DataBase -- Count & Group by

SQL Count()函数：

• SQL COUNT(column_name)语法：COUNT(column_name)函数返回指定列的值得数目（NULL不计入）

select count(column_name) from table_name

• SQL COUNT(DISTINCT column_name)语法：COUNT(DISTINCT column_name)函数返回指定列的不同值得数目

select count(distinct column_name) from table_name

• SQL COUNT(*)语法：COUNT(*)函数返回表中的记录数

select count(*) from table_name

SQL GROUP BY 语句：

合计函数如SUM函数，常常需要添加GROUP BY语句。

语法：

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name

实例：

Question：

Write a SQL query to find all duplicate emails in a table named Person.

+----+---------+
| Id | Email   |
+----+---------+
| 1  | a@b.com |
| 2  | c@d.com |
| 3  | a@b.com |
+----+---------+

For example, your query should return the following for the above table:

+---------+
| Email   |
+---------+
| a@b.com |
+---------+

Note: All emails are in lowercase.

Analysis：

写一个SQL语句，找出Person表中所有重复的email

Answer：

select p1.Email from Person p1
group by p1.Email having count(*) > 1;