MySQL CLAUSE can become a value? - mysql

Question

I am new in mysql. What I would to do is create a new table which is a copy of the original one table with one more column under a specific condition. Which condition appears as a new column is the new table. I mean:
Let table be a sequence of given point (x,y) I want to create the table temp being (x,y,r) where r = x^2 + y^2<1 But what I did is
CREATE temp LIKE table;
ALTER TABLE temp ADD r FLOAT;
INSERT INTO temp (x,y) SELECT * FROM table WHERE x*x+y*y<1;
UPDATE temp SET r=x*x+y*y;
It is ok, it gives what I want, but my database is much more bigger than this simple example and here I calculate twice the radius r in two table. It is not so good about optimization.
Is there a way to pass the clause into the new column directly?
Thanks in advance.

Answer 1

You should (almost) never store calculated data in a database. It ends up creating maintenance and application nightmares when the calculated values end up out of sync with the values from which they are calculated.
At this point you're probably saying to yourself, "Well, I'll do a really good job keeping them in sync." It doesn't matter, because down the road at some point, for whatever reason, they will get out of sync.
Luckily, SQL provides a nice mechanism to handle what you want - views.
CREATE VIEW temp
AS
SELECT
x,
y,
x*x + y*y AS r
FROM My_Table
WHERE
x*x + y*y < 1

Answer 2

You don't actually need to worry about doing the calculation twice. There is more overhead to doing an insert and update. So, you should do those calculations at the same time.
MySQL extends the use of the having clause, so this is easy:
CREATE temp LIKE table;
ALTER TABLE temp ADD r FLOAT;
INSERT INTO temp(x, y, r)
SELECT x, y, x*x+y*y as r
FROM table
HAVING r < 1;
It is quite possible that an additional table is not actually necessary, but it depends on how you are using the data. For instance, if you have rather complicated processing and are referring to temp multiple times and temp is rather smaller than the original data, then this could be a useful optimization.
Also, materializing the calculation in a table not only saves time (when the calculation is expensive, which this isn't), but it also allows building an index on the computed value -- something you cannot otherwise do in MySQL.
Personally, my preference is for more complicated queries rather than a profusion of temporary tables. As with many things with extremes, the best solution often lies in the middle (well, not really in the middle but temporary tables aren't all bad).

Answer 3

Rather than:
INSERT INTO temp (x,y) SELECT * FROM table WHERE x*x+y*y<1;
UPDATE temp SET r=x*x+y*y;
Try this:
INSERT INTO temp (x,y,r)
SELECT x,
y,
x*x+y*y AS r
FROM table
WHERE x*x+y*y<1;