Constraints on building tables

1. Non-null constraints

The ① field cannot be null

②null not equal to ""; Oracle does not allow "" to be written to a non-empty string field

2. Unique constraints

① specifies that data for one or more columns is unique

② A table can have multiple unique constraints

③unique

3. PRIMARY KEY constraints

① to specify a column or columns of the primary key for multiple columns

② primary key columns must have both a non-null constraint and a UNIQUE constraint

③ A table can have only one PRIMARY KEY constraint

④ the role of the PRIMARY KEY constraint: The PRIMARY KEY constraint is able to determine a unique row of data, automatically generate indexes according to the primary key column, improve the efficiency of data retrieval, ensure the uniqueness and standardization of data

How to ⑤ PRIMARY KEY constraints:

A. Unique Code fields

B. Auto-growing integer value field: Oracle is implemented by ordinal column; Other databases have fields that support self-growing types; suitable for single databases; not for distributed databases

C. GUI D globally uniquely identifies: 32-bit-length 16-in-one string; Suitable for distributed database

4. Check constraints

By checking the rule statement: the size of a qualified value; A list of qualified values

5. FOREIGN KEY constraints

Constraint relationships between two tables

Constraints on building tables