为什么这个sql查询不会返回比较浮点数的任何结果? [英] Why doesn't this sql query return any results comparing floating point numbers?
问题描述
id
和 bolag_id
是 int
。 lat
和 lngitude
是 double
。
如果我使用 lngitude
列,则不会返回任何结果:
lngitude
查询: SELECT * FROM location_forslag WHERE
lngitude = 13.8461208
但是,如果我使用 lat
列,它
lat
查询: SELECT * FROM location_forslag WHERE
lat = 58.3902782
lngitude
列?
通常不是一个比较好的主意浮点数与 =
等于运算符。
对于您的应用程序,您需要考虑您希望答案的接近程度。
1度约112公里,0.00001度约为1.1米(赤道)。你是否真的希望你的应用程序说不相等,如果两点不同,那么0.00000001度= 1mm?
set @EPSLION = 0.00001 / *赤道1.1米* /
SELECT * FROM location_forslag
WHERE`lngitude`> = 13.8461208 - @ EPSILON
AND`lngitude`< = 13.8461208 + @EPSILON
这将返回点数,其中lngitude在 @epsilon
所需值的度数。
您应该选择适合您的应用程序的epsilon值。
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 WHERE
lngitude= 13.8461208
However, if I use the lat
column, it does return results:
lat
Query: SELECT * FROM location_forslag WHERE
lat= 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.
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.
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