Db2 简明教程

DB2 - Constraints

本章描述了数据库中的各种约束。

Introduction

为了强制执行数据库的完整性，定义了一组规则，称为约束。这些约束允许或禁止列中的值。

在实时数据库活动中，应该根据某些限制添加数据。例如，在销售数据库中，销售 ID 或交易 ID 应当唯一。约束类型有：

NOT NULL
Unique
Primary key
Foreign Key
Check
Informational

约束仅与表相关。它们仅应用于特定表。它们在创建表时定义和应用于表。

Explanation of each constraint:

NOT NULL

这是禁止表中一个或多个列的空值规则。

Syntax:

db2 create table <table_name>(col_name col_type not null,..)

Example : [要创建一个销售表，其中有四列（id、itemname、qty、price），在此向所有列添加“非空”约束以避免在表中形成任何空单元。]

db2 create table shopper.sales(id bigint not null, itemname
varchar(40) not null, qty int not null,price double not null)

Inserting NOT NULL values into table

你可以按如下方式在表中插入值：

Example: [错误查询]

db2 insert into shopper.sales(id,itemname,qty)
values(1,'raagi',12)

Output: [正确查询]

DB21034E  The command was processed as an SQL statement because
it was not a

valid Command Line Processor command.  During SQL processing
it returned:

SQL0407N  Assignment of a NULL value to a NOT NULL column
"TBSPACEID=5,

TABLEID=4, COLNO=3" is not allowed.  SQLSTATE=23502

Example: [正确查询]

db2 insert into shopper.sales(id,itemname,qty,price)
values(1,'raagi',12, 120.00)

db2 insert into shopper.sales(id,itemname,qty,price)
values(1,'raagi',12, 120.00)

Output:

DB20000I The SQL command completed successfully.

Unique constraints

使用这些约束，你可以唯一地设置列的值。为此，在创建表时使用“非空”约束声明唯一约束。

Syntax:

db2 create table <tab_name>(<col> <col_type> not null unique, ...)

Example:

db2 create table shopper.sales1(id bigint not null unique,
itemname varchar(40) not null, qty int not null,price
double not null)

Inserting the values into table

Example: 插入具有唯一 ID 1、2、3 和 4 的四行。

db2 insert into shopper.sales1(id, itemname, qty, price)
values(1, 'sweet', 100, 89)

db2 insert into shopper.sales1(id, itemname, qty, price)
values(2, 'choco', 50, 60)

db2 insert into shopper.sales1(id, itemname, qty, price)
values(3, 'butter', 30, 40)

db2 insert into shopper.sales1(id, itemname, qty, price)
values(4, 'milk', 1000, 12)

Example: 插入一个具有“id”值 3 的新行

db2 insert into shopper.sales1(id, itemname, qty, price)
values(3, 'cheese', 60, 80)

Output : 当你尝试插入一行已存在的 id 值时，它将显示此结果：

DB21034E  The command was processed as an SQL statement
because it was not a

valid Command Line Processor command.  During
SQL processing it returned:

SQL0803N  One or more values in the INSERT statement,
UPDATE statement, or foreign key update caused by a
DELETE statement are not valid because the primary key,
unique constraint or unique index identified by "1" constrains
table "SHOPPER.SALES1" from having duplicate values for the
index key. SQLSTATE=23505

Primary key

与唯一约束类似，你可以使用“主键”和“外键”约束来声明多个表之间的关系。

Syntax:

db2 create table <tab_name>( ,.., primary
key ())

Example : 创建“salesboys”表，其中“sid”为主键

db2 create table shopper.salesboys(sid int not null, name
varchar(40) not null, salary double not null, constraint
pk_boy_id primary key (sid))

Foreign key

外键是一张表中的一组列，必须匹配另一张表中一行的至少一个主键。它是一种引用约束或引用完整性约束。它是一条关于一个或多个表中多列值的逻辑规则。它使表之间存在必需的关系。

先前，你创建了一张名为“shopper.salesboys”的表。对于此表，主键为“sid”。现在你正在创建一张具有不同模式的销售人员个人详细信息的新表，名为“employee”和表名为“salesboys”。在这种情况下，“sid”是外键。

Syntax:

db2 create table <tab_name>(<col> <col_type>,constraint
<const_name> foreign key (<col_name>)
                  reference <ref_table> (<ref_col>)

Example : [要创建一个名为“salesboys”的表，其中包含外键列“sid”]

db2 create table employee.salesboys(
            sid int,
            name varchar(30) not null,
            phone int not null,
            constraint fk_boy_id
            foreign key (sid)
            references shopper.salesboys (sid)
			 on delete restrict
                       )

Example : [将值插入到主键表“shopper.salesboys”]

db2 insert into shopper.salesboys values(100,'raju',20000.00),
(101,'kiran',15000.00),
(102,'radha',10000.00),
(103,'wali',20000.00),
(104,'rayan',15000.00)

Example : [将值插入外键表 “employee.salesboys” [无错误]]

db2 insert into employee.salesboys values(100,'raju',98998976),
(101,'kiran',98911176),
(102,'radha',943245176),
(103,'wali',89857330),
(104,'rayan',89851130)

如果您输入了一个未知的数字（未存储在 “shopper.salesboys” 表中），这将向您显示 SQL 错误。

Example : [错误执行]

db2 insert into employee.salesboys values(105,'rayan',89851130)

Output:

DB21034E  The command was processed as an SQL statement because it
was not a valid Command Line Processor command.  During SQL
processing it returned: SQL0530N  The insert or update value of
the FOREIGN KEY "EMPLOYEE.SALESBOYS.FK_BOY_ID" is not equal to any
value of the parent key of the parent table.  SQLSTATE=23503

Checking constraint

您需要使用此约束来为表中的特定列添加条件限制。

Syntax:

db2 create table
 (
  primary key (),
  constraint  check (condition or condition)
 )

Example : [使用约束值创建 emp1 表]

db2 create table empl
 (id           smallint not null,
  name         varchar(9),
  dept         smallint check (dept between 10 and 100),
  job          char(5)  check (job in ('sales', 'mgr', 'clerk')),
  hiredate     date,
  salary       decimal(7,2),
  comm         decimal(7,2),
  primary key (id),
  constraint yearsal check (year(hiredate) > 1986 or salary > 40500)
 )

Inserting values

您可以按如下所示将值插入表中：

db2 insert into empl values (1,'lee', 15, 'mgr', '1985-01-01' ,
40000.00, 1000.00)

Dropping the constraint

以下列出删除各种约束的语法。

Dropping UNIQUE constraint

Syntax:

db2 alter table <tab_name> drop unique <const_name>

Dropping primary key

Syntax:

db2 alter table <tab_name> drop primary key

Dropping check constraint

Syntax:

db2 alter table <tab_name> drop check <check_const_name>

Dropping foreign key

Syntax:

db2 alter table <tab_name> drop foreigh key <foreign_key_name>