MySQL Aggregate Functions

Aggregate functions calculate one value from many rows.

Function	Returns
COUNT()	Number of rows
SUM()	Total of a column
AVG()	Average of a column
MIN()	Smallest value
MAX()	Biggest value

SELECT
  COUNT(*)   AS total_students,
  SUM(marks) AS total_marks,
  AVG(marks) AS average_marks,
  MIN(marks) AS lowest,
  MAX(marks) AS highest
FROM students;

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COUNT(*) and COUNT(column)

SELECT COUNT(*) AS all_rows, COUNT(course_id) AS with_course FROM students;

COUNT(*) counts all rows. COUNT(column) counts only the rows where that column is not NULL. Vikram has no course, so he is not counted in the second one.

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With WHERE

SELECT AVG(marks) AS delhi_average FROM students WHERE city = 'Delhi';

The average of Robin (85) and Vikram (65) is 75.

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Summary

• COUNT, SUM, AVG, MIN and MAX give one value for many rows.
• Use them with WHERE and with GROUP BY.

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