more like a constraint you are placing.
Given this example:
CREATE TABLE IF NOT EXISTS user2domain (
userID INTEGER NOT NULL,
domainID INTEGER NOT NULL,
PRIMARY KEY (userID, domainID),
UNIQUE(userID, domainID) ON CONFLICT IGNORE
)
You don't actually have to specify the unique constraint as the primary key would cover the same fields.
CREATE TABLE IF NOT EXISTS user2domain (
userID INTEGER NOT NULL,
domainID INTEGER NOT NULL,
PRIMARY KEY (userID, domainID)
)
-- add this way
INSERT OR IGNORE INTO user2domain(userID, domainID) VALUES(#userID, #domainID)
This would be enough.
Use the Unique constraint when you absolutely feel like any fields
must be unique but not necessarily a row identifier.
The expected behaviour is overwriting. Primary is of a higher order precedence.
So I'm making things complicated ...I think. A primary key basically is to make the row unique. Is that correct? Anyone want to show me an insert statement with the values for PK?
The SQLite documentation says:
On an INSERT, if the ROWID or INTEGER PRIMARY KEY column is not
explicitly given a value, then it will be filled automatically with an
unused integer, usually one more than the largest ROWID currently in
use. This is true regardless of whether or not the AUTOINCREMENT
keyword is used.
So, on a table like
CREATE TABLE test(id INTEGER PRIMARY KEY, descr TEXT);
an insert with a valid id could be
INSERT INTO test(descr) VALUES('this is a test');
A primary key, also called a primary keyword, is a key in a relational database that is unique for each record. It is a unique identifier, such as a driver license number, telephone number (including area code), or vehicle identification number (VIN). A relational database must always have one and only one primary key.
if you are using CREATE TABLE, if you are creating the primary key on a single field, you can use:
CREATE TABLE mytable (
field1 TEXT,
field2 INTEGER PRIMARY KEY,
field3 BLOB,
);
Reference more at: https://www.sqlite.org/lang_createtable.html & http://sqlite.org/faq.html#q11
This is my code:
CREATE TABLE table(
id int(1) UNIQUE,
name varchar(128) NOT NULL)
ALTER TABLE table ADD UNIQUE KEY id (id);
I'm not sure if I need the second statement or not, but either way it doesn't work when inserting INSERT INTO table( name ) VALUES( "test" ). It says table.id may not be NULL. What do I need to do to make it work with NULL(and Auto Increment?)
Replacing the first column with id int(1) NOT NULL AUTO_INCREMENT, makes it say syntax error.
The column must be an INTEGER PRIMARY KEY for autoincrement behavior.
The AUTOINCREMENT keyword imposes extra CPU, memory, disk space, and disk I/O overhead and should be avoided if not strictly needed. It is usually not needed.
In SQLite, a column with type INTEGER PRIMARY KEY is an alias for the ROWID (except in WITHOUT ROWID tables) which is always a 64-bit signed integer
On an INSERT, if the ROWID or INTEGER PRIMARY KEY column is not explicitly given a value, then it will be filled automatically with an unused integer ..
If the AUTOINCREMENT keyword appears after INTEGER PRIMARY KEY, that changes the automatic ROWID assignment algorithm to prevent the reuse of ROWIDs over the lifetime of the database. In other words, the purpose of AUTOINCREMENT is to prevent the reuse of ROWIDs from previously deleted rows.
Unique is insufficient to trigger this behavior; and using AUTOINCREMENT is required to ensure a monotonic increment scheme.
Thus, to expose the surrogate ROWID without a guarantee that the ID will never be reused, use:
id INTEGER PRIMARY KEY
To ensure the ID is never reused over the table/database lifetime (which is the MySQL autoincrement and SQL Server identity column behavior), use:
id INTEGER PRIMARY KEY AUTOINCREMENT
No other types or can be used for this behavior - this includes INT, even though it ends up with the same affinity.