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Engineering LibreTexts

15.9: Constraints in Database Tables

  • Page ID
    3262
  • As we design our table structures, we can tell the database system that we would like it to enforce a few rules on us. These rules help us from making mistakes and introducing incorrect data into out tables. When we create our tables:

    cur.execute('''CREATE TABLE IF NOT EXISTS People
        (id INTEGER PRIMARY KEY, name TEXT UNIQUE, retrieved INTEGER)''')
    cur.execute('''CREATE TABLE IF NOT EXISTS Follows
        (from_id INTEGER, to_id INTEGER, UNIQUE(from_id, to_id))''')

    We indicate that the name column in the People table must be UNIQUE. We also indicate that the combination of the two numbers in each row of the Follows table must be unique. These constraints keep us from making mistakes such as adding the same relationship more than once.

    We can take advantage of these constraints in the following code:

    cur.execute('''INSERT OR IGNORE INTO People (name, retrieved)
        VALUES ( ?, 0)''', ( friend, ) )

    We add the OR IGNORE clause to our INSERT statement to indicate that if this particular INSERT would cause a violation of the "name must be unique" rule, the database system is allowed to ignore the INSERT. We are using the database constraint as a safety net to make sure we don't inadvertently do something incorrect.

    Similarly, the following code ensures that we don't add the exact same Follows relationship twice.

    cur.execute('''INSERT OR IGNORE INTO Follows
        (from_id, to_id) VALUES (?, ?)''', (id, friend_id) )

    Again, we simply tell the database to ignore our attempted INSERT if it would violate the uniqueness constraint that we specified for the Follows rows.