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This article mainly explains "sql how to find duplicate data", the content of the article is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in depth, together to study and learn "sql how to find duplicate data" bar!

Write a SQL query to find all duplicate student names in the student table.

[problem-solving ideas]

1. To see the keyword "find repetition", first use the grouping function (group by), and then count the name column with the count function count () in the aggregate function.

two。 After grouping and summarizing, a table is generated as follows. Selecting a name with a count greater than 1 from this table is a duplicate name.

[problem solving steps]

Method one

1) create an auxiliary table to summarize the columns of last names in row groups

Select name, count (name) as count

From student table

Group by name

2) Select the names with a count greater than 1 in the auxiliary table

Select name from Auxiliary form

Where count > 1

3) combine the first two steps, put the step of "creating auxiliary table" into the subquery

Select name from

(

Select name, count (name) as count

From student table

Group by name

) as Auxiliary Table

Where count > 1

Method two

At this time, some students may wonder, why bother to create a subquery, can't you use this statement (put count in the where sentence) to get the answer directly?

Select name

From student table

Group by name

Where count (name) > 1

If we run this sql statement, we will report the following error. What is the problem?

The aggregate function (count) was mentioned earlier, and the where sentence cannot be used with the aggregate function. Because the where clause is in the second running order, the table has not been grouped when it comes to where.

If you want to filter the results of a grouped query, you can use the having clause. Therefore, the best way to solve this problem is as follows:

Select name

From student table

Group by name

Having count (name) > 1

Thank you for your reading, the above is the content of "how to find duplicate data in sql". After the study of this article, I believe you have a deeper understanding of how to find duplicate data in sql, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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