Sql updating tables with foreign keys
Sql updating tables with foreign keys - Granny free dating no registration
Indices are not required for child key columns but they are almost always beneficial.Returning to the example in section 1, each time an application deletes a row from the artist table (the parent table), it performs the equivalent of the following SELECT statement to search for referencing rows in the track table (the child table). in the above is replaced with the value of the artistid column of the record being deleted from the artist table (recall that the trackartist column is the child key and the artistid column is the parent key).
If OMIT_FOREIGN_KEY is defined, then foreign key definitions cannot even be parsed (attempting to specify a foreign key definition is a syntax error).
If the database schema contains foreign key errors that require looking at more than one table definition to identify, then those errors are not detected when the tables are created.
Instead, such errors prevent the application from preparing SQL statements that modify the content of the child or parent tables in ways that use the foreign keys.
Or, more generally: If this SELECT returns any rows at all, then SQLite concludes that deleting the row from the parent table would violate the foreign key constraint and returns an error.
Similar queries may be run if the content of the parent key is modified or a new row is inserted into the parent table.
Assuming the library is compiled with foreign key constraints enabled, it must still be enabled by the application at runtime, using the PRAGMA foreign_keys command.
For example: Foreign key constraints are disabled by default (for backwards compatibility), so must be enabled separately for each database connection.
Attempting to do so does not return an error; it simply has no effect.
Usually, the parent key of a foreign key constraint is the primary key of the parent table.
CREATE TABLE child6(p, q, FOREIGN KEY(p, q) REFERENCES parent(b, c)); -- Error!
CREATE TABLE child7(r REFERENCES parent(c)); -- Error!
CREATE TABLE child10(x,y,z, FOREIGN KEY(x,y,z) REFERENCES parent2); -- Error!