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This article introduces the relevant knowledge of "how to realize the troubleshooting processing of mysql duplicate record data". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Catalogue

Preface

Analysis.

Total data

Proportion of repetition times

The difference between where and having

Summary

Analysis.

At that time, the client was directly in front of me with a pile of box codes with repeated labels, and I panicked. I'm afraid it wasn't trouble.

After a little thought, what we need to do now is to query the duplicate tags in the database, that is, to count a tag and determine if the count is greater than 1.

Emmm, grammar error. I remember there was a Having. Try it on.

Boy, there are more than 1500 duplicate tags. Count the total number of records of the problems, and then group them to see the proportion of the number of repeated tags.

By the way, first throw these duplicate tag data to the customer to trace the product (fortunately, navicat supports copying data)

Total data

The result of the above query record is a temporary table, on this basis, sum with sum ()

There are a lot of duplicate records, and the problem is a little bit big.

Proportion of repetition times

Change the query mode to the previous query table, that is, group the count data again

From the results, most of the problem data was repeated twice.

The difference between where and having

Where is a constraint declaration that constrains the query conditions in the database before the results of the query database are returned, that is, it works before the results are returned, and aggregate functions cannot be used after where

Having is a filtering declaration that filters after the results of the query database are returned, that is, after the results are returned, and the aggregate function can be used after the having.

The so-called aggregate function is a function that calculates a set of values and returns a single value: sum--- summation, count--- count, max--- maximum, avg--- average, etc.

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