OVERVIEW
The SQLite library is capable of parsing SQL foreign key constraints
supplied as part of CREATE TABLE statements, but it does not actually
implement them. However, most of the features of foreign keys may be
implemented using SQL triggers, which SQLite does support. This text
file describes a feature of the SQLite shell tool (sqlite3) that
extracts foreign key definitions from an existing SQLite database and
creates the set of CREATE TRIGGER statements required to implement
the foreign key constraints.
CAPABILITIES
An SQL foreign key is a constraint that requires that each row in
the "child" table corresponds to a row in the "parent" table. For
example, the following schema:
CREATE TABLE parent(a, b, c, PRIMARY KEY(a, b));
CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent(a, b));
implies that for each row in table "child", there must be a row in
"parent" for which the expression (child.d==parent.a AND child.e==parent.b)
is true. The columns in the parent table are required to be either the
primary key columns or subject to a UNIQUE constraint. There is no such
requirement for the columns of the child table.
At this time, all foreign keys are implemented as if they were
"MATCH NONE", even if the declaration specified "MATCH PARTIAL" or
"MATCH FULL". "MATCH NONE" means that if any of the key columns in
the child table are NULL, then there is no requirement for a corresponding
row in the parent table. So, taking this into account, the expression that
must be true for every row of the child table in the above example is
actually:
(child.d IS NULL) OR
(child.e IS NULL) OR
(child.d==parent.a AND child.e==parent.b)
Attempting to insert or update a row in the child table so that the
affected row violates this constraint results in an exception being
thrown.
The effect of attempting to delete or update a row in the parent table
so that the constraint becomes untrue for one or more rows in the child
table depends on the "ON DELETE" or "ON UPDATE" actions specified as
part of the foreign key definition, respectively. Three different actions
are supported: "RESTRICT" (the default), "CASCADE" and "SET NULL". SQLite
will also parse the "SET DEFAULT" action, but this is not implemented
and "RESTRICT" is used instead.
RESTRICT: Attempting to update or delete a row in the parent table so
that the constraint becomes untrue for one or more rows in
the child table is not allowed. An exception is thrown.
CASCADE: Instead of throwing an exception, all corresponding child table
rows are either deleted (if the parent row is being deleted)
or updated to match the new parent key values (if the parent
row is being updated).
SET NULL: Instead of throwing an exception, the foreign key fields of
all corresponding child table rows are set to NULL.
LIMITATIONS
Apart from those limitiations described above:
* Implicit mapping to composite primary keys is not supported. If
a parent table has a composite primary key, then any child table
that refers to it must explicitly map each column. For example, given
the following definition of table "parent":
CREATE TABLE parent(a, b, c, PRIMARY KEY(a, b));
only the first of the following two definitions of table "child"
is supported:
CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent(a, b));
CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent);
An implicit reference to a composite primary key is detected as an
error when the program is run (see below).
* SQLite does not support recursive triggers, and therefore this program
does not support recursive CASCADE or SET NULL foreign key
relationships. If the parent and the child tables of a CASCADE or
SET NULL foreign key are the same table, the generated triggers will
malfunction. This is also true if the recursive foreign key constraint
is indirect (for example if table A references table B which references
table A with a CASCADE or SET NULL foreign key constraint).
Recursive CASCADE or SET NULL foreign key relationships are *not*
detected as errors when the program is run. Buyer beware.
USAGE
The functionality is accessed through an sqlite3 shell tool "dot-command":
.genfkey ?--no-drop? ?--ignore-errors? ?--exec?
When this command is run, it first checks the schema of the open SQLite
database for foreign key related errors or inconsistencies. For example,
a foreign key that refers to a parent table that does not exist, or
a foreign key that refers to columns in a parent table that are not
guaranteed to be unique. If such errors are found and the --ignore-errors
option was not present, a message for each one is printed to stderr and
no further processing takes place.
If errors are found and the --ignore-errors option is passed, then
no error messages are printed. No "CREATE TRIGGER" statements are generated
for foriegn-key definitions that contained errors, they are silently
ignored by subsequent processing.
All triggers generated by this command have names that match the pattern
"genfkey*". Unless the --no-drop option is specified, then the program
also generates a "DROP TRIGGER" statement for each trigger that exists
in the database with a name that matches this pattern. This allows the
program to be used to upgrade a database schema for which foreign key
triggers have already been installed (i.e. after new tables are created
or existing tables dropped).
Finally, a series of SQL trigger definitions (CREATE TRIGGER statements)
that implement the foreign key constraints found in the database schema are
generated.
If the --exec option was passed, then all generated SQL is immediately
executed on the database. Otherwise, the generated SQL strings are output
in the same way as the results of SELECT queries are. Normally, this means
they will be printed to stdout, but this can be configured using other
dot-commands (i.e. ".output").
The simplest way to activate the foriegn key definitions in a database
is simply to open it using the shell tool and enter the command
".genfkey --exec":
sqlite> .genfkey --exec