Oracle database, non-null constraint, PRIMARY KEY constraint, FOREIGN KEY constraint, UNIQUE constraint

Non-null constraint: When the column is set, the default can be empty, the setting data cannot be empty after removing the checkmark;

Unique constraint: Set in key, UNIQUE constraint name, type unique, column name; After the application is complete, this column data is unique; data is not repeatable

PRIMARY KEY constraint: The primary key column automatically has a non-null constraint and a unique constraint, and a table can have only one PRIMARY KEY constraint (automatically generate indexes to improve query efficiency)

Oracle database, non-null constraint, PRIMARY KEY constraint, FOREIGN KEY constraint, UNIQUE constraint