Returns the coefficient of determination from values of a group where
yExpr are NOT NULL.
yExpr: An numeric expression, the dependent variable.
xExpr: An numeric expression, the independent variable.
cond: An optional boolean expression filtering the rows used for the function.
Any nulls within the group are ignored. If a group is empty or consists only of nulls, the result is NULL.
DISTINCT is specified, the average is computed after duplicates are removed.