Data Engineering Associate with Databricks Practice Exam

What type of data does COUNT(DISTINCT id) return when executed on a table?

1. The total number of NULL values

2. The count of unique IDs in the table

3. The total number of rows including duplicates

4. The sum of all ID values

The correct answer is: The count of unique IDs in the table

The function COUNT(DISTINCT id) is used in SQL to calculate the number of unique, non-null entries in a specified column, in this case, the "id" column. When executed, it performs an aggregation that counts only the distinct values found in that column. This means that any duplicate entries are considered only once, and any NULL values in the column are excluded from the count since they do not represent unique data. This characteristic makes COUNT(DISTINCT id) particularly useful for understanding the diversity of data represented in that column, providing insight into how many different entities (identified by their IDs) exist in the dataset without counting multiples. The other choices do not align with the functionality of the COUNT(DISTINCT id) operation. For instance, counting NULL values, counting total rows (which would include duplicates and NULLs), or summing ID values does not reflect the goal of determining the number of unique IDs present in the table.