分组过滤查询【查找重复的电子邮箱】leetCode

SELECT email FROM Person
GROUP BY email having count(email)>1

 

 分组过滤

select email from Person

group by email

having count(Email) >1;

数据库person表数据有10条，如下所示

表中数据如图

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使用group by 和having，优先顺序：where>group by>having>order by

分组显示数量

 group by分组是优先于count函数的

考虑到可能会有不重复的数据，没有相同email邮箱的，num为1，我们只要找到不重复的数据就可以了，所以还要过滤一遍

 COUNT(DISTINCT expr,[expr...])

DISTINCT去重

1. DISTINCT 表示对后面的所有参数的拼接取 不重复的记录，即查出的参数拼接每行记录都是唯一的

SELECT DISTINCT p1.email AS Email

FROM Person p1 JOIN Person p2 ON p1.email=p2.email

WHERE p1.id!=p2.id;

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子查询

select email from
(
    select email,count(email) as num

    from Person

    group by email

)as temp

where num >1;