The COUNT function has three variations. COUNT(*) counts all the rows in the target table whether they include nulls or not. COUNT(expression) computes the number of rows with non-NULL values in a specific column or expression. COUNT(DISTINCT expression) computes the number of distinct non-NULL values in a column or expression.
This query counts all rows in a table:
SELECT COUNT(*) FROM publishers;
The following query finds the number of different countries where publishers are located:
SELECT COUNT(DISTINCT country) "Count of Countries" FROM publishers