1
2OVERVIEW
3
4 The SQLite library is capable of parsing SQL foreign key constraints
5 supplied as part of CREATE TABLE statements, but it does not actually
6 implement them. However, most of the features of foreign keys may be
7 implemented using SQL triggers, which SQLite does support. This text
8 file describes a feature of the SQLite shell tool (sqlite3) that
9 extracts foreign key definitions from an existing SQLite database and
10 creates the set of CREATE TRIGGER statements required to implement
11 the foreign key constraints.
12
13CAPABILITIES
14
15 An SQL foreign key is a constraint that requires that each row in
16 the "child" table corresponds to a row in the "parent" table. For
17 example, the following schema:
18
19 CREATE TABLE parent(a, b, c, PRIMARY KEY(a, b));
20 CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent(a, b));
21
22 implies that for each row in table "child", there must be a row in
23 "parent" for which the expression (child.d==parent.a AND child.e==parent.b)
24 is true. The columns in the parent table are required to be either the
25 primary key columns or subject to a UNIQUE constraint. There is no such
26 requirement for the columns of the child table.
27
28 At this time, all foreign keys are implemented as if they were
29 "MATCH NONE", even if the declaration specified "MATCH PARTIAL" or
30 "MATCH FULL". "MATCH NONE" means that if any of the key columns in
31 the child table are NULL, then there is no requirement for a corresponding
32 row in the parent table. So, taking this into account, the expression that
33 must be true for every row of the child table in the above example is
34 actually:
35
36 (child.d IS NULL) OR
37 (child.e IS NULL) OR
38 (child.d==parent.a AND child.e==parent.b)
39
40 Attempting to insert or update a row in the child table so that the
41 affected row violates this constraint results in an exception being
42 thrown.
43
44 The effect of attempting to delete or update a row in the parent table
45 so that the constraint becomes untrue for one or more rows in the child
46 table depends on the "ON DELETE" or "ON UPDATE" actions specified as
47 part of the foreign key definition, respectively. Three different actions
48 are supported: "RESTRICT" (the default), "CASCADE" and "SET NULL". SQLite
49 will also parse the "SET DEFAULT" action, but this is not implemented
50 and "RESTRICT" is used instead.
51
52 RESTRICT: Attempting to update or delete a row in the parent table so
53 that the constraint becomes untrue for one or more rows in
54 the child table is not allowed. An exception is thrown.
55
56 CASCADE: Instead of throwing an exception, all corresponding child table
57 rows are either deleted (if the parent row is being deleted)
58 or updated to match the new parent key values (if the parent
59 row is being updated).
60
61 SET NULL: Instead of throwing an exception, the foreign key fields of
62 all corresponding child table rows are set to NULL.
63
64LIMITATIONS
65
66 Apart from those limitiations described above:
67
68 * Implicit mapping to composite primary keys is not supported. If
69 a parent table has a composite primary key, then any child table
70 that refers to it must explicitly map each column. For example, given
71 the following definition of table "parent":
72
73 CREATE TABLE parent(a, b, c, PRIMARY KEY(a, b));
74
75 only the first of the following two definitions of table "child"
76 is supported:
77
78 CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent(a, b));
79 CREATE TABLE child(d, e, f, FOREIGN KEY(d, e) REFERENCES parent);
80
81 An implicit reference to a composite primary key is detected as an
82 error when the program is run (see below).
83
84 * SQLite does not support recursive triggers, and therefore this program
85 does not support recursive CASCADE or SET NULL foreign key
86 relationships. If the parent and the child tables of a CASCADE or
87 SET NULL foreign key are the same table, the generated triggers will
88 malfunction. This is also true if the recursive foreign key constraint
89 is indirect (for example if table A references table B which references
90 table A with a CASCADE or SET NULL foreign key constraint).
91
92 Recursive CASCADE or SET NULL foreign key relationships are *not*
93 detected as errors when the program is run. Buyer beware.
94
95USAGE
96
97 The functionality is accessed through an sqlite3 shell tool "dot-command":
98
99 .genfkey ?--no-drop? ?--ignore-errors? ?--exec?
100
101 When this command is run, it first checks the schema of the open SQLite
102 database for foreign key related errors or inconsistencies. For example,
103 a foreign key that refers to a parent table that does not exist, or
104 a foreign key that refers to columns in a parent table that are not
105 guaranteed to be unique. If such errors are found and the --ignore-errors
106 option was not present, a message for each one is printed to stderr and
107 no further processing takes place.
108
109 If errors are found and the --ignore-errors option is passed, then
110 no error messages are printed. No "CREATE TRIGGER" statements are generated
111 for foriegn-key definitions that contained errors, they are silently
112 ignored by subsequent processing.
113
114 All triggers generated by this command have names that match the pattern
115 "genfkey*". Unless the --no-drop option is specified, then the program
116 also generates a "DROP TRIGGER" statement for each trigger that exists
117 in the database with a name that matches this pattern. This allows the
118 program to be used to upgrade a database schema for which foreign key
119 triggers have already been installed (i.e. after new tables are created
120 or existing tables dropped).
121
122 Finally, a series of SQL trigger definitions (CREATE TRIGGER statements)
123 that implement the foreign key constraints found in the database schema are
124 generated.
125
126 If the --exec option was passed, then all generated SQL is immediately
127 executed on the database. Otherwise, the generated SQL strings are output
128 in the same way as the results of SELECT queries are. Normally, this means
129 they will be printed to stdout, but this can be configured using other
130 dot-commands (i.e. ".output").
131
132 The simplest way to activate the foriegn key definitions in a database
133 is simply to open it using the shell tool and enter the command
134 ".genfkey --exec":
135
136 sqlite> .genfkey --exec
137
138