find rows that multiple columns are identical using SQL query

Question

I have to write a query to get a result set from table T

where table T is defined as

• Primary Key
• column A
• column B
• column C

I need to get the rows that have the same value in column As and also have same value in column Cs. How to write the query? (using generic SQL query)

Answer 1

Use:

SELECT a.PrimaryKey, b.PrimaryKey
  FROM T a
 INNER JOIN T b
    ON a.columnA = b.columnA
   AND a.columnC = b.columnC
   AND a.PrimaryKey < b.PrimaryKey

This will give all couples of rows (only one time with the inequality clause).

If that is too much (having three pairs A–B, A–C, B–C) it is also possible with standard SQL to restrict to the case where the left key is the minimal key for the group (you will then get only A–B and A–C):

SELECT a.PrimaryKey, b.PrimaryKey
  FROM T a
 INNER JOIN T b
    ON b.columnA = a.columnA
   AND b.columnC = a.columnC
  LEFT JOIN T c
    ON c.columnA = a.columnA
   AND c.columnC = a.columnC
   AND c.PrimaryKey < a.PrimaryKey
 WHERE a.PrimaryKey < b.PrimaryKey
   AND c.PrimaryKey IS NULL

Answer 2

To find the tuples A,C that have duplicate in table you can use

SELECT A, C, count(*)
FROM T
GROUP BY A, C
HAVING count(*) >=2

Now you can select all rows from table T that have A, C in this "duplicates" set.

Select PrimaryKey, A, B, C 
FROM T JOIN 
 (SELECT A, C, count(*)
  FROM T
  GROUP BY A, C
  HAVING count(*) >=2
 ) dupl
on T.A = dupl.A and T.C = dupl.C