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-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 |
- |