MySQL column-level constraints and table-level constraints

First, the six constraints

• NOT NULL non-empty
• PRIMARY KEY primary key
• UNIQUE unique
• CHECK check constraint, MySQL does not support, syntax error but there was no effect
• FOREIGN KEY foreign key. Two limit table relationships, introduced from the normal value table foreign key constraints, the introduction of the value of a column in the primary table, to ensure that the value of this field must be derived from the primary table associated with the table in the column
• DEFAULT default value / setpoint system

Second, the table level and column-level constraint definition:

• Table-level constraints: constraints on the establishment of multiple data columns, called table-level constraints can only table-level constraints after the column definition statement

• Column-level constraints: constraints on the establishment of a data column, called a column-level constraint column-level constraints may be declared in the column definition, can also be a statement after the column definition

 ps: the word is simple to understand for column-level constraints are single row, table-level constraints for the entire table, so corollary

Six column-level constraints in the constraint can be used, but no effect of foreign keys (foreign key constraint is a relationship between two tables)

Obviously, non-empty by default, and can not be used at the table level constraints

Third, the timing of adding constraints:

• When you create a table:

　　# Create the main table, the foreign key experiment

　　CREAT  TABLE  IF  NOT  EXISTS  major(

　　　　　　　　id INT,

　　　　　　　　major_name varchar(20)

　　);

　　# Created from a table, table-level experiment / column-level constraint

　　CREAT  TABLE  IF  NOT  EXISTS  students(

　　　　　　　　# Add a table-level constraints, can be added directly after the type

　　　　　　　　id INT PRIMARY KEY,

　　　　　　　　student_name VARCHAR(20),

　　　　　　　　sex CHAR(1),

　　　　　　　　age INT DEFAULT 18,

　　　　　　　　majorid INT,

　　　　　　

　　　　　　　　# Add a table-level constraints, added directly after the end of the column is created

　　　　　　　　CONSTRAINT pk PRIMARY KEY (id). # Primary key, used to name the constraint, may be applied without

　　　　　　　　CONSTRAINT fk_student_majory FOREIGN KEY(majorid) REFERENCES major(id)#外键

　　);

• When modify the table:

　　# Add a non-empty, the other column-level constraints can follow this method

　　ALTER  TABLE  students  MODIFY  COLUMN  student_name  VARCHAR(20)  NOT  NULL;  

　　# Add foreign key, the other table-level constraints may be added using the same procedure

 　　ALTER  TABLE  students ADD  FOREIGN  KEY(majorid)  REFERENCES  major(id) ;