每个约束的解释：
非空
禁止表中一个或多个列的空值是一条规则。
语法:
db2 create table <table_name>(col_name col_type not null,..)
例子：[创建一个销售表，其中包含四列（id、itemname、qty、price），为所有列添加“非空”约束以避免在表中形成任何空单元格。]
db2 create table shopper.sales(id bigint not null, itemname
varchar(40) not null, qty int not null,price double not null)
将 NOT NULL 值插入表中
您可以在表中插入值，如下所示：
例子:[错误查询]
db2 insert into shopper.sales(id,itemname,qty)
values(1,'raagi',12)
输出:[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
例子:[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)
输出:
DB20000I The SQL command completed successfully.
独特的约束
使用这些约束，您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。
语法:
db2 create table <tab_name>(<col> <col_type> not null unique, ...)
例子:
db2 create table shopper.sales1(id bigint not null unique,
itemname varchar(40) not null, qty int not null,price
double not null)
将值插入表中
例子:插入四个不同的行，其唯一 ID 为 1、2、3 和 4。
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)
例子:插入一个“id”值为 3 的新行
db2 insert into shopper.sales1(id, itemname, qty, price)
values(3, 'cheese', 60, 80)
输出：当您尝试插入具有现有 id 值的新行时，它将显示以下结果：
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
主键
与唯一约束类似，您可以使用“主键”和“外键”约束来声明多个表之间的关系。
语法:
db2 create table <tab_name>( ,.., primary
key ())
例子: 创建以“sid”为主键的“salesboys”表
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))
外键
外键是表中的一组列，它们需要匹配另一个表中行的至少一个主键。它是一个参照约束或参照完整性约束。它是关于一个或多个表中的多个列中的值的逻辑规则。它启用了表之间的所需关系。
之前，您创建了一个名为“shopper.salesboys”的表。对于这个表，主键是“sid”。现在，您正在创建一个新表，其中包含名为“employee”的不同模式和名为“salesboys”的表的销售男孩的个人详细信息。在这种情况下，“sid”是外键。
语法:
db2 create table <tab_name>(<col> <col_type>,constraint
<const_name> foreign key (<col_name>)
reference <ref_table> (<ref_col>)
例子: [使用外键列 'sid' 创建一个名为 'salesboys' 的表]
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
)
例子: [将值插入主键表“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)
例子：[将值插入外键表“employee.salesboys”[没有错误]]
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存储在“shopper.salesboys”表中的未知号码，则会显示 SQL 错误。
例子: [错误执行]
db2 insert into employee.salesboys values(105,'rayan',89851130)
输出:
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
检查约束
您需要使用此约束来为表中的特定列添加条件限制。
语法:
db2 create table
(
primary key (),
constraint check (condition or condition)
)
例子: [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)
)
插入值
您可以将值插入到表中，如下所示：
db2 insert into empl values (1,'lee', 15, 'mgr', '1985-01-01' ,
40000.00, 1000.00)
