An aggregate function that returns the number of rows, or the number of non-NULL rows, that meet certain conditions:

• The notation COUNT(*) includes NULL values in the total.
• The notation COUNT(column_name) only considers rows where the column contains a non-NULL value.
• You can also combine COUNT with the DISTINCT operator to eliminate duplicates before counting, and to count the combinations of values across multiple columns.

When the query contains a GROUP BY clause, returns one value for each combination of grouping values.

Return type: BIGINT

Examples:

-- How many rows total are in the table, regardless of NULL values?
select count(*) from t1;
-- How many rows are in the table with non-NULL values for a column?
select count(c1) from t1;
-- Count the rows that meet certain conditions.
-- Again, * includes NULLs, so COUNT(*) might be greater than COUNT(col).
select count(*) from t1 where x > 10;
select count(c1) from t1 where x > 10;
-- Can also be used in combination with DISTINCT and/or GROUP BY.
-- Combine COUNT and DISTINCT to find the number of unique values.
-- Must use column names rather than * with COUNT(DISTINCT ...) syntax.
-- Rows with NULL values are not counted.
select count(distinct c1) from t1;
-- Rows with a NULL value in _either_ column are not counted.
select count(distinct c1, c2) from t1;
-- Return more than one result.
select month, year, count(distinct visitor_id) from web_stats group by month, year;