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NOT NULL Constraints
NOT NULL constraints are in-line constraints that indicate that a column can not contain NULL
values. For example, the PERSON_ID column is defined as NOT NULL in that example.
If you need to add a NOT NULL constraint to a table after the fact, simply use the alter table co
mmand as in this example:
Syntax:-
create table
customer
(status char(3) not null,
val number not null);
Alter Table Table Name Modify( person_id NOT NULL);
When copying tables with CTAS, beware that NULL values many not copy properly.
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Primary Key Constraints
Primary key constraints define a column or series of columns that uniquely
identify a given row in a table. Defining a primary key on a table is optional and
you can only define a single primary key on a table. A primary key constraint can
consist of one or many columns (up to 32). Any column that is defined as a
primary key column is automatically set with a NOT NULL status.
Syntax:-create table
customer
(status_id Number CONSTRAINT CONSTRAINT _NAME
);
If you need to primary key constraint to a table after the fact, simply use the alter t
able command.
ALTER TABLE customer ADD CONSTRAINT pk_my_status
PRIMARY KEY (status_id);
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Unique Constraints
Unique constraints are like alternative primary key constraints.
A unique constraint defines a column, or series of columns, that must be unique in value.
You can have a number of unique constraints defined and the columns can have NULL values
in them, unlike a column that belongs to a primary key constraint.
If you need to add unique key constraints to a table after the fact, simply use the alter table
command.
ALTER TABLE
my_status
ADD CONSTRAINT
uk_my_status
UNIQUE
(status_id,
person_id);
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Foreign Key Constraints
A foreign key constraint is used to enforce a relationship between two tables. As an example,
take the case of two tables, ITEM and PART. These tables have a relationship (an item can have
none, one or many parts). Foreign key constraints help to enforce that relationship
Well, foreign key constraints help to enforce the types of relationships between tables we have
just demonstrated. In this example we will create the ITEM and PART table. In the process of
doing so, we will create a foreign key relationship between the two:
CREATE TABLE part
( Part_no NUMBER PRIMARY KEY,
Part_desc VARCHAR2(200) NOT NULL );
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Cont..
CREATE TABLE item (Item_no NUMBER,Part_no NUMBER,Item_desc varchar2(200) NOT
NULL, CONSTRAINT fk_item_part FOREIGN KEY (part_no) REFERENCES PART (part_no),
CONSTRAINT pk_item PRIMARY KEY (item_no, part_no) );
In this example, what we are really interested in is the creation of the ITEM table. First, note
that we defined the primary key as an out of line primary key. This is because it is a composite
primary key and composite primary keys have to be defined out of line.
Now, we are interested in the foreign key definition. You must define foreign key constraints as
out of line constraints, as we have done in our example. Here is a snippet of the command that
we used:
CONSTRAINT fk_item_part FOREIGN KEY (part_no) REFERENCES part (part_no);
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Check Constraints
Check constraints validate that values in a given column meet a specific criteria.
For example, you could create a check constraint on a varchar2 column so it
only can contain the values T or F as in this example:
Create table my_status
( status_id NUMBER PRIMARY KEY,
person_id NUMBER NOT NULL,
active_record VARCHAR2(1) NOT NULL
CHECK (UPPER(active_record)='T' or
UPPER(active_record)='F'),
person_ssn VARCHAR2(20) CONSTRAINT un_person_ssn UNIQUE
);
