JOINS

This SQL tutorial explains how to use SQL joins with syntax, visual illustrations, and examples.

DESCRIPTION

SQL JOINS are used to retrieve data from multiple tables. A SQL JOIN is performed whenever two or more tables are joined in a SQL statement.

There are 4 different types of SQL joins:

• SQL INNER JOIN (or sometimes called simple join)
• SQL LEFT OUTER JOIN (or sometimes called LEFT JOIN)
• SQL RIGHT OUTER JOIN (or sometimes called RIGHT JOIN)
• SQL FULL OUTER JOIN (or sometimes called FULL JOIN)

So let's discuss SQL JOIN syntax, look at visual illustrations of SQL JOINS, and explore SQL JOIN examples.

SQL INNER JOIN (SIMPLE JOIN)

Chances are, you've already written a SQL statement that uses an SQL INNER JOIN. It is the most common type of SQL join. SQL INNER JOINS return all rows from multiple tables where the join condition is met.

Syntax

The syntax for the SQL INNER JOIN is:

SELECT columns
FROM table1 
INNER JOIN table2
ON table1.column = table2.column;

Visual Illustration

In this visual diagram, the SQL INNER JOIN returns the shaded area:

The SQL INNER JOIN would return the records where table1 and table2 intersect.

Example

Here is an example of a SQL INNER JOIN:

SELECT s.supplier_id, s.supplier_name, od.order_date
FROM suppliers AS s
INNER JOIN order_details AS od
ON s.supplier_id = od.supplier_id;

This SQL INNER JOIN example would return all rows from the suppliers and orders tables where there is a matching supplier_id value in both the suppliers and orders tables.

Let's look at some data to explain how the INNER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have another table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2003/05/12
500126	10001	2003/05/13
500127	10004	2003/05/14

If we run the SQL statement (that contains an INNER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers
INNER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	name	order_date
10000	IBM	2003/05/12
10001	Hewlett Packard	2003/05/13

The rows for Microsoft and NVIDIA from the supplier table would be omitted, since the supplier_id's 10002 and 10003 do not exist in both tables. The row for 500127 (order_id) from the orders table would be omitted, since the supplier_id 10004 does not exist in the suppliers table.

Old Syntax

As a final note, it is worth mentioning that the SQL INNER JOIN example above could be rewritten using the older implicit syntax as follows (but we still recommend using the INNER JOIN keyword syntax):

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id;

SQL LEFT OUTER JOIN

Another type of join is called a LEFT OUTER JOIN. This type of join returns all rows from the LEFT-hand table specified in the ON condition and only those rows from the other table where the joined fields are equal (join condition is met).

Syntax

The syntax for the SQL LEFT OUTER JOIN is:

SELECT columns
FROM table1
LEFT [OUTER] JOIN table2
ON table1.column = table2.column;

In some databases, the LEFT OUTER JOIN keywords are replaced with LEFT JOIN.

Visual Illustration

In this visual diagram, the SQL LEFT OUTER JOIN returns the shaded area:

The SQL LEFT OUTER JOIN would return the all records from table1 and only those records from table2 that intersect withtable1.

Example

Here is an example of a SQL LEFT OUTER JOIN:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers
LEFT OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

This LEFT OUTER JOIN example would return all rows from the suppliers table and only those rows from the orders table where the joined fields are equal.

If a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as <null> in the result set.

Let's look at some data to explain how LEFT OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2003/05/12
500126	10001	2003/05/13

If we run the SQL statement (that contains a LEFT OUTER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers
LEFT OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	supplier_name	order_date
10000	IBM	2003/05/12
10001	Hewlett Packard	2003/05/13
10002	Microsoft	<null>
10003	NVIDIA	<null>

The rows for Microsoft and NVIDIA would be included because a LEFT OUTER JOIN was used. However, you will notice that the order_date field for those records contains a <null> value.

Old Syntax

As a final note, it is worth mentioning that the LEFT OUTER JOIN example above could be rewritten using the older implicit syntax that utilizes the outer join operator (+) as follows (but we still recommend using the LEFT OUTER JOIN keyword syntax):

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id(+);

SQL RIGHT OUTER JOIN

Another type of join is called a SQL RIGHT OUTER JOIN. This type of join returns all rows from the RIGHT-hand table specified in the ON condition and only those rows from the other table where the joined fields are equal (join condition is met).

Syntax

The syntax for the SQL RIGHT OUTER JOIN is:

SELECT columns
FROM table1
RIGHT [OUTER] JOIN table2
ON table1.column = table2.column;

In some databases, the RIGHT OUTER JOIN keywords are replaced with RIGHT JOIN.

Visual Illustration

In this visual diagram, the SQL RIGHT OUTER JOIN returns the shaded area:

The SQL RIGHT OUTER JOIN would return the all records from table2 and only those records from table1 that intersect withtable2.

Example

Here is an example of a SQL RIGHT OUTER JOIN:

SELECT orders.order_id, orders.order_date, suppliers.supplier_name
FROM suppliers
RIGHT OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

This RIGHT OUTER JOIN example would return all rows from the orders table and only those rows from the suppliers table where the joined fields are equal.

If a supplier_id value in the orders table does not exist in the suppliers table, all fields in the suppliers table will display as <null>in the result set.

Let's look at some data to explain how RIGHT OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	Apple
10001	Google

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2013/08/12
500126	10001	2013/08/13
500127	10002	2013/08/14

If we run the SQL statement (that contains a RIGHT OUTER JOIN) below:

SELECT orders.order_id, orders.order_date, suppliers.supplier_name
FROM suppliers
RIGHT OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

order_id	order_date	supplier_name
500125	2013/08/12	Apple
500126	2013/08/13	Google
500127	2013/08/14	<null>

The row for 500127 (order_id) would be included because a RIGHT OUTER JOIN was used. However, you will notice that the supplier_name field for that record contains a <null> value.

Old Syntax

As a final note, it is worth mentioning that the RIGHT OUTER JOIN example above could be rewritten using the older implicit syntax that utilizes the outer join operator (+) as follows (but we still recommend using the RIGHT OUTER JOIN keyword syntax):

SELECT orders.order_id, orders.order_date, suppliers.supplier_name
FROM suppliers, orders
WHERE suppliers.supplier_id(+) = orders.supplier_id;

SQL FULL OUTER JOIN

Another type of join is called a SQL FULL OUTER JOIN. This type of join returns all rows from the LEFT-hand table and RIGHT-hand table with nulls in place where the join condition is not met.

Syntax

The syntax for the SQL FULL OUTER JOIN is:

SELECT columns
FROM table1
FULL [OUTER] JOIN table2
ON table1.column = table2.column;

In some databases, the FULL OUTER JOIN keywords are replaced with FULL JOIN.

Visual Illustration

In this visual diagram, the SQL FULL OUTER JOIN returns the shaded area:

The SQL FULL OUTER JOIN would return the all records from both table1 and table2.

Example

Here is an example of a SQL FULL OUTER JOIN:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers
FULL OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

This FULL OUTER JOIN example would return all rows from the suppliers table and all rows from the orders table and whenever the join condition is not met, <nulls> would be extended to those fields in the result set.

If a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as <null> in the result set. If a supplier_id value in the orders table does not exist in the suppliers table, all fields in the suppliers table will display as <null> in the result set.

Let's look at some data to explain how FULL OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2013/08/12
500126	10001	2013/08/13
500127	10004	2013/08/14

If we run the SQL statement (that contains a FULL OUTER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers
FULL OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	supplier_name	order_date
10000	IBM	2013/08/12
10001	Hewlett Packard	2013/08/13
10002	Microsoft	<null>
10003	NVIDIA	<null>
<null>	<null>	2013/08/14

The rows for Microsoft and NVIDIA would be included because a FULL OUTER JOIN was used. However, you will notice that the order_date field for those records contains a <null> value.

The row for supplier_id 10004 would be also included because a FULL OUTER JOIN was used. However, you will notice that the supplier_id and supplier_name field for those records contain a <null> value.

Old Syntax

As a final note, it is worth mentioning that the FULL OUTER JOIN example above could not have been written in the old syntax without using a UNION query.

INDEXES

This Oracle tutorial explains how to create, rename and drop indexes in Oracle with syntax and examples.

WHAT IS AN INDEX IN ORACLE?

An index is a performance-tuning method of allowing faster retrieval of records. An index creates an entry for each value that appears in the indexed columns. By default, Oracle creates B-tree indexes.

CREATE AN INDEX

Syntax

The syntax for creating an index in Oracle/PLSQL is:

CREATE [UNIQUE] INDEX index_name
  ON table_name (column1, column2, ... column_n)
  [ COMPUTE STATISTICS ];

UNIQUE

It indicates that the combination of values in the indexed columns must be unique.

index_name

The name to assign to the index.

table_name

The name of the table in which to create the index.

column1, column2, ... column_n

The columns to use in the index.

COMPUTE STATISTICS

It tells Oracle to collect statistics during the creation of the index. The statistics are then used by the optimizer to choose a "plan of execution" when SQL statements are executed.

Example

Let's look at an example of how to create an index in Oracle/PLSQL.

For example:

CREATE INDEX supplier_idx
  ON supplier (supplier_name);

In this example, we've created an index on the supplier table called supplier_idx. It consists of only one field - the supplier_name field.

We could also create an index with more than one field as in the example below:

CREATE INDEX supplier_idx
  ON supplier (supplier_name, city);

We could also choose to collect statistics upon creation of the index as follows:

CREATE INDEX supplier_idx
  ON supplier (supplier_name, city)
  COMPUTE STATISTICS;

CREATE A FUNCTION-BASED INDEX

In Oracle, you are not restricted to creating indexes on only columns. You can create function-based indexes.

Syntax

The syntax for creating a function-based index in Oracle/PLSQL is:

CREATE [UNIQUE] INDEX index_name
  ON table_name (function1, function2, ... function_n)
  [ COMPUTE STATISTICS ];

UNIQUE

It indicates that the combination of values in the indexed columns must be unique.

index_name

The name to assign to the index.

table_name

The name of the table in which to create the index.

function1, function2, ... function_n

The functions to use in the index.

COMPUTE STATISTICS

It tells Oracle to collect statistics during the creation of the index. The statistics are then used by the optimizer to choose a "plan of execution" when SQL statements are executed.

Example

Let's look at an example of how to create a function-based index in Oracle/PLSQL.

For example:

CREATE INDEX supplier_idx
  ON supplier (UPPER(supplier_name));

In this example, we've created an index based on the uppercase evaluation of the supplier_name field.

However, to be sure that the Oracle optimizer uses this index when executing your SQL statements, be sure that UPPER(supplier_name) does not evaluate to a NULL value. To ensure this, add UPPER(supplier_name) IS NOT NULL to your WHERE clause as follows:

SELECT supplier_id, supplier_name, UPPER(supplier_name)
FROM supplier
WHERE UPPER(supplier_name) IS NOT NULL
ORDER BY UPPER(supplier_name);

RENAME AN INDEX

Syntax

The syntax for renaming an index in Oracle/PLSQL is:

ALTER INDEX index_name
  RENAME TO new_index_name;

index_name

The name of the index that you wish to rename.

new_index_name

The new name to assign to the index.

Example

Let's look at an example of how to rename an index in Oracle/PLSQL.

For example:

ALTER INDEX supplier_idx
  RENAME TO supplier_index_name;

In this example, we're renaming the index called supplier_idx to supplier_index_name.

COLLECT STATISTICS ON AN INDEX

If you forgot to collect statistics on the index when you first created it or you want to update the statistics, you can always use the ALTER INDEX command to collect statistics at a later date.

Syntax

The syntax for collecting statistics on an index in Oracle/PLSQL is:

ALTER INDEX index_name
  REBUILD COMPUTE STATISTICS;

index_name

The index in which to collect statistics.

Example

Let's look at an example of how to collect statistics for an index in Oracle/PLSQL.

For example:

ALTER INDEX supplier_idx
  REBUILD COMPUTE STATISTICS;

In this example, we're collecting statistics for the index called supplier_idx.

DROP AN INDEX

Syntax

The syntax for dropping an index in Oracle/PLSQL is:

DROP INDEX index_name;

index_name

The name of the index to drop.

Example

Let's look at an example of how to drop an index in Oracle/PLSQL.

For example:

DROP INDEX supplier_idx;

In this example, we're dropping an index called supplier_idx

CHECK CONSTRAINTS

This Oracle tutorial explains how to use the check constraints in Oracle with syntax and examples.

WHAT IS A CHECK CONSTRAINT IN ORACLE?

A check constraint allows you to specify a condition on each row in a table.

NOTE

• A check constraint can NOT be defined on a SQL View.
• The check constraint defined on a table must refer to only columns in that table. It can not refer to columns in other tables.
• A check constraint can NOT include a SQL Subquery.
• A check constraint can be defined in either a SQL CREATE TABLE statement or a SQL ALTER TABLE statement.

USING A CREATE TABLE STATEMENT

The syntax for creating a check constraint using a CREATE TABLE statement in Oracle is:

CREATE TABLE table_name
(
  column1 datatype null/not null,
  column2 datatype null/not null,

  ...

  CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE]

);

The DISABLE keyword is optional. If you create a check constraint using the DISABLE keyword, the constraint will be created, but the condition will not be enforced.

Example

CREATE TABLE suppliers
(
  supplier_id numeric(4),
  supplier_name varchar2(50),
  CONSTRAINT check_supplier_id
  CHECK (supplier_id BETWEEN 100 and 9999)
);

In this first example, we've created a check constraint on the suppliers table called check_supplier_id. This constraint ensures that the supplier_id field contains values between 100 and 9999.

CREATE TABLE suppliers
(
  supplier_id numeric(4),
  supplier_name varchar2(50),
  CONSTRAINT check_supplier_name
  CHECK (supplier_name = upper(supplier_name))
);

In this second example, we've created a check constraint called check_supplier_name. This constraint ensures that the supplier_name column always contains uppercase characters.

USING AN ALTER TABLE STATEMENT

The syntax for creating a check constraint in an ALTER TABLE statement in Oracle is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE];

The DISABLE keyword is optional. If you create a check constraint using the DISABLE keyword, the constraint will be created, but the condition will not be enforced.

Example

ALTER TABLE suppliers
ADD CONSTRAINT check_supplier_name
  CHECK (supplier_name IN ('IBM', 'Microsoft', 'NVIDIA'));

In this example, we've created a check constraint on the existing suppliers table called check_supplier_name. It ensures that the supplier_name field only contains the following values: IBM, Microsoft, or NVIDIA.

DROP A CHECK CONSTRAINT

The syntax for dropping a check constraint is:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

Example

ALTER TABLE suppliers
DROP CONSTRAINT check_supplier_id;

In this example, we're dropping a check constraint on the suppliers table called check_supplier_id.

ENABLE A CHECK CONSTRAINT

The syntax for enabling a check constraint in Oracle is:

ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;

Example

ALTER TABLE suppliers
ENABLE CONSTRAINT check_supplier_id;

In this example, we're enabling a check constraint on the suppliers table called check_supplier_id.

DISABLE A CHECK CONSTRAINT

The syntax for disabling a check constraint in Oracle is:

ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;

Example

ALTER TABLE suppliers
DISABLE CONSTRAINT check_supplier_id;

In this example, we're disabling a check constraint on the suppliers table called check_supplier_id.

UNIQUE CONSTRAINTS

This Oracle tutorial explains how to create, drop, disable, and enable unique constraints in Oracle with syntax and examples.

WHAT IS A UNIQUE CONSTRAINT IN ORACLE?

A unique constraint is a single field or combination of fields that uniquely defines a record. Some of the fields can contain null values as long as the combination of values is unique.

NOTE

• In Oracle, a unique constraint can not contain more than 32 columns.
• A unique constraint can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.

WHAT IS THE DIFFERENCE BETWEEN A UNIQUE CONSTRAINT AND A PRIMARY KEY?

Primary Key	Unique Constraint
None of the fields that are part of the primary key can contain a null value.	Some of the fields that are part of the unique constraint can contain null values as long as the combination of values is unique.

Oracle does not permit you to create both a primary key and unique constraint with the same columns.

CREATE UNIQUE CONTRAINT - USING A CREATE TABLE STATEMENT

The syntax for creating a unique constraint using a CREATE TABLE statement in Oracle is:

CREATE TABLE table_name
(
  column1 datatype [ NULL | NOT NULL ],
  column2 datatype [ NULL | NOT NULL ],
  ...

  CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);

table_name

The name of the table that you wish to create.

column1, column2

The columns that you wish to create in the table.

constraint_name

The name of the unique constraint.

uc_col1, uc_col2, ... uc_col_n

The columns that make up the unique constraint.

Example

Let's look at an example of how to create a unique constraint in Oracle using the CREATE TABLE statement.

CREATE TABLE supplier
( supplier_id numeric(10) NOT NULL,
  supplier_name varchar2(50) NOT NULL,
  contact_name varchar2(50),
  CONSTRAINT supplier_unique UNIQUE (supplier_id)
);

In this example, we've created a unique constraint on the supplier table called supplier_unique. It consists of only one field - the supplier_id field.

We could also create a unique constraint with more than one field as in the example below:

CREATE TABLE supplier
( supplier_id numeric(10) NOT NULL,
  supplier_name varchar2(50) NOT NULL,
  contact_name varchar2(50),
  CONSTRAINT supplier_unique UNIQUE (supplier_id, supplier_name)
);

CREATE UNIQUE CONTRAINT - USING AN ALTER TABLE STATEMENT

The syntax for creating a unique constraint using an ALTER TABLE statement in Oracle is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);

table_name

The name of the table to modify. This is the table that you wish to add a unique constraint to.

constraint_name

The name of the unique constraint.

column1, column2, ... column_n

The columns that make up the unique constraint.

Example

Let's look at an example of how to add a unique constraint to an existing table in Oracle using the ALTER TABLE statement.

ALTER TABLE supplier
ADD CONSTRAINT supplier_unique UNIQUE (supplier_id);

In this example, we've created a unique constraint on the existing supplier table called supplier_unique. It consists of the field called supplier_id.

We could also create a unique constraint with more than one field as in the example below:

ALTER TABLE supplier
ADD CONSTRAINT supplier_name_unique UNIQUE (supplier_id, supplier_name);

DROP UNIQUE CONSTRAINT

The syntax for dropping a unique constraint in Oracle is:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

table_name

The name of the table to modify. This is the table that you wish to remove the unique constraint from.

constraint_name

The name of the unique constraint to remove.

Example

Let's look at an example of how to remove a unique constraint from a table in Oracle.

ALTER TABLE supplier
DROP CONSTRAINT supplier_unique;

In this example, we're dropping a unique constraint on the supplier table called supplier_unique.

DISABLE UNIQUE CONSTRAINT

The syntax for disabling a unique constraint in Oracle is:

ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;

table_name

The name of the table to modify. This is the table whose unique constraint you wish to disable.

constraint_name

The name of the unique constraint to disable.

Example

Let's look at an example of how to disable a unique constraint in Oracle.

ALTER TABLE supplier
DISABLE CONSTRAINT supplier_unique;

In this example, we're disabling a unique constraint on the supplier table called supplier_unique.

ENABLE UNIQUE CONSTRAINT

The syntax for enabling a unique constraint in Oracle is:

ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;

table_name

The name of the table to modify. This is the table whose unique constraint you wish to enable.

constraint_name

The name of the unique constraint to enable.

Example

Let's look at an example of how to enable a unique constraint in Oracle.

ALTER TABLE supplier
ENABLE CONSTRAINT supplier_unique;

In this example, we're enabling a unique constraint on the supplier table called supplier_unique.

FOREIGN KEYS

This Oracle tutorial explains how to use Foreign Keys in Oracle with syntax and examples.

WHAT IS A FOREIGN KEY IN ORACLE?

A foreign key is a way to enforce referential integrity within your Oracle database. A foreign key means that values in one table must also appear in another table.

The referenced table is called the parent table while the table with the foreign key is called the child table. The foreign key in the child table will generally reference a primary key in the parent table.

A foreign key can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.

USING A CREATE TABLE STATEMENT

Syntax

The syntax for creating a foreign key using a CREATE TABLE statement is:

CREATE TABLE table_name
(
  column1 datatype null/not null,
  column2 datatype null/not null,
  ...

  CONSTRAINT fk_column
    FOREIGN KEY (column1, column2, ... column_n)
    REFERENCES parent_table (column1, column2, ... column_n)
);

Example

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)
);

In this example, we've created a primary key on the supplier table called supplier_pk. It consists of only one field - the supplier_id field. Then we've created a foreign key called fk_supplier on the products table that references the supplier table based on the supplier_id field.

We could also create a foreign key with more than one field as in the example below:

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, supplier_name)
);

CREATE TABLE products
( product_id numeric(10) not null,
  supplier_id numeric(10) not null,
  supplier_name varchar2(50) not null,
  CONSTRAINT fk_supplier_comp
    FOREIGN KEY (supplier_id, supplier_name)
    REFERENCES supplier(supplier_id, supplier_name)
);

In this example, our foreign key called fk_foreign_comp references the supplier table based on two fields - the supplier_id and supplier_name fields.

USING AN ALTER TABLE STATEMENT

Syntax

The syntax for creating a foreign key in an ALTER TABLE statement is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name
   FOREIGN KEY (column1, column2, ... column_n)
   REFERENCES parent_table (column1, column2, ... column_n);

Example

ALTER TABLE products
ADD CONSTRAINT fk_supplier
  FOREIGN KEY (supplier_id)
  REFERENCES supplier(supplier_id);

In this example, we've created a foreign key called fk_supplier that references the supplier table based on the supplier_id field.

We could also create a foreign key with more than one field as in the example below:

ALTER TABLE products
ADD CONSTRAINT fk_supplier
  FOREIGN KEY (supplier_id, supplier_name)
  REFERENCES supplier(supplier_id, supplier_name);