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