# UniqueValueRatio

Checks the *unique value ratio* of a column against a
given expression. A unique value ratio is the fraction of unique values divided by the
number of all distinct values in a column. Unique values occur exactly one time, while
distinct values occur *at least* once.

For example, the set `[a, a, b]`

contains one unique value (`b`

)
and two distinct values (`a`

and `b`

). So the unique value ratio of the
set is ½ = 0.5.

**Syntax**

`UniqueValueRatio`

`<COL_NAME>`

`<EXPRESSION>`

**COL_NAME**– The name of the column that you want to evaluate the data quality rule against.**Supported column types**: Any column type**EXPRESSION**– An expression to run against the rule type response in order to produce a Boolean value. For more information, see Expressions.

**Example: Unique value ratio**

This example checks the unique value ratio of a column against a range of values.

`UniqueValueRatio "test_score" between 0 and 0.5 UniqueValueRatio "Customer_ID" between 0 and 0.9 where "Customer_ID < 10"`

**Sample dynamic rules**

`UniqueValueRatio "colA" > avg(last(10))`

`UniqueValueRatio "colA" <= index(last(10),2) + std(last(5))`