为 SQL Server 中的列中的每个值计算冗余

Question

If I have the following table in a SQL Server 2019 database as follows:

|id   | name   | count  |
+-----+--------+--------+
| 1   | rose   |   1    |
| 2   | peter  |   1    |
| 3   | ann    |   1    |
| 4   | rose   |   2    |
| 5   | ann    |   2    |
| 6   | ann    |   3    |
| 7   | mike   |   1    |

I would like to find out if an inserted name already exists in the column "name" and how many times and right a count next to it as shown in "count" column. For example when ann was inserted the second time I put count value bext to it which is 2, and ann was inserted the third time I put 3 next to it.

How to do that using SQL?

Thank you

Answer 1

Here is one approach using the insert ... select syntax:

insert into mytable (name, cnt)
select v.name, (select count(*) + 1 from mytable t where t.name = v.name)
from (values (@name)) v(name)

The value to insert is given as @name in derived table values() . Then we use a subquery to count how many such names already exist in the table.

Answer 2

I would suggest that you calculate this information when you query the table, rather than when you insert rows:

select t.*,
       row_number() over (partition by name order by id) as count
from t;

The value will always be correct -- even when the data is updated or rows are deleted.

Answer 3

Select Name, Count(*) 
From Table
Group By Name