可以在创建表时规定约束(通过 CREATE TABLE 语句),或者在表创建之后也可以(通过 ALTER TABLE 语句)。 我们将主要探讨以下几种约束: NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT SQL NOT NULL 约束 NOT NULL 约束强制列不接受 NULL 值。 SQL UNIQUE 约束 SQL UNIQUE 约束 UNIQUE 约束唯一标识数据库表...
CREATETABLEPersons(Id_PintNOTNULL,LastNamevarchar(255)NOTNULL,FirstNamevarchar(255),Addressvarchar(255),Cityvarchar(255),PRIMARYKEY(Id_P)) 1. 2. 3. 4. 5. 6. 7. 8. 9. SQL Server / Oracle / MS Access: CREATETABLEPersons(Id_PintNOTNULLPRIMARYKEY,LastNamevarchar(255)NOTNULL,FirstNamevar...
The login for the current connection must be associated with an existing user ID in the database specified by database_name, and that user ID must have CREATE TABLE permissions. schema_name The name of the schema to which the new table belongs. table_name The name of the new table. ...
– 维护外键关联关系 alter table t_student add constraints fk_student_classid foreign key(classid) references t_class(id); 6.check约束:在字段类型的基础上。更进一步的提升数据的精度 DROP TABLE t_student; CREATE TABLE t_student( id number(5) unique not null , – 学生编号 name varchar2(20) ...
Introduction to CREATE TABLE CREATE TABLE - Creating Managed Tables CREATE TABLE - Creating Managed Tables CREATE TABLE - Creating a table with schema CREATE TABLE - Creating a table from a query CREATE EXTERNAL TABLE ALTER TABLE ALTER TABLE - Adding and removing vertical partition buckets ...
Create a table with constraints Now, let us understand how we can create a table with a constraint. A table can have the following constraints. NOT NULL: This constraint ensures that a query does not insert a NULL value in a column on which the constraint has been created. ...
There can be only one primary key constraint at a time in a table. If you try to create two primary key constraints on the same table, the command will fail. 同一时间里一张表里只允许有一个主键约束。如果你企图在同一个表上创建两个主键约束,这个命令将失败。
Once we created both the tables using the SQL Create table, the database diagram will be the same as above. SQL Create Statement | FOREIGN KEY, PRIMARY KEY, CHECK constraints Now if we want to create a table called laboratory with two foreign keys and a primary key constraint, also using...
另外,如果满足以下一个或多个条件,则还需要 ALTER TABLE 权限: 存在约束但未指定 CHECK_CONSTRAINTS 选项。 禁用约束是默认行为。 若要显式检查约束,请使用 CHECK_CONSTRAINTS 选项。 存在触发器但未指定 FIRE_TRIGGER 选项。 默认情况下,不会激发触发器。 若要显式激发触发器,请使用 FIRE_TRIGGER 选项。
CREATE [ OR ALTER ] FUNCTION [ schema_name. ] function_name ( { @parameter_name [ AS ] [ type_schema_name. ] parameter_data_type [ NULL ] [ = default ] } [ , ...n ] ) RETURNS TABLE <clr_table_type_definition> [ WITH <clr_function_option> [ , ...n ] ] [ ORDER ( <...