SQL constraint Usage

NOT null: Non-empty

Default: Defaults;

Unique: only value;

Primary key: Primary key;

FOREIGN key: foreign key;

Check: Inspection conditions;

Constraints with parameters are not required at the back: NOT NULL

A constraint with parameters is required after (the keyword is appended with parentheses): Primary key, foreign key, check, unique

No parentheses are required after the constraint: default

CREATE TABLE Test1 (
UID INT not NULL,
' Name ' VARCHAR (50),
' Password ' VARCHAR (a) not NULL DEFAULT ' 0 ',
class_id INT not NULL,
UNIQUE (UID),
PRIMARY KEY (UID),
FOREIGN KEY (class_id) REFERENCES class_table (class_id),
CHECK (class_id < 1000)

);

SQL constraint Usage