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

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