The snippet library.
Small, practical examples. Search by task or narrow things down below.
Apply a default when an insert omits a column
SQL: A default supplies an omitted value; an explicit NULL still follows the column’s nullability rules.
SQL✓ Tested
Block deletion of a referenced parent
SQL: The default foreign-key action prevents deletion while a child still refers to the parent.
SQL✓ Tested
Enforce a bounded numeric range
SQL: A named CHECK constraint keeps numeric values inside an inclusive interval.
SQL✓ Tested
Keep children while clearing a deleted relationship
SQL: ON DELETE SET NULL requires a nullable foreign-key column.
SQL✓ Tested
Make a pair of columns unique together
SQL: A composite primary key permits repeated individual values while forbidding a duplicate combination.
SQL✓ Tested
Reference a parent identified by two columns
SQL: A composite foreign key checks the complete referenced key rather than either column independently.
SQL✓ Tested
Reject a negative quantity with CHECK
SQL: Combine NOT NULL and CHECK because an unknown NULL check result is normally accepted.
SQL✓ Tested
Reject duplicate business identifiers
SQL: A UNIQUE constraint protects a business key independently of the primary key.
SQL✓ Tested
Remove dependent rows with a declared cascade
SQL: ON DELETE CASCADE removes children when their referenced parent is deleted.
SQL✓ Tested
Require a value with NOT NULL
SQL: Declare required fields at the database boundary, not only in application validation.
SQL✓ Tested