Removing duplicates from a SQL query not just use distinct

Question

It s probably simple  here is my query   SELECT DISTINCT U NAME  P PIC ID FROM USERS U  PICTURES P  POSTINGS P1 WHERE U EMAIL ID   P1 EMAIL ID AND P1 PIC ID   P PIC ID AND P CAPTION LIKE   car      but this will only remove duplicates where a row has both the same u name and p pic id  I want it so if there is any duplicates of the names  it just leaves out the other rows  It s a weird query  but in general  how can I apply the distinct to a single column of the SELECT clause

User · Answer

You need to tell the query what value to pick for the other columns  MIN or MAX seem like suitable choices    SELECT    U NAME  MIN P PIC ID   FROM    USERS U     PICTURES P     POSTINGS P1  WHERE    U EMAIL ID   P1 EMAIL ID AND    P1 PIC ID   P PIC ID AND    P CAPTION LIKE   car    GROUP BY    U NAME

User · Answer

Arbitrarily choosing to keep the minimum PIC ID  Also  avoid using the implicit join syntax   SELECT U NAME  MIN P PIC ID      FROM USERS U         INNER JOIN POSTINGS P1             ON U EMAIL ID   P1 EMAIL ID         INNER JOIN PICTURES P             ON P1 PIC ID   P PIC ID     WHERE P CAPTION LIKE   car       GROUP BY U NAME

User · Answer

Your question is kind of confusing  do you want to show only one row per user  or do you want to show a row per picture but suppress repeating values in the U NAME field  I think you want the second  if not there are plenty of answers for the first   Whether to display repeating values is display logic  which SQL wasn t really designed for  You can use a cursor in a loop to process the results row-by-row  but you will lose a lot of performance  If you have a  smart  frontend language like a  NET language or Java  whatever construction you put this data into can be cheaply manipulated to suppress repeating values before finally displaying it in the UI   If you re using Microsoft SQL Server  and the transformation HAS to be done at the data layer  you may consider using a CTE  Computed Table Expression  to hold the initial query  then select values from each row of the CTE based on whether the columns in the previous row hold the same data  It ll be more performant than the cursor  but it ll be kinda messy either way  Observe   USING CTE  Row  Name  PicID  AS       SELECT ROW NUMBER   OVER  ORDER BY U NAME  P PIC ID          U NAME  P PIC ID     FROM USERS U         INNER JOIN POSTINGS P1             ON U EMAIL ID   P1 EMAIL ID         INNER JOIN PICTURES P             ON P1 PIC ID   P PIC ID     WHERE P CAPTION LIKE   car       ORDER BY U NAME  P PIC ID    SELECT     CASE WHEN current Name    previous Name THEN    ELSE current Name END      current PicID FROM CTE current LEFT OUTER JOIN CTE previous    ON current Row   previous Row   1 ORDER BY current Row   The above sample is TSQL-specific  it is not guaranteed to work in any other DBPL like PL SQL  but I think most of the enterprise-level SQL engines have something similar

User · Answer

If I understand you correctly  you want to list to exclude duplicates on one column only  inner join to a sub-select  select u    whatever joined values  from users u inner join  select name from users group by name having count    1  uniquenames on uniquenames name   u name

User · Answer

If I understand you correctly  you want a list of all pictures with the same name  and their different ids  such that their name occurs more than once in the table  I think this will do the trick   SELECT U NAME  P PIC ID FROM USERS U  PICTURES P  POSTINGS P1 WHERE U EMAIL ID   P1 EMAIL ID AND P1 PIC ID   P PIC ID AND U Name IN   SELECT U Name  FROM USERS U  PICTURES P  POSTINGS P1 WHERE U EMAIL ID   P1 EMAIL ID AND P1 PIC ID   P PIC ID AND P CAPTION LIKE   car    GROUP BY U Name HAVING COUNT U Name   gt  1    I haven t executed it  so there may be a syntax error or two there

[sql] Removing duplicates from a SQL query (not just "use distinct")

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