COUNT
Syntax
COUNT(expr)
Contents
Description
Returns a count of the number of non-NULL values of expr in the rows retrieved by a SELECT statement. The result is a BIGINT value. It is an aggregate function, and so can be used with the GROUP BY clause.
COUNT(*) counts the total number of rows in a table.
COUNT() returns 0 if there were no matching rows.
COUNT() can be used as a window function.
Examples
CREATE TABLE student (name CHAR(10), test CHAR(10), score TINYINT); INSERT INTO student VALUES ('Chun', 'SQL', 75), ('Chun', 'Tuning', 73), ('Esben', 'SQL', 43), ('Esben', 'Tuning', 31), ('Kaolin', 'SQL', 56), ('Kaolin', 'Tuning', 88), ('Tatiana', 'SQL', 87), ('Tatiana', 'Tuning', 83); SELECT COUNT(*) FROM student; +----------+ | COUNT(*) | +----------+ | 8 | +----------+
COUNT(DISTINCT) example:
SELECT COUNT(DISTINCT (name)) FROM student; +------------------------+ | COUNT(DISTINCT (name)) | +------------------------+ | 4 | +------------------------+
As a window function
CREATE OR REPLACE TABLE student_test (name CHAR(10), test CHAR(10), score TINYINT); INSERT INTO student_test VALUES ('Chun', 'SQL', 75), ('Chun', 'Tuning', 73), ('Esben', 'SQL', 43), ('Esben', 'Tuning', 31), ('Kaolin', 'SQL', 56), ('Kaolin', 'Tuning', 88), ('Tatiana', 'SQL', 87); SELECT name, test, score, COUNT(score) OVER (PARTITION BY name) AS tests_written FROM student_test; +---------+--------+-------+---------------+ | name | test | score | tests_written | +---------+--------+-------+---------------+ | Chun | SQL | 75 | 2 | | Chun | Tuning | 73 | 2 | | Esben | SQL | 43 | 2 | | Esben | Tuning | 31 | 2 | | Kaolin | SQL | 56 | 2 | | Kaolin | Tuning | 88 | 2 | | Tatiana | SQL | 87 | 1 | +---------+--------+-------+---------------+
See Also
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