SQL de-weight

SQL de-weight

Now there is a table T (Id,name), the ID is the primary key, name can be duplicated, and now you want to delete the duplicate data, preserving the data with the smallest ID. Please write out SQL.

Table: T

ID Name

1 sheets of three

2 sheets of three

3 John Doe

4 John Doe

5 John Doe

Analysis:

First, by name grouping, select each group ID minimum record. All data except those records is then deleted.

1:select min (id) id,name from t Groud by name.

Focus: Min (), groud by, not exists ()

Full sql:

Delete  from where  not exists Select *  from Select min  from Group  by where a.id=b.id)

SQL de-weight