Integrity constraints (Chapter Summary), integrity constraints Chapter Summary

Source: Internet
Author: User

Integrity constraints (Chapter Summary), integrity constraints Chapter Summary
1. The Integrity Constraint of the database is a check measure for data updating. One or more constraints exist on a data table.

2. constraints are divided into five types: non-NULL constraint (not null), UNIQUE constraint (UNIQUE), primary key constraint (primary key), CHECK constraint (CHECK ), foreign key constraint (foreign key ).

3. When defining constraints, you can use the CONSTRAINT keyword to set the CONSTRAINT name.

4. When setting foreign key constraints, you can perform cascade data update operations as follows.
On delete cascade: when the primary table data is deleted, the corresponding sub-table data is also deleted;
On delete set null: when the primary table data is deleted, the corresponding sub-table data is SET to null.

5. constraints can be set when the table is defined, or you can add constraints separately for existing data tables. The ALTER command is used to modify the constraints.

6. When a constraint is temporarily not in use, you can set it to disabled and then re-enable it.
What are the differences between the column-level integrity constraints and the table-level integrity constraints?

When creating a table, you can also define integrity constraints related to the table. These integrity constraints are stored in the system's data dictionary, when you operate data in a table, DBMS automatically checks whether the operation violates these integrity constraints. If the integrity constraints involve multiple attribute columns of the table, they must be defined at the table level. Otherwise, they can be defined at either the column level or the table level.
Example 1 creates a Student table consisting of five attributes: Student ID Sno, name Sname, Gender Ssex, age Sage, and department Sdept. The student ID cannot be blank, the value is unique, and the name value is also unique.
Create table Student
(Sno CHAR (5) not null unique,/* constraints on column-level integrity. The Sno value is UNIQUE,
Sname CHAR (20) UNIQUE. null values are not allowed */
Ssex CHAR (1 ),
Sage INT,
Sdept CHAR (15 ));

Common integrity constraints:
Master code constraint: PRIMARY KEY
Uniqueness constraint: UNIQUE
Non-NULL constraint: NOT NULL
Integrity constraints

Experiment 4 database integrity constraints and Relationship Diagram

% [Z, A] = X (0, pi/2, 0 );
N = 3; % recursion times
Len = 400/3 ^ n;
Hold on;

If n> = N
Plot ([z, z + len * exp (I * A)]/I );
Z = z + len * exp (I * );
[Z, A] = X (z, A, n + 1); % X
A = A + pi/3; % +
[Z, A] = Y (z, A, n + 1); % YF

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