SQL中的 CONSTRAINT示例 在SQL中,CONSTRAINT是用于指定表中的某些列的规则的关键字。这些规则确保表中的数据的完整性和准确性。约束可以应用于单个列或多个列,最常见的约束类型有:主键 (PRIMARY KEY)、唯一 (UNIQUE)、非空 (NOT NULL)、外键 (FOREIGN KEY)、和检查 (CHECK)。 场景 假设我们要创建一个 books表...
字段定义constraint 约束名约否类型(字段名)-->unique,primary key,check 字段定义constraint 约否名foreingn key (字段名)references 表名(字段名)--->foreign 三、建表时约束定义 1.定义各种不同的约束 --创建一个用于作外键的表tb_dept SQL> CREATE TABLE tb_dept 2 ( 3 deptno NUMBER(4) PRIMARY KEY...
We can also add thePRIMARY KEYconstraint to a column in an existing table using theALTER TABLEcommand. For example, For a Single Column ALTERTABLECollegesADDPRIMARYKEY(college_id); Here, the SQL command adds thePRIMARY KEYconstraint to thecollege_idcolumn in the existingCollegestable. For Multip...
常见的Constraint包括:主键约束(PRIMARY KEY)、唯一性约束(UNIQUE)、非空约束(NOT NULL)、外键约束(FOREIGN KEY)以及检查约束(CHECK)。以主键约束(PRIMARY KEY)为例,当我们在创建数据表时,可以设置某一列(或者列的组合)为主键,主键具有唯一性和非空性,即主键列的每个值都是唯一的,且不能为NULL,这样可以确保每条...
Here, thecustomer_idcolumn in theProductstable references theidcolumn in theCustomerstable. Foreign Key Syntax in SQL The syntax of the SQLFOREIGN KEYconstraint is: CREATE TABLE table_name ( column1 data_type, column2 data_type, ..., ...
问题现象使用如下的 sql 语句创建自关联外键表:drop table self_f_key;create table self_f_key(t1 number primary key not null..., t2 number);create index i_s_1 on self_f_key(t2);alter ...
sql语句如下:test.sql ===主键约束=== create table t_emp(empno int not null primary key, ename varchar(20), esex char(2));普通字段也可以设置为not null /* 等价于 */ create table t_emp(empno int primary key, ename varchar(20), esex char(2)); /*...
ERROR 1062 (23000): Duplicate entry '932834897@qq.com' for key 'email' 2、使用表级约束,给多个字段联合约束 联合约束,表示两个或以上的字段同时与另一条记录相等,则报错 mysql> create table t_user( -> id int(10), -> name varchar(32) not null, ...
TheFOREIGN KEYconstraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the parent table. SQL FOREIGN KEY on CREATE TABLE The following SQL creates aFOREIGN KEYon the "PersonID" column when the "Orders" table is cr...
MySQL / SQL Server / Oracle / MS Access: CREATETABLEPersons ( ID intNOTNULL, LastName varchar(255)NOTNULL, FirstName varchar(255), Age int, CONSTRAINTPK_PersonPRIMARYKEY(ID,LastName) ); Note:In the example above there is only ONEPRIMARY KEY(PK_Person). However, the VALUE of the prima...