# Correlation(CORR) Analytic Functions in Oracle SQL

## Correlation(CORR) Analytic Functions in Oracle SQL

CORR returns the coefficient of correlation of a set of number data.
Correlation coefficients are used in statistics to measure how strong a relationship is between two variables.
Correlation coefficient formulas are used to find how strong a relationship is between data. The formulas return a value between -1 and 1.

Output means:
1 indicates a strong positive relationship.
-1 indicates a strong negative relationship.
0 indicates no relationship at all.

Oracle Make following computation:
``` COVAR_POP(expr1, expr2) / (STDDEV_POP(expr1) * STDDEV_POP(expr2)) ```

Example 1:
Correlation between duration at the company and salary by the employee’s position by job posisiton. It implicit convert to numeric value.
``` SELECT employee_id, job_id, TO_CHAR((SYSDATE - hire_date) YEAR TO MONTH ) "Yrs-Mns", salary, CORR(SYSDATE-hire_date, salary) OVER(PARTITION BY job_id) AS "Correlation" FROM employees WHERE department_id in (50, 80) ORDER BY job_id, employee_id;```

``` EMPLOYEE_ID JOB_ID Yrs-Mns SALARY Correlation ----------- ---------- ------- ---------- ----------- 145 SA_MAN +14-05 14000 .912385598 146 SA_MAN +14-02 13500 .912385598 147 SA_MAN +14-00 12000 .912385598 148 SA_MAN +11-05 11000 .912385598 149 SA_MAN +11-02 10500 .912385598 150 SA_REP +14-01 10000 .80436755 151 SA_REP +14-00 9500 .80436755 152 SA_REP +13-07 9000 .80436755 153 SA_REP +12-11 8000 .80436755 154 SA_REP +12-03 7500 .80436755 155 SA_REP +11-04 7000 .80436755 156 SA_REP +15-01 10000 .80436755 157 SA_REP +15-00 9500 .80436755 158 SA_REP +14-07 9000 .80436755 ```

Example 2:
Correlation between duration at the company and salary by the employee’s position department wise.
``` SELECT employee_id, department_id, TO_CHAR((SYSDATE - hire_date) YEAR TO MONTH ) "Yrs-Mns", salary, CORR(SYSDATE-hire_date, salary) OVER(partition by department_id) AS "Correlation" FROM employees WHERE department_id in (50, 80) ORDER BY department_id, employee_id;```

``` EMPLOYEE_ID DEPARTMENT_ID Yrs-Mns SALARY Correlation ----------- ------------- ------- ---------- ----------- 120 50 +14-08 8000 .500062192 121 50 +13-11 8200 .500062192 122 50 +15-10 7900 .500062192 123 50 +13-05 6500 .500062192 124 50 +11-04 5800 .500062192 125 50 +13-08 3200 .500062192 126 50 +12-06 2700 .500062192 127 50 +12-02 2400 .500062192 128 50 +11-00 2200 .500062192 ```