Searching and deleting duplicate data

1. Search for duplicate records

1. Check the repeated values of one or more columns.(Only the value of the record can be found, and the information of the entire record cannot be found)

For example, you can find records with duplicate stuid and stuname values.

Select stuid, stuname from stuinfo

Group by stuid, stuname

Having (count (*)> 1

2. query records with duplicate values in a column(This method is used to identify all duplicate records. If there are two duplicate records, we will find two records)

For example, find the record with duplicate stuid

Select * From stuinfo

Where stuid in (

Select stuid from stuinfo

Group by stuid

Having (count (*)> 1

)

3. query records with duplicate values in a column(Only Redundant records are displayed. That is to say, two records are displayed if there are three duplicate records)

Note: A non-repeated column is required. In this example, recno is used.

For example, find the record with duplicate stuid

Select * From stuinfo S1

Where recno not in (

Select max (recno) from stuinfo S2

Where s1.stuid = s2.stuid

2. Delete duplicate data [SQL]View
Plaincopy

1. Delete A from tablea A where a. id not in (select Min (B. ID) from tablea B group by B. Name)

[SQL]View
Plaincopy

1. Delete A from tablea A where a. ID <> (select Min (B. ID) from tablea B where a. Name = B. Name)