<s>
In	O
SQL	B-Language
(	O
Structured	B-Language
Query	I-Language
Language	I-Language
)	O
,	O
the	O
term	O
cardinality	B-Application
refers	O
to	O
the	O
uniqueness	O
of	O
data	O
values	O
contained	O
in	O
a	O
particular	O
column	O
(	O
attribute	O
)	O
of	O
a	O
database	B-Application
table	I-Application
.	O
</s>
<s>
The	O
lower	O
the	O
cardinality	B-Application
,	O
the	O
more	O
duplicated	O
elements	O
in	O
a	O
column	O
.	O
</s>
<s>
Thus	O
,	O
a	O
column	O
with	O
the	O
lowest	O
possible	O
cardinality	B-Application
would	O
have	O
the	O
same	O
value	O
for	O
every	O
row	O
.	O
</s>
<s>
SQL	B-Language
databases	I-Language
use	O
cardinality	B-Application
to	O
help	O
determine	O
the	O
optimal	O
query	B-Language
plan	I-Language
for	O
a	O
given	O
query	O
.	O
</s>
<s>
When	O
dealing	O
with	O
columnar	O
value	O
sets	O
,	O
there	O
are	O
three	O
types	O
of	O
cardinality	B-Application
:	O
high-cardinality	O
,	O
normal-cardinality	O
,	O
and	O
low-cardinality	O
.	O
</s>
<s>
High-cardinality	O
refers	O
to	O
columns	O
with	O
values	O
that	O
are	O
very	O
uncommon	O
or	O
unique	O
.	O
</s>
<s>
High-cardinality	O
column	O
values	O
are	O
typically	O
identification	O
numbers	O
,	O
email	O
addresses	O
,	O
or	O
user	O
names	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
data	O
table	B-Application
column	O
with	O
high-cardinality	O
would	O
be	O
a	O
USERS	O
table	B-Application
with	O
a	O
column	O
named	O
USER_ID	O
.	O
</s>
<s>
This	O
column	O
would	O
contain	O
unique	O
values	O
of	O
1-n	O
.	O
Each	O
time	O
a	O
new	O
user	O
is	O
created	O
in	O
the	O
USERS	O
table	B-Application
,	O
a	O
new	O
number	O
would	O
be	O
created	O
in	O
the	O
USER_ID	O
column	O
to	O
identify	O
them	O
uniquely	O
.	O
</s>
<s>
Since	O
the	O
values	O
held	O
in	O
the	O
USER_ID	O
column	O
are	O
unique	O
,	O
this	O
column	O
's	O
cardinality	B-Application
type	O
would	O
be	O
referred	O
to	O
as	O
high-cardinality	O
.	O
</s>
<s>
Normal-cardinality	O
refers	O
to	O
columns	O
with	O
values	O
that	O
are	O
somewhat	O
uncommon	O
.	O
</s>
<s>
Normal-cardinality	O
column	O
values	O
are	O
typically	O
names	O
,	O
street	O
addresses	O
,	O
or	O
vehicle	O
types	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
data	O
table	B-Application
column	O
with	O
normal-cardinality	O
would	O
be	O
a	O
CUSTOMER	O
table	B-Application
with	O
a	O
column	O
named	O
LAST_NAME	O
,	O
containing	O
the	O
last	O
names	O
of	O
customers	O
.	O
</s>
<s>
Since	O
there	O
is	O
a	O
variety	O
of	O
possible	O
values	O
held	O
in	O
this	O
column	O
,	O
its	O
cardinality	B-Application
type	O
would	O
be	O
referred	O
to	O
as	O
normal-cardinality	O
.	O
</s>
<s>
Low-cardinality	O
refers	O
to	O
columns	O
with	O
few	O
unique	O
values	O
.	O
</s>
<s>
Low-cardinality	O
column	O
values	O
are	O
typically	O
status	O
flags	O
,	O
Boolean	O
values	O
,	O
or	O
major	O
classifications	O
such	O
as	O
gender	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
data	O
table	B-Application
column	O
with	O
low-cardinality	O
would	O
be	O
a	O
CUSTOMER	O
table	B-Application
with	O
a	O
column	O
named	O
NEW_CUSTOMER	O
.	O
</s>
<s>
Since	O
there	O
are	O
only	O
two	O
possible	O
values	O
held	O
in	O
this	O
column	O
,	O
its	O
cardinality	B-Application
type	O
would	O
be	O
referred	O
to	O
as	O
low-cardinality	O
.	O
</s>
