**Description**: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword, but it can also be used to match tags or timestamp. The first occurrence of a timestamp or tag is used.
**Description**: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword, but it can also be used to match tags or timestamp which correspond to first occurrence of the record.
**Return value type**:Same as the data type of the column being operated upon
**Applicable column types**: Any data types except for timestamp