Databases: deduplication and querying duplicate data

In a database table, duplicate values may be included. That's not a problem, but sometimes you might want to just list different values (distinct). Keyword DISTINCT is used to return only different values.

Table A:

1. Acting on a single row

Select distinct  from A

The following results are performed:

Acting on multi-column Example 2.1

Select distinct  from A

The following results are performed:

In fact, it is the same way that access and SQL Server support it, depending on the name and ID two fields.

There are times when you need to query for records of duplicate data, which you can summarize with GROUP by:

Select ID  from A
Group  by  having Count (1>=2

Databases: deduplication and querying duplicate data