SQL to duplicate query

SQL Single or group de-duplication Query method

The following data are used for testing:

Table Name: T_demo

Id	Name	Project	Grade
1	Xiao ming	Mathematical	59
1	Xiao ming	Chinese	59
1	Xiao ming	English	59
2	Little Red	Mathematical	100
2	Little Red	Chinese	99
3	Small white	Mathematical	100
3	Small white	Chinese	90
3	Small white	English	80
3	Small white	Political	70
3	Small white	History	60
4	Little Black	Mathematical	89
4	Little Black	Chinese	89
4	Little Black	English	90

1. Single field to repeat query

We now want to get the ID field information table in the table above and remove the duplicate values, which means we now want to get the number of IDs in the table.

Select ID from T_demo GROUP by ID

Query Result:

Id

1

2

3

4

2. Multi-field to repeat the query

If you need to get the ID and name remove the duplicate value information table, the equivalent of getting the number of people in this table now.

Select Id,name from T_demo GROUP by Id,name

Query Result:

Id	Name
1	Xiao ming
2	Little Red
3	Small white
4	Little Black

3. Multiple fields to repeat and get the number of duplicates

Now we need to get the number of everyone in the table.

Select Id,name,count (DISTINCT Project) as Num from T_demo GROUP by Id,name

Query Result:

Id	Name	Num
1	Xiao ming	3
2	Little Red	2
3	Small white	5
4	Little Black	3

This article is from the "World is the same" blog, please be sure to keep this source http://970076933.blog.51cto.com/9767314/1890703

SQL to duplicate query