MySQL Others--约束(Constraint)示例

ENUM约束

--使用ENUM来限制用户输入
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200),
        Sex ENUM('Male','Female')
)

默认值约束

--创建表时指定默认值
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200) DEFAULT ''
)

--创建表后再创建默认约束
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200)
)

ALTER TABLE Student
ALTER COLUMN StudentName SET DEFAULT ''

--查看默认约束
SHOW CREATE TABLE Student;

外键约束

MYSQL 对外键约束只支持即时检查(immediate check)， 与SQL SERVER 相同。

ORACLE 对外键约束支持延时检查(deferred check)。

在创建外键约束时，MYSQL 默认为外键约束列添加索引，以避免外键约束带来的死锁问题。

--创建被引用的班级表
CREATE TABLE Class
(
        ClassID INT AUTO_INCREMENT PRIMARY KEY,
        ClassName VARCHAR(200)
);

--创建学生表时外键引用班级表
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200),
        FOREIGN KEY(ClassID) REFERENCES Class(ClassID)
)

--创建学生表，然后再增加外键
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200),
);
ALTER TABLE Student ADD FOREIGN KEY(ClassID) REFERENCES Class(ClassID);

--查看外键
SELECT *
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS
WHERE CONSTRAINT_SCHEMA='testdb1'
AND TABLE_NAME='Student' \G

*************************** 1. row ***************************
       CONSTRAINT_CATALOG: def
        CONSTRAINT_SCHEMA: testdb1
          CONSTRAINT_NAME: student_ibfk_1
UNIQUE_CONSTRAINT_CATALOG: def
UNIQUE_CONSTRAINT_SCHEMA: testdb1
   UNIQUE_CONSTRAINT_NAME: PRIMARY
             MATCH_OPTION: NONE
              UPDATE_RULE: RESTRICT
              DELETE_RULE: RESTRICT
               TABLE_NAME: student
    REFERENCED_TABLE_NAME: class

在数据导入过程中，为提高导入效率，可以先禁用外键检查，导入完后再开启。

--禁用外键检查
SET @@foreign_key_checks=0;

--启用外键检查
SET @@foreign_key_checks=1;

唯一约束

--创建学生表时外键引用班级表
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200),
        UNIQUE KEY (StudentName)
)

--创建表后再增加唯一约束
CREATE TABLE Student
(
        StudentID INT AUTO_INCREMENT PRIMARY KEY,
        ClassID INT,
        StudentName VARCHAR(200)
)
ALTER TABLE Student ADD UNIQUE KEY(StudentName);

--查看约束
SELECT *
FROM information_schema.TABLE_CONSTRAINTS
WHERE CONSTRAINT_SCHEMA='testdb1'
AND TABLE_NAME='Student' \G

*************************** 1. row ***************************
CONSTRAINT_CATALOG: def
CONSTRAINT_SCHEMA: testdb1
   CONSTRAINT_NAME: PRIMARY
      TABLE_SCHEMA: testdb1
        TABLE_NAME: student
   CONSTRAINT_TYPE: PRIMARY KEY
*************************** 2. row ***************************
CONSTRAINT_CATALOG: def
CONSTRAINT_SCHEMA: testdb1
   CONSTRAINT_NAME: StudentName
      TABLE_SCHEMA: testdb1
        TABLE_NAME: student
   CONSTRAINT_TYPE: UNIQUE