Question

For example I have a table MyTable like this:

Id Value Rate

1 10

2 30

3 40

4 60

5 100

Rate is defined as: Rate = [ Value(id) – Value(5) ] / [ 5 – id ] , for id from 1 to 4.

I’m thinking of doing this:

INSERT INTO MyTable (Id, Rate)

SELECT Id,

??? real work goes here

FROM MyTable

LIMIT 4

ON DUPLICATE KEY UPDATE Rate=VALUES(Rate);

But can someone help me how to do the “rate” part? Thanks!

Another thing, if I define Id like this:

Id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY

Is it possible the program could assign the value of Id not continuous, for example, the values of Id like this: 1, 2, 4, 5, 6? I’m asking because if this is possible, my program would fail because I assume the Id values are continuous.

In addition, is it possible the value of Id not starting from 1? For example, the values of Id like this: 2, 3, 4, 5, 6? I’m asking because if this is possible, my program would fail because I assume the Id value always starts from 1 and if there are five rows the last one would be 5.

Thanks for helping!

Answer 1

You can write:

UPDATE MyTable
   SET Rate =
         ( Value - (SELECT Value From (SELECT Value FROM MyTable WHERE id = 5) AS t) )
         / (5 - id)
 WHERE id BETWEEN 1 AND 4
;