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Definition and Characteristics

Definition

Allowing multiple data values to be stored in a single attribute is an extended feature of relational database. Each element of a collection is possible to have different data type each other except View.

Type

Description

Definition

Input Data

Stored Data

SET

A union which does not allow duplicates

col_name SET VARCHAR(20)
col_name SET (VARCHAR(20))

{'c','c','c','b','b','a'}
{'c','c','c','b','b', 'a'}

{'a','b','c'}
{'a','b','c'}

MULTISET

A union which allows duplicates

col_name MULTISET VARCHAR(20)
col_name MULTISET (VARCHAR(20))

{'c','c','c','b','b','a'}
{'c','c','c','b','b','a'}

{'a','b','b','c','c','c'}
{'a','b','b', 'c','c','c'}

LIST
SEQUENCE

A union which allows duplicates and stores data in the order of input

col_name LIST VARCHAR(20)
col_name LIST (VARCHAR(20))

{'c','c','c','b','b','a'}
{'c','c','c','b','b', 'a'}

{'c','c','c','b','b','a'}
{'c','c','c','b','b','a'}

As you see the table above, the value specified as a collection type can be inputted with curly braces ('{', '}') each value is separated with a comma (,).

Characteristics

Coercions

If the specified collection types are identical, the collection types can be cast explicitly by using the CAST operator. The following table shows the collection types that allow explicit coercions.

Explicit Coercions

 

TO

FROM

 

SET

MULTISET

LIST

SET

-

O

O

MULTISET

O

-

X

LIST

O

O

-