Many-To-Many
In order to support a child having zero or more dogs and a dog belonging to zero or more children, your database table structure needs to support a Many-To-Many relationship. This requires three tables:
CREATE TABLE child (
id INTEGER PRIMARY KEY AUTOINCREMENT,
name TEXT
);
CREATE TABLE dog (
id INTEGER PRIMARY KEY AUTOINCREMENT,
dog TEXT
);
CREATE TABLE child_dog {
child_id INTEGER,
dog_id INTEGER,
FOREIGN KEY(child_id) REFERENCES child(id),
FOREIGN KEY(dog_id) REFERENCES dog(id)
};
How to Insert
An insert into each of the three tables must be separate SQL statements, but can take place in the context of same transaction. Inserts into the child_dog table (known as the mapping table) must happen after inserts into the child and dog tables. This is for two related reasons:
- You need to know the identifiers of both the child and the dog in order to
to insert into this table.
- Because of the foreign key constraint, an insert into the child_dog table would fail if the child and/or dog referenced do not exist in the database or transaction.
Here are some example SQL statements for insert:
INSERT INTO child VALUES(NULL, 'bobby');
SELECT last_insert_rowid(); -- gives the id of bobby, assume 2 for this example
INSERT INTO dog VALUES(NULL, 'spot');
SELECT last_insert_rowid(); -- gives the id of spot, assume 4 for this example
INSERT INTO child_dog VALUES(2, 4);
Inserting In Python
Although your question did not mention python, there is a python tag on this question so I'll assume you want to know how to do this in python. The sqlite3 module in python provides a nice little shortcut which saves you from having to run the 'last_insert_rowid()' function explicitly.
# Import the sqlite3 module
import sqlite3
# Create a connection and cursor to your database
conn = sqlite3.connect('example.db')
c = conn.cursor()
# Insert bobby
c.execute("""INSERT INTO child VALUES(NULL, 'bobby')""")
# The python module puts the last row id inserted into a variable on the cursor
bobby_id = c.lastrowid
# Insert spot
c.execute("""INSERT INTO dog VALUES(NULL, 'spot')""")
spot_id = c.lastrowid
# Insert the mapping
c.execute("""INSERT INTO child_dog VALUES(?, ?)""", (bobby_id, spot_id));
# Commit
conn.commit()
conn.close()
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