I am having a brain fart on figuring this out. I have the following two Tables:

Table: parts
    part_id          INT IDENTITY(1,1) NOT NULL,
    part_number      VARCHAR(50) UNIQUE NOT NULL,
    part_description VARCHAR(MAX) NOT NULL,
    information      VARCHAR(MAX) NULL,
    manufacturer_id  INT NOT NULL,
    subcategory_id   INT NOT NULL

Table: part_temp
    part_num  VARCHAR(50) NOT NULL,
    part_desc VARCHAR(MAX) NULL,
    info      VARCHAR(MAX) NULL,
    man_id    INT NULL,
    sub_id    INT NULL

part_temp is my temporary table that contains data from a CSV file, that is why only one column is set to be NOT NULL. I need to insert the data from part_temp into parts.

I have properly cleaned the data in the table so there are no null values trying to be inserted into rows which require a value. My issue however is with my UNIQUE constraint for my part_number columns in the parts table. There are duplicate values within the part_temp table so I need a way to be able to skip over them during my insert. This is what I have tried so far, but it does not work:

SELECT DISTINCT pt.part_num, pt.part_desc, pt.info, m.manufacturer_id, s.subcategory_id
  FROM part_temp                AS pt
       FULL OUTER JOIN man_temp AS mt ON pt.man_id = mt.man_id
       INNER JOIN manufacturers AS m  ON mt.man_name = m.manufacturer_name
       FULL OUTER JOIN cat_temp AS ct ON pt.sub_id = ct.category_id
       INNER JOIN subcategories AS s  ON ct.category_name = s.subcategory_name
 WHERE NOT EXISTS(SELECT part_number FROM parts WHERE part_number = pt.part_num)

These are the tables included in the joins not listed above

Table: man_temp
    man_id INT NOT NULL,
    man_name VARCHAR(100) NOT NULL

Table: manufacturers
    manufacturer_id   INT IDENTITY(1,1) NOT NULL,
    manufacturer_name VARCHAR(100) NOT NULL

Table: cat_temp
    category_id   INT NOT NULL,
    category_name VARCHAR(100) NOT NULL

Table: subcategories
    subcategory_id   INT IDENTITY(1,1) NOT NULL,
    subcategory_name VARCHAR(100) NOT NULL

What is it that is wrong with my INSERT query?

The specific error I am getting is:

Msg 2627, Level 14, State 1, Line 1

Violation of UNIQUE KEY constraint. Cannot insert duplicate key in object 'dbo.parts'. The duplicate key value is (31335A11)

part_num 31335A11 appears in the csv file more than once. As such it appears in the part_temp table more than once. It would be easy if it was just this entry, but I have more than 1,000 repeat entries, so it would take forever to remove all the duplicates. Nothing exists in parts as it is a brand new empty table I am trying to put values into.

  • In that case, I believe your DISTINCT isn't helping because other attributes you are selecting out are making it non-unique and therefore looks to be a bog standard data quality issue. If you stick a WHERE partnum = '31335A11' what output do you get? Aug 16, 2018 at 20:20
  • @sqldavedb If I do WHERE part_num = '31335A11' I get the same error, but for the next duplicate item in the table Aug 16, 2018 at 20:25
  • 1
    I mean don't run the insert, just run the SELECT and you should be able to visually see why 31335A11 shows up twice - unless of course if it is an invisible control character in one of the text columns. Aug 16, 2018 at 20:29
  • To further clarify this: you most likely have two records with the same part_num but different part_desc, info, manufacturer_id, or subcategory_id. So you need to decide which one is the right one. Or pick a random one of you don't care.
    – Nick.Mc
    Aug 17, 2018 at 2:39

2 Answers 2


If I run into a Violation of UNIQUE KEY constraint the first thing I do is ask myself the following questions:

  1. Does the duplicate key already exist in the target table?
  2. Is the key duplicated in any source table?

From answering the above you will have a good idea of where the duplicate value is sourced from and a reasonable idea of the cause. Example possibilities are:

  • Duplicate exists in source table
  • Duplicate does not exist at source, but a transform of the data causes duplicates to be returned

Ultimately you need to strip this problem back to basics, as you have the duplicate value in question you need to start digging into the data and pay close attention to the source query. Luckily in your example it is a simple query and you can easily strip out joins to see if any of those introduce the duplicate, or if indeed the duplicate exists due to non-key attributes being different therefore causing your DISTINCT to not do what you expect it to do.

I guess what I am trying to say is that there isn't necessarily a quick win for this kind of problem and you need to do some good ol' fashion detective work using the clues provided by SQL.

Moot point, but I like to explicitly list the columns on an insert so that nothing breaks if the order of columns on the target table is different to that of the source query.


First of all: there is no CONFLICT command in SQL SERVER.


You are going to the wrong direction using DISTINCT and WHERE NOT EXISTS. Neither commands will prevent you from dupplicating data:

Suppose you have the following data on part_temp:

part_num    part_desc         info        man_id     sub_id
000345      something1        some info   2          1
000345      something2        some info1  4          6
000345      something3        some info2  5          8

Suppose part_num 000345 does not exists in parts table; the query will try to insert the 3 records (which are distinct each other).


For accomplish what you want. SQL Server provides a command called: MERGE

Using MERGE, you can decide what to do everytime you hit a conflict: this is either to update the previous inserted record or simply skip the new candidate for insertion

Use the following code for skip inserting conflict cases:



) B
ON(A.part_num = B.part_num)
WHEN NOT MATCHED THEN INSERT(part_number,part_description,....) 

Good luck!

If you want to store repeated data into a second table include the OUTPUT Clause at the end of the UPDATE statement (which works as a DO NOTHING actually)....

   UPDATE SET A.part_num=A.part_num
   OUTPUT DELETED.part_id, B.* INTO part_duplicates
  • 1
    Personally I wouldn't use MERGE purely to avoid inserting a duplicate.I think people would be better off looking at why the duplicate exists in the first place and correcting it, otherwise perhaps use a window function like DENSE_RANK so that you can reliably pick a candidate row for insert. Otherwise you are going to pseudo-randomly pick which row is the end state for the row in the target table Aug 16, 2018 at 22:47
  • My solution focus on a recurrent loading process. If this is a single unique loading task, I agree with cleaning up the data on every table. But if you face several input csv files every day, the best you can do is to stablish rules of what to do with repeated data, and process them accordingly. Otherwise you will lose hours of work doing this check.
    – AMG
    Aug 17, 2018 at 13:55
  • @AMG Let's say I used MERGE and wanted to take all of the duplicates and put them in a table called part_duplicates that has all the same columns as the parts table (so I can find specifically what entries are causing problems, and potentially edit them in the future). How would I do that? Aug 17, 2018 at 15:03
  • @Skitzafreak, check my answer extension... But important thing here.... you can have several repeated parts for a single part_id... so don't treat this column as a primary key in the part_duplicates table
    – AMG
    Aug 17, 2018 at 15:41
  • @AMG agreed you can’t fix them one by one, but by using merge I think you will have an unreliable end state for the row which could cause more pain down the line with reconciliation issues. In which case I think youd be better off using a mechanism that will reliably/predictably return a specific row. I guess ultimately the OP needs to define rules on what is acceptable for the data processing they’re doing. Aug 17, 2018 at 16:12

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