Integrity constraint

 Oracle/PLSQL: ORA-02291 Error
Error      : ORA-02291: integrity constraint <constraint name> violated - parent key not found

Cause    : You tried to reference a table using a unique or primary key, but the columns that you listed did not    match the primary key, or a primary key does not exist for this table.


Action :
The options to resolve this Oracle error are:

1. This error commonly occurs when you have a parent-child relationship established between two tables through a foreign key. You then have tried to insert a value into the child table, but the corresponding value does not exist in the parent table.

To correct this problem, you need to insert the value into the parent table first and then you can insert the corresponding value into the child table.

For example, if you had created the following foreign key (parent-child relationship).

CREATE TABLE supplier
(	supplier_id	numeric(10)	not null,
	supplier_name	varchar2(50)	not null,
	contact_name	varchar2(50),
	CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);

CREATE TABLE products
(	product_id	numeric(10)	not null,
	supplier_id	numeric(10)	not null,
	CONSTRAINT fk_supplier
	FOREIGN KEY (supplier_id)
	REFERENCES supplier (supplier_id)
);

Then you try inserting into the products table as follows:

INSERT INTO products
(product_id, supplier_id)
VALUES (1001, 5000);

You would receive the following error message:


Since the supplier_id value of 5000 does not yet exist in the supplier table, you need to first insert a record into the supplier table as follows:

INSERT INTO supplier
(supplier_id, supplier_name, contact_name)
VALUES (5000, 'Microsoft', 'Bill Gates');

Then you can insert into the products table:

INSERT INTO products
(product_id, supplier_id)
VALUES (1001, 5000);

Decode Function

---

In Oracle/PLSQL, the decode function has the functionality of an IF-THEN-ELSE statement.
The syntax for the decode function is:

decode( expression , search , result [, search , result]... [, default] )

expression is the value to compare.
search is the value that is compared against expression.
result is the value returned, if expression is equal to search.
default is optional. If no matches are found, the decode will return default. If default is omitted, then the decode statement will return null (if no matches are found).

Applies To:

• Oracle 9i, Oracle 10g, Oracle 11g

For example:

You could use the decode function in an SQL statement as follows:

SELECT supplier_name,
decode(supplier_id,	10000,	'IBM',
	10001,	'Microsoft',
	10002,	'Hewlett Packard',
		'Gateway') result
FROM suppliers;

The above decode statement is equivalent to the following IF-THEN-ELSE statement:

IF supplier_id = 10000 THEN
     result := 'IBM';
ELSIF supplier_id = 10001 THEN
    result := 'Microsoft';
ELSIF supplier_id = 10002 THEN
    result := 'Hewlett Packard';
ELSE
    result := 'Gateway';
END IF;

The decode function will compare each supplier_id value, one by one.