I'm creating a table with foreign key references. I'm wondering about the required syntax. Mostly I've seen the following (from http://www.sqlite.org/foreignkeys.html#fk_basics):
CREATE TABLE artist(
artistid INTEGER PRIMARY KEY,
artistname TEXT
);
CREATE TABLE track(
trackid INTEGER,
trackname TEXT,
trackartist INTEGER,
FOREIGN KEY(trackartist) REFERENCES artist(artistid)
);
However, from the same site (http://www.sqlite.org/foreignkeys.html#fk_actions) I see this:
CREATE TABLE artist(
artistid INTEGER PRIMARY KEY,
artistname TEXT
);
CREATE TABLE track(
trackid INTEGER,
trackname TEXT,
trackartist INTEGER REFERENCES artist(artistid) ON UPDATE CASCADE
);
The latter syntax is a little more concise, but I want to know if the result is somehow different (aside from the ON UPDATE CASCADE, which of course has an effect; I only included it because I copied the code exactly from the referenced site, and because I don't know that the above syntax doesn't apply only when making such a specification). I am working in Android, in case that matters.
This answer might not be related to yours but i thought it should be helpful for others who are working with android database.
IN SQLite Foreign key constraints are disabled by default (for backwards compatibility). You have to enable it explicitly using
PRAGMA foreign_keys = 1
after you establishing your connection with the database.
Here's the link to the official docs that explains it in more depth.
http://sqlite.org/foreignkeys.html
Please navigate to enabling foreign key support in the above link.
See the syntax diagrams.
The first syntax is a table constraint, while the second syntax is a column constraint.
In these examples, they behave the same.
You would need a table constraint for a key over multiple columns (where you do not have a single column you could attach it to).
Related
I create a table, lets name it CUSTOMERS in SQLite:
CREATE TABLE "CUSTOMERS" (
"tel" INTEGER NOT NULL,
"customer" VARCHAR ,
);
When I see the table from a GUI (I use SQLite Manager from Firefox), I noticed that there is an extra column rowid which is working like auto-increment. My question is, in tables where I don't use a primary key should I specify a column like:
ROWID INTEGER PRIMARY KEY AUTOINCREMENT
If I execute this query PRAGMA table_info(CUSTOMERS); I get only the two columns tel,customer.
Sqlite usually adds a rowid automatically as #laato linked in the comments SqLite : ROWID
That can be removed, but does not need to be specified. So there is no need to add it to the Create Table.
The hidden rowid allows delete's to be targetted at a single row, bu
t if you are using the ROWID as a specific foreign key, it would be better to name a column explicitly. That will then become a synonym with the rowid.
You can see here, as it was commented in your question.
However, if you are in a dilemma what to choose between these options, SQLite recommends that you should not use AUTOINCREMENT attribute because:
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.
More info you can read here.
I only use primary key integer ID for it's "auto-increment function".
What if I don't need an "auto-increment"? Do I still need primary key if I don't care the uniqueness of record?
Example: Lets compare this table:
create table if not exists `table1`
(
name text primary key,
tel text,
address text
);
with this:
create table if not exists `table2`
(
name text,
tel text,
address text
);
table1 applies primary key and table2 don't. Is there any bad thing happen to table2?
I don't need the record to be unique.
SQLite is a relational database system. So it's all about relations. You build relations between tables on keys.
You can have tables without a primary key; it is not necessary for a table to have a primary key. But you will almost always want a primary key to show what makes a record unique in that table and to build relations.
In your example, what would it mean to have two identical records? They would mean the same person, no? Then how would you count how many persons named Anna are in the database? If you count five, how many of them are unique, how many are mere duplicates? Such queries can be done properly, but get overly complicated because of the lacking primary key. And how would you build relations, say the cars a person drives? You would have a car table and then how to link it to the persons table in your example?
There are cases when you want a table without a primary key. These are usually log tables and the like. They are rare. Whenever you are creating a table without a primary key, ask yourself why this is the case. Maybe you are about to build something messy ;-)
You get auto-incrementing primary keys only when a column is declared as INTEGER PRIMARY KEY; other data types result in plain primary keys.
You are not required to declare a PRIMARY KEY.
But even if you do not do this, there will be some column(s) used to identify and look up records.
The PRIMARY KEY declaration helps to document this, enforces uniqueness, and optimizes lookups through the implicit index.
Using the C API, I don't see a way to determine the foreign key constraints for a named table?
Given this example:
CREATE TABLE artist(
artistid INTEGER PRIMARY KEY,
artistname TEXT
);
CREATE TABLE track(
trackid INTEGER,
trackname TEXT,
trackartist INTEGER,
FOREIGN KEY(trackartist) REFERENCES artist(artistid)
);
sqlite3_table_column_metadata() will tell you it's a primary key, autoincrement, etc. but how
do I get the foreign key constraints?
FOREIGN KEY(trackartist) REFERENCES artist(artistid)
I want to be able to get a list back for table "track" that there are foreign keys back to table Artist column artistid?
I don't see an api to do this? I need to do this programmaticlly upon opening the database, for purposes of aggregation.
Thanks.
After using PRAGMA foreign_key_list(Valuation);
I got back:
PRAGMA foreign_key_list(Valuation);
0|0|Stock|StockId|Id|NO ACTION|NO ACTION|NONE
I understand I need to split on the vertical bar, but what are the first two columns? 0|0 ?
Please note that (foreign) keys can consist of multiple columns, so it would not make sense to return this as column information.
To get information about a table's foreign keys, use this:
PRAGMA foreign_key_list(table-name);
This pragma returns one row for each foreign key constraint created by a REFERENCES clause in the CREATE TABLE statement of table "table-name".
Sorry for novice question.
I have created my tables using SQLite database browser, but:
I do not know how can I specify my foreign keys using the application?
How can I create a relationship diagram between tables?
I know this question has been asked long ago but I found it. Its built right into GUI. You just need to drag and make those Name, Type tabs little bit small to make space for the Foreign Key tab. Place your mouse pointer at the end and drag the header.
My version of SQLite Browser is Version 3.7.0.
I couldn't find a way of defining foreign key constraints using the "Database Structure" tab. I'd strongly recommend defining table definitions and constraints using a script rather than building them using the graphical editor - it makes it much easier to create new databases and to track changes to the schema.
By way of an example, assume we have two tables: one defining file names and one specifying the method used for compression, we can add a foreign key constraint to the file_definition table when defining it.
CREATE TABLE [compression_state] (
[compression_state_id] INTEGER PRIMARY KEY NOT NULL,
[value] TEXT NOT NULL
);
CREATE TABLE [file_definition] (
[file_id] INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
[compression_state_id] INTEGER NOT NULL,
[name] TEXT NOT NULL,
FOREIGN KEY(compression_state_id) REFERENCES compression_state(compression_state_id)
);
However, by default, SQLite will not enforce the constraint, therefore every time you connect to the database, you must issue the following command to enable constraint checking.
PRAGMA foreign_keys = ON;
Further details in the documentation.
If the tables already exist and you don't want to build a complete script then you're out of luck, SQLite doesn't support adding foreign keys once the table has been generated, see here: SQL Features That SQLite Does Not Implement
Go to edit table definition window
Click on Add field
Name it with Type : Integer
Scroll right and find Foreign Key column
Double click under Foreign Key column in new row
Select master table and its id field
Click OK
Click Write Changes
From the SQLite Documentation :
CREATE TABLE artist(
artistid INTEGER PRIMARY KEY,
artistname TEXT
);
CREATE TABLE track(
trackid INTEGER,
trackname TEXT,
trackartist INTEGER -- Must map to an artist.artistid!
);
and in the end :
CREATE TABLE track(
trackid INTEGER,
trackname TEXT,
trackartist INTEGER,
FOREIGN KEY(trackartist) REFERENCES artist(artistid)
);
In the DB Browser for SQLite Environment (v 3.8.0 - Sqlite v 3.9.2) when you add the DB fields for the track table along with
the PK ,AI and other columns you can find a Foreign Key Column.
In there , and for this example, you just add artist(artistid) in the trackartist row.
Then the foreign key constraint is created.
In DB Browser, in the Edit Table Definition window, you can double click the blank area of Foreign Key and a text box will activate. You can add your Foreign Key there.
it's really very easy , just do this
Go to edit table definition window
Right click on the table that you want to relate ( foreign table )
choose modify table
on Constraints tab select add constraints button and choose foreign key
you can relate tables here and then back to fields tab and do
Name it with Type : Integer
Scroll right and find Foreign Key column
Double click under Foreign Key column in new row
Select master table and its id field
Click OK
Click Write Changes
Triggers in SQLite3 enforces foreign key constraints. Link https://www.sqlite.org/cvstrac/wiki?p=ForeignKeyTriggers would help you out to solve your first question.
I am not sure whether this is entirely right but here's what I did:
I added the variable "UserID" in the fields tab an checked the box "primary key"
The I went to the constraints tab and added a foreign key Type constraint on the "UserID"
Then I went back to fields tab and double clicked on the foreign key field that opened and added the name of the table where that key is and the name of the variable
I know that SQLite does not enforce foreign keys natively, but that's not my primary concern. The question is: If I declare
CREATE TABLE invoice (
invoiceID INTEGER PRIMARY KEY,
clientID INTEGER REFERENCES client(clientID),
...
)
will sqlite at least use the information that clientID is a foreign key to optimize queries and automatically index invoice.clientID, or is this constraint a real no-op?
In the SQLite Documentation it says:
... "an index should be created on the child key columns of each foreign key constraint"
ie. the index is not automatically created, but you should create one in every instance.
Even if it is not actually a no-op (a data structure describing the constraint is added to the table), foreign key related statement doesn't create any index on involved columns.
Indexes are implicitly created only in the case of PRIMARY KEY and UNIQUE statements.
For more details, check it out build.c module on the sqlite source tree:
http://www.sqlite.org/cvstrac/rlog?f=sqlite/src/build.c https://www.sqlite.org/src/file?name=src/build.c&ci=tip