| Term | Definition |
|---|
| NOT NULL | enforces a column to NOT accept NULL values or enforces a field to always contain a value |
| UNIQUE | provide a guarantee for uniqueness for a column or set of columns; constraint uniquely identifies each record in a database table |
| PRIMARY KEY | provide a guarantee for uniqueness for a column or set of columns; automatically has a UNIQUE constraint defined on it; can only have one |
| ALTER TABLE table_name ADD UNIQUE column_name | to create a UNIQUE constraint on a column when the table is already created, |
| ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name) | to allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple column |
| ALTER TABLE table_name DROP CONSTRAINT constraint_name | to drop a UNIQUE constraint |
| ALTER TABLE table_name ADD PRIMARY KEY column_name | to create a PRIMARY KEY constraint on a column when the table is already created, |
| ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY (column_name) | to allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple column |
| ALTER TABLE table_name DROP CONSTRAINT primary key__name | to drop a PRIMARY KEY constraint |
| CREATE TABLE table_name (column_name1, NOT NULL, column_name2, UNIQUE (column_name)) | to create a table with a UNIQUE constraint on a column |
| CREATE TABLE table_name (column_name1, NOT NULL, column_name2, PRIMARY KEY (column_name)) | to create a table with a PRIMARY KEY constraint on a column |
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