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# Symmetric Pairs

JOIN

[Question Link](https://www.hackerrank.com/challenges/symmetric-pairs/problem?isFullScreen=true)

You are given a table, *Functions*, containing two columns: *X* and *Y*.

![](https://s3.amazonaws.com/hr-challenge-images/12892/1443818798-51909e977d-1.png)

Two pairs *(X1, Y1)* and *(X2, Y2)* are said to be *symmetric* *pairs* if *X1 = Y2* and *X2 = Y1*.

Write a query to output all such *symmetric* *pairs* in ascending order by the value of *X*. List the rows such that *X1 ≤ Y1*. 

**Sample Input**

![](https://s3.amazonaws.com/hr-challenge-images/12892/1443818693-b384c24e35-2.png)

**Sample Output**

```
20 20
20 21
22 23
```

```sql
/*
- GROUP BY f1.x, f1.y HAVING COUNT(f1.x) > 1 is used to remove duplicate records 
- f1.x < f1.y is used to keep the rest in the result
*/


SELECT
    f1.x as x1,
    f1.y as y1
--    f2.x as x2,
--    f2.y as y2
FROM functions f1
JOIN functions f2
ON f1.x = f2.y and f2.x = f1.y 
GROUP BY f1.x, f1.y HAVING COUNT(f1.x) > 1 OR f1.x < f1.y  
ORDER BY f1.x
```


---

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