1 2 OVERVIEW 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 13 CAPABILITIES 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 64 LIMITATIONS 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 95 USAGE 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
