Question

I am trying to write a query to removed duplicates records from the following table (valid_columns) and keep only the records with the lowest possible [order] number.

For example in the following table I would like to remove duplicate rows, region 2,3 and job 3 and keep the records with the lowest possible [order].

E.g. The input table, valid_columns looks like this:

name    col_order
-------------
job     1   
job     3   
status  2   
cust    2   
county  1   
state   1   
region  1
region  2
region  3
so      4

Desired Output:

name    col_order
-------------
job     1       
status  2   
cust    2   
county  1   
state   1   
region  1
so      4

I am trying to fix a bug and I can’t figure out the SQL. Currently it uses a delete statment and a subquery. The query used at the moment looks like this:

— 3) Remove duplicated columns

DELETE 
FROM valid_columns
WHERE   NOT ( col_order = ( SELECT  TOP 1 col_order 
            FROM    valid_columns   firstValid
            WHERE   name = firstValid.name
            AND col_order = firstValid.col_order
            ORDER BY col_order ASC ))

However, this only returns the following, which is incorrect:

name    col_order
-------------
job     1
county  1
state   1
region  1

Many thanks

Answer 1

DELETE FROM t1
FROM valid_columns t1
WHERE col_order > 
    (SELECT MIN(col_order) from valid_columns t2 WHERE t1.name = t2.name)

EDIT:
can be simplified to this:

   DELETE FROM valid_columns 
   WHERE col_order > 
        (SELECT MIN(col_order) from valid_columns t2 WHERE valid_columns.name = t2.name)

The DELETE statement can have a FROM clause to delete a record based on the value of a related record in a second table. In this case the FROM is not really required (I sometimes use the FROM to alias the table name because I don’t like the extra typing.)

DELETE FROM TableA
FROM TableA 
JOIN TableB On TableA.CriteriaA = TableB.CriteriaA

You could also try this example (might be faster if you have to do this a lot):

DELETE FROM valid_columns 
WHERE EXISTS
    (SELECT * FROM valid_columns t1 
     WHERE t1.name = valid_columns.name AND valid_columns.col_order > t1.col_order);