Returns the sum of products of
xExpr calculated from values of a group where
yExpr are NOT NULL.
yExpr: A numeric expression, the dependent variable.
xExpr: A numeric expression, the independent variable.
cond: An optional boolean expression filtering the rows used for the function.
The result type is a DOUBLE.
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 result is computed after duplicates are removed.
regr_sxy(y, x) is a synonym for
regr_count(y, x) * covar_pop(y, x).