SQL - Aggregations and Grouping
COUNT, SUM, AVG, MIN, MAX, GROUP BY, HAVING, nested aggregate functions, COALESCE, CASE WHEN
1Which SQL function counts the number of rows in a table?
Which SQL function counts the number of rows in a table?
Answer
COUNT is the SQL aggregate function that counts the number of rows returned by a query. COUNT(*) counts all rows including those with NULL values, while COUNT(column) only counts rows where the specified column is not NULL.
2Which SQL function calculates the sum of values in a numeric column?
Which SQL function calculates the sum of values in a numeric column?
Answer
SUM is the aggregate function that adds up all non-NULL values in a numeric column. It is commonly used to calculate totals such as revenue, quantities sold, or any other cumulative value. If all values are NULL, SUM returns NULL.
3What is the difference between COUNT(*) and COUNT(column)?
What is the difference between COUNT(*) and COUNT(column)?
Answer
COUNT(*) counts all rows in the result, including those containing NULL values in any column. COUNT(column) only counts rows where the specified column has a non-NULL value. This distinction is essential when analyzing incomplete data.
Which SQL function returns the average value of a numeric column?
What is the role of the GROUP BY clause in an SQL query?
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