Does SQLite indeed have a limitation that it is not possible to retrieve the name of a foreign key? I am asking because I couldn't find this limitation mentioned anywhere in their documentation.
For example, I run the following script:
CREATE TABLE
users (
id INTEGER NOT NULL PRIMARY KEY,
first_name TEXT NOT NULL,
last_name TEXT NOT NULL
) ;
CREATE TABLE
orders (
id INTEGER NOT NULL PRIMARY KEY,
user_id INTEGER NOT NULL,
CONSTRAINT fk_users FOREIGN KEY (user_id) REFERENCES users(id)
) ;
Now I would like to check that the key "fk_users" was created indeed, so I run the following PRAGMA:
PRAGMA foreign_key_list(orders);
I would expect to see the name of my foreign key in the first column, but I am seeing some "0" value instead. Moreover, if I create multiple foreign keys with custom names, they are all called either "0" or "1".
Is this indeed a limitation of SQLite, or am I missing something?
There is no mechanism to extract the constraint name.
The table sqlite_master stores a CREATE command in the column "sql". You could query that command and do some parsing to extract the name of the foreign key. An example for a combined foreign key that works for me:
SELECT sql FROM sqlite_master WHERE name = 'song'
yields
CREATE TABLE "song" (
"songid" INTEGER,
"songartist" TEXT,
"songalbum" TEXT,
"songname" TEXT,
CONSTRAINT "fk__song_album" FOREIGN KEY ("songartist", "songalbum") REFERENCES "album" ("albumartist", "albumname")
)
and contains the name "fk__song_album" of the foreign key.
If one alters the foreign key with a query, the content of the sql column is modified/updated:
The text in the sqlite_master.sql column is a copy of the original CREATE statement text that created the object, except normalized as described above and as modified by subsequent ALTER TABLE statements. The sqlite_master.sql is NULL for the internal indexes that are automatically created by UNIQUE or PRIMARY KEY constraints.
https://www.sqlite.org/fileformat2.html
Extra tip:
In order to see the foreign key information in Navicat (Lite) ... right click on a table and choose "Design table". Then select the foreign keys tab.
Related
So i am trying to complete finance. Following is the .schema:
sqlite> .schema
CREATE TABLE users (id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, username TEXT NOT NULL, hash TEXT NOT NULL, cash NUMERIC NOT NULL DEFAULT 10000.00);
CREATE TABLE sqlite_sequence(name,seq);
CREATE TABLE history(
symbol TEXT, name TEXT, shares INTEGER, price NUMERIC, time DATETIME
);
CREATE UNIQUE INDEX username ON users (username);
When i try to add foriegn key to history table it always return error. Here is my code:
sqlite> ALTER TABLE history ADD COLUMN id INT;
sqlite> ALTER TABLE history ADD FOREIGN KEY(id) REFRENCES users(id);
Parse error: near "FOREIGN": syntax error
ALTER TABLE history ADD FOREIGN KEY(id) REFRENCES users(id);
^--- error here
I think based on what I see in the sqlite docs that the statement should be together with the ADD column:
ALTER TABLE history ADD COLUMN id INTEGER REFERENCES users(id);
But you please check me on this syntax! Another option is to take care of creating the constraint at the same time that you create the table.
CREATE TABLE history(
symbol TEXT,
name TEXT,
shares INTEGER,
price NUMERIC,
time DATETIME,
id INTEGER,
FOREIGN KEY (id)
REFERENCES users (id));
It might not be something you have realized (yet) but every database has its unique flavor of SQL, so despite there being a SQL standard there are often little differences in the syntax of SQL for specific db implementations. So you always have to beware of this when looking up commands for your sql db.
Further detail on Sqlite foreign key constraints can be found here:
https://www.sqlitetutorial.net/sqlite-foreign-key/
I keep getting an error "Incorrect index name 'f7'" using MySQL and I've narrowed it down to the following:
First I create the table,
CREATE TABLE testTable (
id INTEGER PRIMARY KEY AUTO_INCREMENT,
f7 INTEGER NOT NULL,
FOREIGN KEY (f7) REFERENCES testTable2 (id) ON DELETE CASCADE ON UPDATE CASCADE,
) ENGINE=InnoDB;
And then elsewhere,
ALTER TABLE testTable ADD UNIQUE f7;
This has led me to believe that this has to do with a duplicate index (?) I just can't figure out how to fix it. Many thanks.
Give it a name, so it doesn't conflict with the foreign Key index
ALTER TABLE `testtable` ADD UNIQUE INDEX `foo` (`f7`);
An incorrect index name error is given when you're attempting to create a new index with the same name as an existing index.
In MySQL, when you create a foreign key, as you're doing with FOREIGN KEY (f7) REFERENCES testTable2 (id) ON DELETE CASCADE ON UPDATE CASCADE, an index is auto-created as well. In this case, the name is defaulted to f7.
The foreign key is created as a non-unique index; your second command: ALTER TABLE testTable ADD UNIQUE (f7); will make this index unique - not add a second one.
To verify what indexes already exist on the table, you can use the following:
SHOW INDEXES FROM testTable;
If you're receiving this error, there is likely additional code elsewhere that is attempting to create an index named f7. You can attempt to find it, or change your CREATE TABLE syntax to name the key something different so that it doesn't cause conflicts:
FOREIGN KEY fk_testTable_f7 (f7) REFERENCES testTable2 (id) ON DELETE CASCADE ON UPDATE CASCADE
In this example, I used fk_testTable_f7 and you should now have a non-unique index on the table named fk_testTable_f7. To make it unique, you can use your existing ALTER command as you want the column to be unique - not the foreign key itself.
I have a SQLite DB that I am trying to use with EF Core database first.
It has a table of users, and a table of groups that users can belong to, and it has a mapping table because users can belong to multiple groups.
-- holds users
CREATE TABLE IF NOT EXISTS user (
_id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
name TEXT,
);
-- holds groups users can belong to
CREATE TABLE IF NOT EXISTS group (
_id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
name TEXT,
);
-- holds user group membership
CREATE TABLE IF NOT EXISTS map_group_user (
group_id INTEGER,
user_id INTEGER,
UNIQUE (group_id,user_id) ON CONFLICT REPLACE,
FOREIGN KEY(group_id) REFERENCES group(_id) ON DELETE CASCADE,
FOREIGN KEY(user_id) REFERENCES user(_id) ON DELETE CASCADE
);
When I scaffold this up I get a warning from dotnet ef scaffold that it could not identify a primary key for map_group_user and it does not generate a model, and neither the User nor Group model contains any reference to the other (expected).
Try adding an explicit primary key to the map_group_user bridge table:
CREATE TABLE IF NOT EXISTS map_group_user (
group_id INTEGER,
user_id INTEGER,
UNIQUE (group_id,user_id) ON CONFLICT REPLACE,
FOREIGN KEY(group_id) REFERENCES "group"(_id) ON DELETE CASCADE,
FOREIGN KEY(user_id) REFERENCES user(_id) ON DELETE CASCADE,
PRIMARY KEY (group_id, user_id)
);
The logical primary key for the map_group_user table is the combination of group_id and user_id, each combination which should ideally appear only once.
By the way, please avoid naming your tables and columns using reserved SQL keywords, such as group. I don't know if this was giving you an error, but I have placed "group" in double quotes to escape it.
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
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".