I am looking for a way to change a value randomly from positive to negative. (I am creating a distortion on a lat/long location, so I would like to offset a given location with +/- some degrees)
I already created the following query which give me a number between -1 and +1, the idea is to multiply my distortion with this number to get a random negative or positive number.
SELECT round(-1+3*RAND(),0);
The only problem is, this also generates the value 0.0 which can't be multiplied. How do I get -1 or +1 only?
TIA
ABBOV
Maybe:
start by rounding, to give 0 or 1
then multiply to give 0 or 2
then subtract, to give -1 or 1
i.e.:
SELECT ROUND(RAND()) * 2 - 1;
Related
I want to retrieve values from a table with figures formatted in float. The decimal places range up to thirteen. I only want to retrieve the rows whose values only have exactly 2 decimal places. How do I do this?
Expected output:
[45.678, 56.236656457, 89.23, 100.89] ==> [89.23, 100.89]
You can use select Length(123.12 % 1) - 2 to get the length of the values after the decimal.
select (123.12 % 1) returns 0.12 so you always know that you have 0. in your result set, you can then get the length of your value minus the two characters and only select the rows where length = 2.
Hey is there any way to create query with simple formula ?
I have a table data with two columns value_one and value_two both are decimal values. I want to select this rows where difference between value_one and value_two is grater then 5. How can i do this?
Can i do something like this ?
SELECT * FROM data WHERE (MAX(value_one, value_two) - MIN(value_one, value_two)) > 5
Example values
value_one, value_two
1,6
9,3
2,3
3,2
so analogical difs are: 5, 6, 1, 1 so the selected row would be only first and second.
Consider an example where smaller number is subtracted with a bigger number:
2 - 5 = -3
So, the result is a difference of two numbers with a negation sign.
Now, consider the reverse scenario, when bigger number is subtracted with the smaller number:
5 - 2 = 3
Pretty simple right.
Basically, the difference of two number remains same, if you just ignore the sign. This is in other words called absolute value of a number.
Now, the question arises how to find the absolute value in MySQL?
Answer to this is the built-in method of MySQL i.e. abs() function which returns an absolute value of a number.
ABS(X):
Returns the absolute value of X.
mysql> SELECT ABS(2);
-> 2
mysql> SELECT ABS(-32);
-> 32
Therefore, without worrying about finding min and max number, we can directly focus on the difference of two numbers and then, retrieving the absolute value of the result. Finally, check if it is greater than 5.
So, the final query becomes:
SELECT *
FROM data
WHERE abs(value_one - value_two) > 5;
You can also do complex operations once the absolute value is calculated like adding or dividing with the third value. Check the code below:
SELECT *
FROM
data
WHERE
(abs(value_one - value_two) / value_three) + value_four > 5;
You can also add multiple conditions using logical operators like AND, OR, NOT to do so. Click here for logical operators.
SELECT *
FROM
data
WHERE
((abs(value_one - value_two) / value_three) + value_four > 5)
AND (value_five != 0);
Here is the link with various functions available in MySQL:
https://dev.mysql.com/doc/refman/5.0/en/mathematical-functions.html
No, you would just use a simple where clause:
select *
from data
where abs(value_one - value_two) > 5;
I have this in a mysql table:
id and bolag_id are int. lat and lngitude are double.
If I use the the lngitude column, no results are returned:
lngitude Query: SELECT * FROM location_forslag WHERElngitude= 13.8461208
However, if I use the lat column, it does return results:
lat Query: SELECT * FROM location_forslag WHERElat= 58.3902782
What is the problem with the lngitude column?
It is not generally a good idea to compare floating point numbers with = equals operator.
Is it correct to compare two rounded floating point numbers using the == operator?
Dealing with accuracy problems in floating-point numbers
For your application, you need to consider how close you want the answer to be.
1 degree is about 112km, and 0.00001 degrees is about 1.1 metres (at the equator). Do you really want your application to say "not equal" if two points are different by 0.00000001 degrees = 1mm?
set #EPSLION = 0.00001 /* 1.1 metres at equator */
SELECT * FROM location_forslag
WHERE `lngitude` >= 13.8461208 -#EPSILON
AND `lngitude` <= 13.8461208 + #EPSILON
This will return points where lngitude is within #epsilon degrees of the desired value.
You should choose a value for epsilon which is appropriate to your application.
Floating points are irritating....
WHERE ABS(lngitude - 13.8461208) < 0.00000005
Convert float to decimal for compare. I had the same problem and solved like this:
SELECT
[dbo].[Story].[Longitude],
[dbo].[Story].[Latitude],
[dbo].[Story].[Location],
FROM
[dbo].[Story],
[dbo].[Places]
WHERE
convert(decimal, [dbo].[Story].[Latitude]) = convert(decimal, [dbo].[Places].[Latitude])
and
convert(decimal, [dbo].[Story].[Longitude]) = convert(decimal, [dbo].[Places].[Longitude])
and
[dbo].[Places].[Id] = #PlacesID
and
[dbo].[Story].IsDraft = 0
ORDER BY
[dbo].[Story].[Time] desc
Look at the first 3 rows after the WHERE clausule.
Hope it helps.
I have a MySQL table that has a row called cur_odds which is a percent number with the percent probability that that row will get selected. How do I make a query that will actually select the rows in approximately that frequency when you run through 100 queries for example?
I tried the following, but a row that has a probability of 0.35 ends up getting selected around 60-70% of the time.
SELECT * FROM table ORDER BY RAND()*cur_odds DESC
All the values of cur_odds in the table add up to 1 exactly.
If cur_odds is changed rarely you could implement the following algorithm:
1) Create another column prob_sum, for which
prob_sum[0] := cur_odds[0]
for 1 <= i <= row_count - 1:
prob_sum[i] := prob_sum[i - 1] + cur_odds[i]
2) Generate a random number from 0 to 1:
rnd := rand(0,1)
3) Find the first row for which prob_sum > rnd (if you create a BTREE index on the prob_sum, the query should work much faster):
CREATE INDEX prob_sum_ind ON <table> (prob_sum);
SET #rnd := RAND();
SELECT MIN(prob_sum) FROM <table> WHERE prob_sum > #rnd;
Given your above SQL statement, whatever numbers you have in cur_odds are not the probabilities that each row is selected, but is instead just an arbitrary weighting (relative to the "weights" of all the other rows) which could instead be best interpreted as a relative tendency to float towards the top of the sorted table. The actual value in each row is meaningless (e.g. you could have 4 rows with values of 0.35, 0.5, 0.75 and 0.99, or you could have values of 35, 50, 75 and 99, and the results would be the same).
Update: Here's what's going on with your query. You have one row with a cur_odds value of 0.35. For the sake of illustration, I'm going to assume that the other 9 rows all have the same value (0.072). Also for the sake of illustration, let's assume RAND() returns a value from 0.0 to 1.0 (it may actually).
Every time you run this SELECT statement, each row is assigned a sorting value by multiplying its cur_odds value by a RAND() value from 0.0 to 1.0. This means that the row with a 0.35 will have a sorting value between 0.0 and 0.35.
Every other row (with a value of 0.072) will have sorting values ranging between 0.0 and 0.072. This means that there is an approximately 80% chance that your one row will have a sorting value greater than 0.072, which would mean that there is no possible chance that any other row could be sorted higher. This is why your row with the cur_odds value of 0.35 is coming up first more often than you expect.
I incorrectly described the cur_odds value as a relative change weighting. It actually functions as a maximum relative weighting, which would then involve some complex math to determine the actual relative probabilities involved.
I'm not sure what you need can be done with straight T-SQL. I've implemented a weighted probability picker many times (I was even going to ask a question about best methods for this this morning, ironically) but always in code.
So I have a column with different numbers and wish to categorize them by range within 30 minute intervals. So 5 would be 0-30, 697 would be 690-720, and 169 would be 150-180. I was first thinking of doing a case statement, but it doesn't look like Access 2003 supports it. Is there perhaps some sort of algorithm that could determine the range? Preferably, this would be done within the query.
Thank you.
Take the integer portion of (number / 30) using the Int function and multiply it by 30 to get your lower bound, then add 30 to that number to get your upper bound.
Examples
Int(5 / 30) = 0 * 30 = 0
Int(697 / 30) = 23 * 30 = 690
Use / (integer division) and * (multiplication).
5/30*30 = 0
697/30*30 = 690
169/30*30 = 150
...
Let x be your column with the values you want to catalogue, the in pseudo-SQL you have:
select ((x/30)*30) as minrange,
(((x/30)+1)*30) as maxrange
from yourtable
(you should take the integer part of the division).
Hope this helps.
This is fairly straight forward. You can just use the following.
(number \ 30) * 30
This will give you the lower index of your range. It does have one problem, which is that 30, 720, 180 etc, will be returned as themselves. This means your ranges either need to be 0-29, 690-719, etc, or have your caller take this into account.
This assumes you are using VBA where the '\' operator returns only the quotient. See more on VB operators here