SQL COUNT()

COUNT() 统计行数：COUNT(*) 数全部行，COUNT(col) 不计 NULL，COUNT(DISTINCT col) 数去重后行数。

Beginner

Key PointsSQL,COUNT,计数,DISTINCT

讲解

COUNT() 统计行数：COUNT(*) 数全部行，COUNT(col) 不计 NULL，COUNT(DISTINCT col) 数去重后行数。

SQL

SELECT COUNT(*) FROM Websites;
SELECT COUNT(country) FROM Websites;
SELECT COUNT(DISTINCT country) FROM Websites;

Output

COUNT(*)
--------
5

COUNT(country)
--------------
5

COUNT(DISTINCT country)
-----------------------
2

COUNT() 统计行数：COUNT(*) 数全部行，COUNT(col) 不计 NULL，COUNT(DISTINCT col) 数去重后行数。

SQL

SELECT COUNT(*) FROM Websites;
SELECT COUNT(country) FROM Websites;
SELECT COUNT(DISTINCT country) FROM Websites;

Output

COUNT(*)
--------
5

COUNT(country)
--------------
5

COUNT(DISTINCT country)
-----------------------
2