L' Eomot Inversé (11/12/2012)

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mbova407 (11/12/2012)

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Thanks for the question

Will someone please explain example C in english

CREATE TABLE T (a int, b int, c int);

GO

INSERT INTO T VALUES (1, 1, -3), (2, 2, 4), (3, 1, NULL), (4, 3, 1), (5, 2, NULL), (6, 1, 5);

SELECT b, c,

LEAD(2*c, b*(SELECT MIN(b) FROM T), -c/2.0) OVER (ORDER BY a) AS i

FROM T;

I am very confused over the first parameter 2*c

It's very easy: 2*c is just what it says - the value in column c multiplied by 2. Which row supplies the value of c that is to be multiplied by 2? The row that is b*(select min(b)) rows ahead of the current row (the value of b used for that is of course the value in the current row; if 2*c calculated by those rules is NULL, the value -c/2.0 (where this time c is taken from the current row) is to be (implicitly converted to an integer and) used instead of that NULL.

More generally, the three arguments of LEAD are treated as follows:

The second and third arguments are expressions where any unqualified row names (or row names using the source table & schama names as qualifers) that are not bound in an aggregate refer to the current row. The first argument is an expression where those unbound column names refer to the row that leads the current row by the number which the second argument evaluates to. The third argument doesn't need to be evaluated unless evaluating the second argument and then the first argument leads to NULL for the first argument, but there's no guarantee that it won't be evaluated even when its value isn't needed. The other two arguments always have to be evaluated, and the first argument can't be evaluated until the second argument has been evaluated, because until then one doesn't know which row the free column names in it refer to.