This function converts its arguments to DOUBLE, and performs the computation in double-precision floating-point arithmetic.
If the function is applied to an empty set, then it returns NULL.

Both dependent-expression and independent-expression are numeric. The function is applied to the set of (dependent-expression, independent-expression) pairs after eliminating all pairs for which either dependent-expression or independent-expression is NULL. The following computation is then made:

( SUM( y * x ) - SUM( x ) * SUM( y ) / n ) / n

where y represents the dependent-expression and x represents the independent-expression.

Syntax 2 represents usage as a window function in a SELECT statement. As such, elements of window-spec can be specified either in the function syntax (inline), or in conjunction with a WINDOW clause in the SELECT statement.
See the window-spec definition provided in WINDOW clause.

For more information about using window functions in SELECT statements, including working examples, see Window functions.