使用外键创建新表需要在数据库中具有CREATE TABLE权限,并对在其中创建表的架构具有ALTER SCHEMA权限。 限制 外键约束不一定要链接到另一个表中的主键约束。 外键还可以定义为引用另一个表中UNIQUE约束的列。 如果在NULL约束的列中输入非FOREIGN KEY值,则此值必须在被引用列中存在。 否则,将返回外键冲突错误消息
SQL定义功能中,用CREATE TABLE建立表时,FOREIGN KEY...REFERENCES…短语的含义是A.说明主关键字B.建立表之间的联系C.说明有效性规则
Visual FoxPro uses the default value if you use the SQL ALTER TABLEcommand to remove autoincrementing for the field. PRIMARY KEY | UNIQUE PRIMARY KEY creates a primary index for the field specified in FieldName1. UNIQUE creates a candidate index for the field specified in FieldName1. The ...
Note:The above code works in all major database systems. However, there may be an alternate syntax to create foreign keys depending on the database. Refer to their respective database documentation for more information. Inserting Records in Table With Foreign Key Lets try to insert records in ...
SQL FOREIGN KEY on ALTER TABLE To create aFOREIGN KEYconstraint on the "PersonID" column when the "Orders" table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEOrders ADDFOREIGNKEY(PersonID)REFERENCESPersons(PersonID); ...
Visual FoxPro uses the default value if you use the ALTER TABLE - SQL command to remove autoincrementing for the field. PRIMARY KEY | UNIQUE PRIMARY KEY creates a primary index for the field specified in FieldName1. UNIQUE creates a candidate index for the field specified in FieldName1. ...
// foreign key constraints in other tables. You can see what // constraints are referencing a table by issuing the following // command: // SELECT * FROM USER_CONSTRAINTS WHERE TABLE_NAME = "tabnam"; 比较清楚地说明了问题,以及解决方法:可以在执行前,先禁用外键约束,执行truncate后再恢复外键约束...
Candidate indexes (created by including the UNIQUE option in CREATE TABLE or ALTER TABLE - SQL) are not the same as indexes created with the UNIQUE option in the INDEX command. An index created with the UNIQUE option in the INDEX command allows duplicate index keys; candidate indexes do not...
The CREATE TABLE statement defines a table. The definition must include its name and the names and attributes of its columns. The definition can include other attributes of the table, such as its primary key and its table space.
CREATE TABLE product (product_id INT PRIMARY KEY AUTO_INCREMENT cat_id INT NOT NULL sku_code VARCHAR(20) UNIQUE stock INT CHECK(stock >= 0)FOREIGN KEY (cat_id) REFERENCES category(cat_id)) ENGINE=InnoDB;这里包含五个约束处理：外键关联分类表保证数据有效性，唯一约束避免重复商品编码，检查约束...