Question

Background

I have a table which has six columns. The first three columns create the pk. I’m tasked with removing one of the pk columns.

I selected (using distinct) the data into a temp table (excluding the third column), and tried inserting all of that data back into the original table with the third column being ’11’ for every row as this is what I was instructed to do. (this column is going to be removed by a DBA after I do this)

However, when I went to insert this data back into the original table I get a pk constraint error. (shocking, I know)

The other three columns are just date columns, so the distinct select didn’t create a unique pk for each record. What I’m trying to achieve is just calling a distinct on the first two columns, and then just arbitrarily selecting the three other columns as it doesn’t matter which dates I choose (at least not on dev).

What I’ve tried

I found the following post which seems to achieve what I want:

How do I (or can I) SELECT DISTINCT on multiple columns?

I tried the answers from both Joel,and Erwin.

Attempt 1:

However, with Joels answer the set returned is too large – the inner join isn’t doing what I thought it would do. Selecting distinct col1 and col2 there are 400 columns returned, however when I use his solution 600 rows are returned. I checked the data and in fact there were duplicate pk’s. Here is my attempt at duplicating Joels answer:

select a.emp_no, 
        a.eec_planning_unit_cde, 
        '11' as area, create_dte, 
        create_by_emp_no, modify_dte,
        modify_by_emp_no
from tempdb.guest.temp_part_time_evaluator b
inner join
(
    select emp_no, eec_planning_unit_cde
    from tempdb.guest.temp_part_time_evaluator
    group by emp_no, eec_planning_unit_cde
) a
ON b.emp_no = a.emp_no AND b.eec_planning_unit_cde = a.eec_planning_unit_cde

Now, if I execute just the inner select statement 400 rows are returned. If I select the whole query 600 rows are returned? Isn’t inner join supposed to only show the intersection of the two sets?

Attempt 2:

I also tried the answer from Erwin. This one has a syntax error and I’m having trouble googling the spec on the where clause (specifically, the trick he is using with (emp_no, eec_planning_unit_cde))

Here is the attempt:

select emp_no, 
    eec_planning_unit_cde, 
    '11' as area, create_dte, 
    create_by_emp_no, 
    modify_dte,
    modify_by_emp_no
where (emp_no, eec_planning_unit_cde) IN
(
    select emp_no, eec_planning_unit_cde
    from tempdb.guest.temp_part_time_evaluator
    group by emp_no, eec_planning_unit_cde
)

Now, I realize that the post I referenced is for postgresql. Doesn’t T-SQL have something similar? Trying to google parenthesis isn’t working too well.

Overview of Questions:

1. Why doesn’t inner join return an intersection of two sets? From googling this is what I thought it was supposed to do
2. Is there another way to achieve the same method that I was trying in attempt 2 in t-sql?
3. It doesn’t matter to me which one of these I use, or if I use another solution… how should I go about this?

Answer 1

A select distinct will be based on all columns so it does not guarantee the first two to be distinct

select pk1, pk2, '11', max(c1), max(c2), max(c3) 
from table 
group by pk1, pk2